Global warming in the pipeline

James E. Hansen (1), Makiko Sato (1), Leon Simons (2), Larissa S. Nazarenko (3 and 4), Karina von Schuckmann (5), Norman G. Loeb (6), Matthew B. Osman (7), Pushker Kharecha (1), Qinjian Jin (8), George Tselioudis (3), Andrew Lacis (3), Reto Ruedy (3 and 9), Gary Russell (3), Junji Cao (10), Jing Li (11) ((1) Climate Science, Awareness and Solutions, Columbia University Earth Institute, New York, NY, USA, (2) The Club of Rome Netherlands, 's-Hertogenbosch, The Netherlands, (3) NASA Goddard Institute for Space Studies, New York, NY, USA, (4) Center for Climate Systems Research, Columbia University Earth Institute, New York, NY, USA, (5) Mercator Ocean International, Ramonville St.-Agne, France, (6) NASA Langley Research Center, Hampton, VA, USA, (7) Department of Geosciences, University of Arizona, Tucson, AZ, USA, (8) Department of Geography and Atmospheric Science, University of Kansas, Lawrence, KS, USA, (9) Business Integra, Inc., New York, NY, USA, (10) Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China, (11) Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China)

Improved knowledge of glacial-to-interglacial global temperature change implies that fast-feedback equilibrium climate sensitivity is at least ~4°C for doubled CO2 (2xCO2), with likely range 3.5-5.5°C. Greenhouse gas (GHG) climate forcing is 4.1 W/m2 larger in 2021 than in 1750, equivalent to 2xCO2 forcing. Global warming in the pipeline is greater than prior estimates. Eventual global warming due to today's GHG forcing alone -- after slow feedbacks operate -- is about 10°C. Human-made aerosols are a major climate forcing, mainly via their effect on clouds. We infer from paleoclimate data that aerosol cooling offset GHG warming for several millennia as civilization developed. A hinge-point in global warming occurred in 1970 as increased GHG warming outpaced aerosol cooling, leading to global warming of 0.18°C per decade. Aerosol cooling is larger than estimated in the current IPCC report, but it has declined since 2010 because of aerosol reductions in China and shipping. Without unprecedented global actions to reduce GHG growth, 2010 could be another hinge point, with global warming in following decades 50-100% greater than in the prior 40 years. The enormity of consequences of warming in the pipeline demands a new approach addressing legacy and future emissions. The essential requirement to "save" young people and future generations is return to Holocene-level global temperature. Three urgently required actions are: 1) a global increasing price on GHG emissions, 2) purposeful intervention to rapidly phase down present massive geoengineering of Earth's climate, and 3) renewed East-West cooperation in a way that accommodates developing world needs.

Comments:	48 pages, 27 figures. Correction of formatting error on page 21, which messed up placement of all following figures
Subjects:	Atmospheric and Oceanic Physics (physics.ao-ph)
Cite as:	arXiv:2212.04474 [physics.ao-ph]
	(or arXiv:2212.04474v2 [physics.ao-ph] for this version)
	https://doi.org/10.48550/arXiv.2212.04474

Submission history

James E. Hansen, 1 Makiko Sato, 1 Leon Simons, 2 Larissa S. Nazarenko, 3,4 Karina von

Schuckmann, 5 Norman G. Loeb, 6 Matthew B. Osman, 7 Pushker Kharecha, 1 Qinjian Jin, 8

George Tselioudis, 3 Andrew Lacis, 3 Reto Ruedy, 3,9 Gary Russell, 3 Junji Cao, 10 Jing Li 11

Improved knowledge of glacial-to-interglacial global temperature change implies that fast-

feedback equilibrium climate sensitivity is at least ~4°C for doubled CO 2 (2×CO 2 ), with likely

range 3.5-5.5°C. Greenhouse gas (GHG) climate forcing is 4.1 W/m 2 larger in 2021 than in

1750, equivalent to 2×CO 2 forcing. Global warming in the pipeline is greater than prior

estimates. Eventual global warming due to today’s GHG forcing alone – after slow feedbacks

operate – is about 10°C. Human-made aerosols are a major climate forcing, mainly via their

effect on clouds. We infer from paleoclimate data that aerosol cooling offset GHG warming for

several millennia as civilization developed. A hinge-point in global warming occurred in 1970

as increased GHG warming outpaced aerosol cooling, leading to global warming of 0.18°C per

decade. Aerosol cooling is larger than estimated in the current IPCC report, but it has declined

since 2010 because of aerosol reductions in China and shipping. Without unprecedented global

actions to reduce GHG growth, 2010 could be another hinge point, with global warming in

following decades 50-100% greater than in the prior 40 years. The enormity of consequences of

warming in the pipeline demands a new approach addressing legacy and future emissions. The

essential requirement to "save" young people and future generations is return to Holocene-level

global temperature. Three urgently required actions are: 1) a global increasing price on GHG

emissions, 2) purposeful intervention to rapidly phase down present massive geoengineering of

Earth’s climate, and 3) renewed East-West cooperation in a way that accommodates developing

Climate Science, Awareness and Solutions, Columbia University Earth Institute, New York, NY, USA

4 Center for Climate Systems Research, Columbia University Earth Institute, New York, NY, USA

8 Department of Geography and Atmospheric Science, University of Kansas, Lawrence, KS, USA

10 Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

11 Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China

It has been known since the 1800s that infrared-absorbing (greenhouse) gases (GHGs) warm

Earth’s surface and that the abundance of GHGs changes naturally as well as from human

actions. 1,2 Roger Revelle wrote in 1965 that we are conducting a “vast geophysical experiment”

by burning fossil fuels that accumulated in Earth’s crust over hundreds of millions of years. 3

Carbon dioxide (CO 2 ) in the air is now increasing and already has reached levels that have not

existed for millions of years, with consequences that have yet to be determined. Jule Charney led

a study in 1979 by the United States National Academy of Sciences that concluded that doubling

of atmospheric CO 2 was likely to cause global warming of 3 ± 1.5°C. 4 Charney added:

“However, we believe it is quite possible that the capacity of the intermediate waters of the

After U.S. President Jimmy Carter signed the 1980 Energy Security Act, which included a focus

on unconventional fossil fuels such as coal gasification and rock fracturing (“fracking”) to

extract shale oil and tight gas, the U.S. Congress asked the National Academy of Sciences again

to assess potential climate effects. Their Changing Climate report has a measured tone on energy

policy – amounting to a call for research. 5 Was not enough known to caution lawmakers against

taxpayer subsidy of the most carbon-intensive fossil fuels? Perhaps the equanimity was due in

part to a major error: the report assumed that the delay of global warming caused by the ocean’s

thermal inertia is 15 years, independent of climate sensitivity. With that assumption, they

concluded that climate sensitivity for 2×CO 2 is near or below the low end of Charney’s 1.5-

4.5°C range. If climate sensitivity was low and the lag between emissions and climate response

Simultaneous with preparation of Changing Climate, a symposium was held 25-27 October 1982

at Columbia University’s Lamont Doherty Geophysical Observatory, with papers published in

January 1984 as Climate Processes and Climate Sensitivity, a monograph of the American

Geophysical Union. 6 The symposium focused on the ocean’s role in climate change and on

climate change information contained in the paleoclimate record. Paleoclimate data showed that

climate sensitivity is in the range 2.5-5°C for 2×CO 2 , thus at the upper end of Charney’s range.

In turn, this implied that the climate response time to a forcing is of the order of a century, not 15

years. Thus, the concept that a large amount of additional human-made warming is already “in

the pipeline” was introduced. 7 E.E. David, Jr., President of Exxon Research and Engineering, in

his keynote talk at the symposium insightfully noted: “The critical problem is that the

environmental impacts of the CO 2 buildup may be so long delayed. A look at the theory of

feedback systems shows that where there is such a long delay, the system breaks down, unless

Thus, the danger caused by climate’s delayed response and the need for anticipatory action to

alter the course of fossil fuel development was apparent to scientists and the fossil fuel industry

40 years ago. 9 Yet industry chose to long deny the need to change energy course, 10 and now,

while governments and financial interests connive, most industry adopts a “greenwash” approach

that threatens to lock in perilous consequences for humanity. Scientists will share responsibility,

if we allow governments to rely on goals for future global GHG levels as if targets had meaning

in the absence of policies required to achieve them. In the final section of this perspective article,

The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 to provide

policymakers with regular scientific assessments on the current state of knowledge about climate

change 11 and almost all nations agreed to the 1992 United Nations Framework Convention on

Climate Change 12 with the objective to avert “dangerous anthropogenic interference with the

climate system,” The current IPCC Working Group 1 report 13 describes shutdown of the

overturning ocean circulations and large sea level rise on the century time scale as “high impact,

low probability” even under extreme GHG growth scenarios. This contrasts with “high impact,

high probability” assessments reached in a paper – hereafter abbreviated Ice Melt – that several

of us published in 2016. 14 Recently, the first author (JEH) of our present paper published a

qualitative description of the decade-long investigation that led to the conclusion that most

climate models are unrealistically insensitive to freshwater injected by melting ice and also that

ice sheet models are unrealistically lethargic in the face of rapid, large climate change. 15

Eelco Rohling, editor-in-chief of Oxford Open Climate Change, invited one of us (JEH) to write

a perspective article on these scientific issues. We had noted in our papers that global warming in

the past century does not imply a unique climate sensitivity because the warming at Earth’s

surface depends on three major unknowns with only two fundamental constraints. Unknowns are

ECS, net climate forcing (because aerosol forcing is unmeasured), and ocean mixing of heat.

Constraints are observed global temperature change and EEI. In our investigation noted above,

we assumed the canonical climate sensitivity 3°C for 2×CO 2 , thus leaving two unknowns and

two constraints. This allowed us to confirm that most climate models mix heat excessively into

the deeper ocean and compensate for this by using a less strong aerosol forcing (less negative)

A fresh look at this problem is demanded by two recent developments. First, improved analyses

of global temperature during the last glacial maximum and during the prior (Eemian) interglacial

period allow inference that ECS is higher than the canonical estimate. Second, although aerosol

climate forcing remains unmeasured, there is evidence that human-made aerosol amount is on

the decline, implying that acceleration of global warming may be in the offing. We clarify the

physics by use of “response functions” for both global temperature and EEI, which reveal that

climate response time is not simply a function of ocean mixing. We infer that ultrafast cloud

feedbacks affect global temperature and EEI in opposite senses – slowing the warming of the

ocean while speeding up partial restoration of planetary energy balance. We will describe

implications in two papers. This first paper – Global Warming in the Pipeline – focuses on

climate sensitivity, climate response time, and aerosols. The second paper – Sea Level Rise in the

Pipeline – presents evidence that continued warming and increasing ice melt can cause shutdown

of the overturning ocean circulations within decades and large sea level rise within a century.

Charney defined an equilibrium climate sensitivity (ECS): the eventual global temperature

change caused by doubled CO 2 in the idealized case in which ice sheets, vegetation and long-

lived GHGs are fixed (except for the specified CO 2 doubling). All other quantities are allowed to

change. The ones deemed most significant – clouds, aerosols, water vapor, snow cover and sea

ice – change rapidly in response to climate change. Thus, the Charney ECS is also called the

“fast feedback” climate sensitivity. Feedbacks can interact in many ways, so their changes are

usually calculated in global climate models (GCMs) that can simulate such interactions. Charney

implicitly assumed that change of the ice sheets on Greenland and Antarctica – which we will

categorize as a “slow feedback” – was not important on the time scale of most public interest.

ECS defined by Charney is a useful concept that helps us understand how human-made and

natural climate forcings affect climate. We must also consider an Earth system sensitivity, 16 ESS,

in which all feedbacks are allowed to respond to a climate forcing. ECS and ESS both depend on

the initial climate state 17,18 and direction (warming or cooling) of climate change, but at the

present climate state – with ice sheets on Antarctica and Greenland – climate should be about as

sensitive in the warmer direction as in the cooler direction. Paleoclimate data indicate that ESS

substantially exceeds ECS, i.e., when feedbacks that Charney kept fixed are allowed to change,

climate sensitivity increases. As Earth warms, ice sheets shrink and the atmosphere contains

The time scale of climate feedbacks is crucial, but poorly understood, especially for the unique

human-made forcing. As quantified below, the human-made GHG climate forcing is already 4

W/m 2 , equivalent to 2×CO 2 , and a GHG forcing as large as 8 W/m 2 (equivalent to 4×CO 2 ) is

possible, perhaps likely, within a century. Such forcing is larger than estimates of the forcing that

drove the largest known rapid global warming, the Paleocene Eocene Thermal Maximum

(PETM), 19 which occurred ~56 MyBP. The CO 2 increase that drove PETM global warming was

introduced over a few thousand years. 20 The net human-made climate forcing has been growing

rapidly only since about 1970, i.e., for about half a century, but within another century it could

match or exceed the PETM forcing, while being introduced 20 times faster. There is no known

paleoclimate analogue of such a forcing. In Sea level rise in the Pipeline it will be argued that

such a large, rapid forcing will cause nonlinear growth of ice melt, that excessive small-scale

ocean mixing in most GCMs has caused underestimate of the effect of ice melt on overturning

ocean circulations, that the world is nearing ice melt rates that will affect these circulations, that

increasing ice melt increases Earth’s energy imbalance thus accelerating ice melt and creating

the danger of collapse of the West Antarctic ice sheet on a century time scale. Such increased ice

melt and shutdown of ocean circulations, if they occur, will cool the North Atlantic and Southern

Oceans. That type of cooling is not helpful, as it increases Earth’s energy imbalance and thus the

rate at which energy is pumped into the ocean. The cooling needed to slow and stop global

warming and ice melt requires reducing and eliminating Earth’s energy imbalance caused by the

human-made climate forcing. Despite the danger of transitioning into nonlinear climate change –

indeed, because of that danger – improved understanding of ECS is important. High ECS

increases climate response time and the amount of global warming presently “in the pipeline”

If knowledge of ECS was based only on models, it would be difficult to narrow the range of

estimated climate sensitivity – or to have high confidence in any range – because we do not

know how well feedbacks are modeled or even if the models include all significant real-world

feedbacks. Cloud and aerosol interactions are complex, and even small cloud changes can have a

substantial effect. That is why data on Earth’s paleoclimate history are so valuable; they allow us

to compare different equilibrium climate states, knowing that all feedbacks were in operation.

In our paper 7 for the AGU Geophysical Monograph we compared the Last Glacial Maximum

(LGM) with the current interglacial period (the Holocene). We ran GCM simulations introducing

one-by-one LGM surface conditions provided by the CLIMAP project 21 and analyzed the effect

of individual feedbacks on global change. With all CLIMAP surface conditions incorporated in

the GCM – including ice sheet sizes and sea surface temperature (SST) – the calculated global

mean surface temperature was 3.6°C colder in the LGM than in the Holocene. From analysis of

We recognized the potential to get a more certain and accurate evaluation of ECS by using the

fact that Earth had to be in energy balance during the LGM. With CLIMAP surface conditions,

we found that the model Earth was out of energy balance by 2.1 W/m 2 , radiating more energy to

space than it receives from the Sun. Such a large energy imbalance is impossible; averaged over

millennia, the planet had to be in energy balance within less than 0.1 W/m 2 . Earth (i.e., the

climate model with CLIMAP SSTs) was trying to cool off; it would need to cool at least 1-2°C

to achieve energy balance. When we employed CLIMAP’s “maximal extent” ice sheet area –

assuming that maximum ice sheet size was obtained simultaneously on all continents in both

hemispheres – the increased reflection of sunlight only reduced the imbalance to 1.6 W/m 2 .

Something was wrong with either CLIMAP surface conditions or our assumed change of

atmospheric composition between the LGM and today. Indeed, we did not realize that – in

addition to reduced CO 2 – CH 4 and N 2 O were less abundant in the LGM than today. However,

the effect of that change is moderate and the sense is to make the energy imbalance even larger.

A likely explanation was that CLIMAP SSTs were unrealistically warm. We noted independent

evidence for that conclusion, including a then-ongoing study of proxy temperature data by Rind

and Peteet that indicated low latitude CLIMAP SSTs were too warm by as much as 2-3°C. 22

Colder SSTs during the LGM implied a higher climate sensitivity. Our calculated climate forcing

for the Holocene relative to the LGM (due to ice sheet, vegetation and CO 2 change) was almost 6

W/m 2 . Forcing by 2×CO 2 is ~4 W/m 2 , two-thirds of the LGM-to-Holocene forcing, so

CLIMAP’s estimate of 3.6°C temperature change implied an ECS at the low end of the 2.5-5°C

range estimated from our feedback analysis. But if the LGM was cooler – as implied by the

calculated energy imbalance – ECS would be in the upper part of the 2.5-5°C range.

CLIMAP project members would not concede such large errors in LGM SSTs. Therefore, we

concluded only that climate sensitivity was 2.5-5°C for 2×CO 2 . Even so, that range was more

precise and reliable than climate models alone can ever provide. Today, advanced techniques for

analysis allow more definitive assessment of climate sensitivity. Tierney et al. 23 used a large

collection of geochemical proxies for SST constrained by isotope measurements and climate

change patterns defined by GCMs to find cooling of 6.1°C (95% confidence: 5.7-6.5°C) for the

interval 23-19 ky BP. A further dynamically-constrained full-field analysis of climate evolution

since the LGM by Osman, Tierney, et al. 24 sets LGM cooling at 21-19 ky BP as 6.8 ± 1°C with

95% confidence. 25 Seltzer et al. 26 use the temperature-dependent solubility of dissolved noble

gases in ancient groundwater to find that global land areas between 45°S and 35°N cooled by 5.8

± 0.6°C in the LGM; given the polar amplification of LGM cooling due in part to enhanced ice

Here we accept the conclusion that the LGM was at least 6°C cooler than the preindustrial

Holocene and infer implications for climate sensitivity. First, however, we must clarify the

Progress in narrowing the uncertainty in climate sensitivity was slow in the first five assessment

reports of the IPCC. The fifth assessment report 27 (AR5) in 2014 concluded only – with 66%

probability – that ECS was in the range 1.5-4.5°C, the same as Charney’s report 35 years earlier.

Actually, much progress was being made in understanding of climate change. We estimate that

thousands of papers on relevant climate processes were published that affect estimates of climate

sensitivity. The broad spectrum of information – especially constraints imposed by paleoclimate

data – at last affected the AR6 estimate of ECS. AR6 13 concludes with 66% probability that ECS

We avoid review of the literature on climate sensitivity by relying on the recent comprehensive

review by Sherwood and 24 co-authors, 28 who used multiple lines of evidence to infer that

climate sensitivity to doubled CO 2 is 2.6-3.9°C with 66% probability. This range refers to an

“effective sensitivity,” S, that the authors anticipate will differ from ECS by only several percent.

S is intended to be relevant to the 150-year time scale. We focus on the equilibrium climate

sensitivity (ECS) to allow us to evaluate climate sensitivity and climate response time

independently. Understanding of response time is needed for the sake of assessing the urgency

and the nature of actions required to maintain a propitious climate. Also, ECS can potentially be

derived precisely from data on past stable climate states that were necessarily in near energy

balance. Over the LGM-to-Holocene transition, which required energy to melt ice equivalent to

130 m of sea level and raise ocean temperature several degrees, energy imbalance averaged only

about +0.2 W/m 2 . 29 Thus during the LGM and Holocene, when global ocean temperature and sea

level were relatively stable, EEI averaged over several ky was much less than 0.1 W/m 2 .

We will estimate ECS using pairs of equilibrium climate states that bound glacial-to-interglacial

climate changes. First, though, we need to discuss climate forcing definitions and comment on

Equilibrium global surface temperature change, at least nominally, is related to ECS by

where λ is a widely-used abbreviation of ECS, ΔT S is the global mean equilibrium surface

temperature change in response to climate forcing F, which is an imposed perturbation of the

planet’s energy imbalance measured in W/m 2 averaged over the entire planetary surface. There

are alternative ways to define F, as discussed in Chapter 8 30 of AR5 and in a paper 31 hereafter

called Efficacy. Objectives are to find a definition of F such that different forcing mechanisms of

the same magnitude yield a similar global temperature change, but also a definition that can be

computed easily and reliably. The first four IPCC reports used adjusted forcing, F a , which is

Earth’s energy imbalance after stratospheric temperature adjusts to presence of the forcing agent.

F a usually yields a consistent response among different forcing agents, but there are exceptions

such as black carbon aerosols; F a exaggerates their impact. Also, F a is awkward to compute and

depends on definition of the tropopause, which varies among models. F s , the fixed SST forcing

(including fixed sea ice), is much more robust than F a as a predictor of climate response, 31,32 but

where F o is Earth’s energy imbalance after atmosphere and land surface adjust to the presence of

the forcing agent with SST fixed. A GCM run of about 100 years is needed to accurately define

F o because of unforced atmospheric variability. The GCM run also defines δT o , the global mean

surface air temperature change caused by the forcing with SST fixed. λ is the model’s ECS in °C

per W/m 2 . δT o /λ is the portion of the total forcing (F s ) that is “used up” in causing the δT o

warming; radiative flux to space increases by δT o /λ due to warming of the land surface and

global air. The term δT o /λ is usually less than 10% of F o , but not necessarily negligible.

IPCC AR5 and AR6 define effective radiative forcing ERF as ERF = F o . Omission of δT o /λ was

intentional 30 and is not a major issue because uncertainty in most forcings is as large as δT o /λ.

However, if the forcing is used to calculate global surface temperature response, the forcing to

use is F s , not F o . It would be useful if both F o and δT o were reported for all climate models.

A further refinement of climate forcing is suggested in Efficacy: an effective forcing (F e ) defined

by a long GCM run with calculated ocean temperature. The resulting global surface temperature

change, relative to that for an equal CO 2 forcing, defines the efficacy of the forcing. Effective

forcings, F e , were found to be within a few percent of F s for most forcing agents, i.e., the results

confirmed that F s is a robust forcing definition. This support, strictly, is for F s , not for F o = ERF,

Another issue was exposed by climate simulations of the Goddard Institute for Space Studies

(GISS) GCM for the CMIP6 33 and AR6 studies. This newer GISS model, 34,35 which we label the

GISS (2020) model, 36 has higher resolution (2°×2.5° and 40 atmospheric layers) and other

changes that yield a moister upper troposphere and lower stratosphere, relative to the GISS

model used in Efficacy and earlier papers. The fixed SST simulation for 2×CO 2 with the GISS

(2020) model yields F o = 3.59 W/m 2 , δT o = 0.27°C and λ = 0.9 °C per W/m 2 . Thus F S = 3.59 +

0.30 = 3.89 W/m 2 , which is 5.4% smaller than the F S = 4.11 W/m 2 for the GISS model used in

Efficacy. The GISS (2020) authors 34,35 attribute reduced CO 2 forcing to infrared blanketing by

increased water vapor. We agree with the sense of this impact of increased water vapor, but

changes introduced in the GISS (2020) cloud parameterization also may be a relevant factor.

Introduction of F s in AR5 to quantify climate forcing is a valuable advance, but it allows rapid

feedbacks to come into play in assessed forcings. In a later section, we find evidence of rapid

adjustments in GISS (2020) that are likely related to the cloud parameterization.

Fig. 1. IPCC AR6 Annex III greenhouse gas forcing, 13 which employs F a for O 3 and F o for other

GHGs, compared with the effective forcing, F e , from Eq. (3). See discussion in text.

Meanwhile, we use existing data to construct and make available formulae for GHG forcing as a

function of gas amounts. Our original formulae, 37 included in Supporting Material, are adjusted

GHG forcing, F a , obtained as numerical fit to calculations with the GISS GCM radiation code,

which uses the correlated k-distribution method 38 based on high spectral resolution laboratory

data. 39 The laboratory data have changed little, so we convert these F a to effective forcings (F e )

via efficacy factors (E a ) from Table 1 of Efficacy. The total GHG forcing is then

F e = F a (CO 2 ) + 1.45 F a (CH 4 ) + 1.04 F a (N 2 O) + 1.32 F a (MPTGs + OTGs) + 0.45 F a (O 3 ). (3)

F a forcings were calculated with a global-mean 1-dimensional (1-D) radiative convective model;

thus coefficients in (3) include effect of conversion to 3-D atmosphere (see Supporting Material).

The coefficient for CH 4 (1.45) includes the effect of changing CH 4 on stratospheric water vapor

and O 3 , as well as the efficacy of CH 4 per se (1.10). Following Prather and Ehhalt, 40 we assume

that CH 4 is responsible for 45% of the O 3 change. Forcing caused by the remaining 55% of the

O 3 change is based on the IPCC AR6 O 3 forcing (F a = 0.47 W/m 2 in 2019); we multiply this AR6

O 3 forcing by 0.55 × 0.82 = 0.45, where 0.82 is the efficacy of O 3 forcing from Table 1 of

Efficacy. Thus, the non-CH 4 portion of the O 3 forcing is 0.21 W/m 2 in 2019. MPTGs and OTGs

are Montreal Protocol Trace Gases and Other Trace Gases. 41 An updated list of these gases and a

table of their annual forcings since 1992 are available as well as the earlier data. 42

The climate forcing from our formulae is slightly larger than IPCC AR6 forcings (Fig. 1). For

example, in 2019, the final year of AR6 data, our GHG forcing is 4.00 W/m 2 , while the AR6

forcing is 3.84 W/m 2 . Our forcing is expected to be larger, because the IPCC forcings are F o for

all gases except O 3 , for which they provide F a (AR6 section 7.3.2.5). Table 1 in Efficacy allows

accurate comparison: δT o for 2×CO 2 for the GISS model used in Efficacy is 0.22°C, λ is 0.67°C

per W/m 2 , so δT o /λ = 0.33 W/m 2 . Thus, the conversion factor from F o to F e (or F s ) is 4.11/(4.11-

0.33). The non-O 3 portion of the AR6 2019 forcing (3.84 – 0.47 = 3.37) W/m 2 increases to 3.664

W/m 2 . The O 3 portion of the AR6 2019 forcing (0.47 W/m 2 ) decreases to 0.385 W/m 2 because

the efficacy of F a (O 3 ) is 0.82. The AR6 GHG forcing in 2019 is thus ~ 4.05 W/m 2 , expressed as

Fig. 2. Antarctic Dome C temperature for past 800 ky from Jouzel et al.(2007) 43 relative to the

This nearly precise agreement is not indicative of the true uncertainty in the GHG forcing, which

IPCC AR6 estimates as 10%, thus about 0.4 W/m 2 . In Supporting Materials, we show that our

CH 4 forcing is larger than that of IPCC AR6, while our MPTG + OTG forcing is smaller than

that of IPCC AR6; these differences approximately offset. The forcing calculation is difficult

because it must account for the complex spectral variability of gaseous absorption and the four-

dimensional variability of water vapor and clouds, but modern computer capability makes more

accurate assessment possible via systematic model intercomparisons. Improved knowledge of

atmospheric radiative properties is needed as humanity works to limit GHGs and otherwise

The stunning conclusion is that the GHG increase since 1750 now produces a climate forcing

equivalent to that of 2×CO 2 (our formulae yield F e ~ F s = 4.09 W/m 2 for 2021; IPCC’s AR6 F s =

4.14 W/m 2 ). The human-made 2×CO 2 climate forcing imagined by Charney, Tyndall and other

greenhouse giants 1 is no longer imaginary. At this moment, humanity is taking its first steps into

the period of consequences. Earth’s paleoclimate history helps us assess potential outcomes.

Air bubbles in Antarctic ice cores – trapped as snowfall piled up and compressed into ice –

preserve a record of long-lived GHGs for at least the past 800,000 years. Isotopic composition of

the ice provides a measure of temperature change in and near Antarctica. 43 In general, CO 2 , CH 4

Changes of Antarctic temperature and GHGs, especially CO 2 , are highly correlated (Fig. 2). This

does not imply that GHGs were the primal cause of the climate oscillations. Hays, Imbrie and

Shackleton 45 showed that small changes of Earth’s orbit about the Sun and the tilt of Earth’s spin

axis relative to the orbital plane are pacemakers of the ice ages. These orbital changes alter the

seasonal and geographical distribution of insolation, which initiates change of ice sheet size and

GHG amounts. Both of these are mechanisms for glacial-interglacial climate change, but the

reason long-term climate is so sensitive is the further role of ice sheets and GHGs as amplifying

feedbacks. As Earth warms, ice sheets shrink, thus exposing a darker surface that absorbs more

sunlight and warms Earth; this effect works in the opposite sense as Earth cools. Also, as Earth

warms, the ocean and continents release GHGs to the air, which amplifies the warming; as Earth

cools, the ocean and continents take up these gases, which amplifies the cooling. 46

The weak orbital forcings oscillate slowly over tens and hundreds of thousands of years. 47 The

picture of how Earth orbital changes drive millennial climate change was first painted clearly in

the 1920s by Milutin Milankovitch, who built on 19 th century hypotheses of James Croll and

Joseph Adhémar. Paleoclimate changes of ice sheet size and GHG amount in response to global

temperature change are sometimes described as slow feedbacks. 48 They change slowly in the

paleoclimate record, because they are paced by the slowly changing Earth orbital forcing.

However, this does not mean that these feedbacks cannot operate more rapidly in response to a

rapid climate forcing. Indeed, we will conclude that GHG and ice sheet feedbacks partially

respond well before the fast-feedback response to a climate forcing is complete.

Today it is possible to evaluate ECS precisely via comparison of stable climate states before and

after a glacial-to-interglacial climate transition. GHG amounts are known from ice cores and ice

sheet sizes can be inferred from sea level and other geologic data. A warm LGM suggested by

CLIMAP and MARGO 49 data (~3°C cooler than the Holocene) can be firmly rejected, because it

is now certain that their SST data yield a planet out of energy balance by more than 2 W/m 2 , as

discussed above. An energy imbalance of +2 W/m 2 is enough to raise the temperature of the

upper kilometer of the ocean 2.2°C or melt ice to raise sea level 22 m in a century 50 – and 10

times those amounts in a millennium. Such change rates did not occur, so the LGM was more

than 3°C cooler than today. As discussed above, we accept the recent paleo analyses concluding

The Holocene is an unusual interglacial. It began as expected: the maximum glacier melt rate

was at 13.2 kyBP (kiloyears before present) 51 and, after peaking early in the Holocene, GHG

amounts began to decline as in most interglacials. However, several ky later, CO 2 and CH 4 began

to increase, which raised a question of whether humans were beginning to affect GHG amounts.

Ruddiman 52 suggested that CO 2 began to be affected by deforestation 8 ky ago and CH 4 by rice

irrigation 5 ky ago. That issue does not prevent us from using the LGM-Holocene comparison to

estimate ECS, but for the sake of clarity we compare the LGM with both the early and late

Holocene. In addition, we compare the prior glacial maximum (PGM) 53 with the subsequent

interglacial (Eemian, about 130-118 kyBP). Based on a review 54 of Eemian data, we estimate

that the Eemian was about +1°C warmer than the average Holocene temperature. The review

includes a robust estimate of peak Eemian SSTs of +0.5 ± 0.3°C relative to 1870-1889, 55 which

is +0.65 ± 0.3°C relative to our base period 1880-1920 and is consistent with our estimate of

CO 2 , CH 4 and N 2 O amounts in the Holocene, LGM, Eemian and PGM are known accurately

from ice cores, with the exception of N 2 O in the PGM when N 2 O reactions with dust in the ice

core corrupt the data. 56 We take PGM N 2 O as the mean of the smallest reported PGM amount

and the LGM amount. The resulting potential error in the N 2 O forcing is of order 0.01 W/m 2 .

We calculate CO 2 , CH 4 , and N 2 O forcings using Eq. (3) and formulae for each gas in Supporting

Material. We provide 57 GHG amounts and calculated forcings for the periods shown by green

bars in Fig. 3. The period chosen in the Eemian avoids the early spike of CO 2 and temperature to

assure that it is a period with Earth in energy balance. Between the LGM (18-24 kyBP) and late

Holocene (1-5 kyBP), GHG forcing increased 2.5 W/m 2 with 2.0 W/m 2 (80%) from CO 2 .

Between the LGM and early Holocene, GHG forcing increased 2.15 W/m 2 with 80% from CO 2 .

Between the PGM and Eemian, GHG forcing increased 2.3 W/m 2 with 79% from CO 2 .

Fig. 3. Antarctic (dome C) temperature (Jouzel et al. 43 ) and multi-ice core GHG amounts (Schilt

et al.). 56 Green bars (1-5, 6-10, 18-24, 120-126, 137-144 kyBP) are periods of calculations.

Earth’s surface changes are the other forcing required to evaluate ECS: (1) change of surface

albedo (reflectivity) and topography due to ice sheets, (2) vegetation change, e.g., interglacial

boreal forests replaced by brighter tundra, and (3) continental shelves exposed by lower sea level

in glacial times. The forcing caused by all three changes can be evaluated at once with a global

climate model. Accurate assessment requires realistic simulation of clouds, which reduce surface

albedo effects. Clouds are the most important and difficult fast feedback (rapid response) in

global climate models. 58 Thus, evaluation of the surface forcing is ideal for PMIP 59

(Paleoclimate Modelling Intercomparison Project) collaboration with CMIP 60 (Coupled Model

Intercomparison Project); a joint study could provide valuable model intercomparisons as well as

Aerosols are a final issue to address before estimating climate sensitivity. Human-made aerosols,

including their effect on clouds, are a climate forcing (an imposed perturbation of Earth’s energy

balance). Natural aerosol changes are, like clouds and water vapor, a fast climate feedback.

Indeed, aerosols and clouds form a continuum and distinction becomes arbitrary as humidity

approaches 100 percent. There are many aerosol types, including VOCs (volatile organic

compounds) produced by trees, sea salt produced by wind and waves, black and organic carbon

produced by forest and grass fires, dust produced by wind and drought, and marine biologic

dimethyl sulfide and its secondary aerosol products. All of these vary geographically and in

response to climate change. We cannot accurately specify their properties in prior eras, and there

is no need to do so, because their changes are feedbacks included in the climate response.

Sherwood et al. 28 review studies of LGM ice sheet forcing and settle on –3.2 ± 0.7 W/m 2 , the

same as the IPCC AR4 estimate. 61 However, some GCMs yield efficacies for ice sheet forcings

as low as ~0.75 62 or even ~0.5, 63 i.e., the response to the ice sheet forcing is a fraction of the

response to an equal CO 2 forcing. For LGM vegetation, we 7 found a forcing of – 0.9 W/m 2 by

using the Koppen 64 scheme to relate vegetation to local climate. Kohler et al. 65 estimate a

continental shelf forcing of – 0.6 W/m 2 . We estimate the net LGM-Holocene surface forcing as

3-5 W/m 2 , with the wide range due to the uncertain efficacy of the surface forcing. For the time

being – until more accurate assessment of surface forcing is available – let’s use the mean

Holocene GHG forcing of 2.3 W/m 2 , which makes the total LGM-Holocene forcing 5.3-7.3

W/m 2 . Taking the LGM-Holocene warming as 6.1°C 23 and 2×CO 2 forcing as 4 W/m 2 yields ECS

= 3.3-4.6°C for 2×CO 2 . Osman, Tierney et al. LGM cooling of 6.8°C for 23-19 ky BP yields

PGM-Eemian climate change provides a check. PGM-Eemian GHG forcing was 2.3 W/m 2 . PGM

sea level was ~10 m higher than LGM sea level. 66 The North American ice sheet was smaller

than in the LGM and the Eurasian ice sheet was probably larger. 53 Redistribution of ice mass

between the two major ice sheets has little effect on their combined climate forcing, but less ice

mass by the equivalent of 10 m of sea level reduces the surface forcing by ~0.3 W/m 2 ; see Fig.

S4 in Target CO 2 paper. 67 PGM-Eemian global warming was at least as great as LGM-Holocene

global warming. The PGM was probably slightly warmer than the LGM, as suggested by the

higher PGM sea level and the temperature inferred at Dome C in Antarctica (Fig. 2). However,

temperature at Dronning Maud Land in Antarctica seems to have been cooler in the PGM than in

the LGM. 68 The global Eemian temperature was about 1°C warmer than the Holocene, as

discussed above. In summary, PGM-Eemian warming was several tenths of a degree greater than

the LGM-Holocene warming, while the forcing maintaining Eemian warmth was a few tenths of

a W/m 2 smaller than the Holocene forcing. Thus, while the LGM-Holocene climate change

We conclude ECS is at least approximately 4°C and is almost surely in the range 3.5-5.5°C. The

IPCC AR6 conclusion that 3°C is the best estimate for ECS is inconsistent with paleoclimate

data. Our conclusion also applies for transition to warmer climates, as discussed in the Summary

below. Charney’s estimate of 3°C for 2×CO 2 , thus ¾°C per W/m 2 forcing, stood as the canonical

ECS estimate for more than 40 years. Precise data for equilibrium paleo climate states point to a

new canonical ECS: 1°C per W/m 2 forcing. The one major caveat is uncertainty in the glacial

surface climate forcing. A well-designed PMIP/CMIP study could narrow that uncertainty.

High climate sensitivity has implications for climate response time and the amount of warming

in-the-pipeline. Slow climate response – delayed climate response – has policy implications.

Climate response time was surprisingly long in our climate simulations 7 for the 1982 Ewing

Symposium. The e-folding time – the time for surface temperature to reach 63% of its

equilibrium response – was about a century. The only published atmosphere-ocean GCM – that

of Bryan and Manabe 69 – had a response time of 25 years, while several simplified climate

models referenced in our Ewing paper had even faster responses. The longer response time of

our climate model was largely a result of high climate sensitivity – our model had an ECS of 4°C

The physics is straightforward. If the delay were a result of a fixed source of thermal inertia, say

the ocean’s well-mixed upper layer, response time would increase linearly with ECS because

most climate feedbacks come into play in response to temperature change driven by the forcing,

not in direct response to the forcing. Thus, a model with ECS of 4°C takes twice as long to reach

full response as a model with ECS of 2°C, if the mixed layer provides the only heat capacity.

However, while the mixed layer is warming, there is exchange of water with the deeper ocean,

which slows the mixed layer warming. The longer response time with high ECS allows more of

the ocean to come into play. If mixing into the deeper ocean is approximated as diffusive, surface

temperature response time is proportional to the square of climate sensitivity. 70

Slow climate response accentuates need for the “anticipation” that E.E. David, Jr. spoke about. If

ECS is 4°C, more warming is in the pipeline than widely assumed. The greater warming could

eventually make much of the planet inhospitable for humanity and cause the loss of coastal cities

to sea level rise. We will argue that these fates can still be avoided via a reasoned policy

response, but we must understand climate response time to define effective policies.

In the Bjerknes lecture 71 at the 2008 American Geophysical Union meeting, the first author

(JEH) argued that the ocean in many 72 GCMs has excessive, unrealistic mixing, and he suggested

that GCM modeling groups report and make available the global temperature response function

of their models. The response function is global temperature response to instantaneous doubling

of carbon dioxide (2×CO 2 ) with the model run long enough to approach equilibrium. The

response function characterizes a climate model and allows rapid (Green’s function) estimate of

T G is the Green’s function estimate of global temperature at time t, λ (°C per W/m 2 ) the model’s

2×CO 2 equilibrium sensitivity, R the dimensionless temperature response function (% of

equilibrium response), and dF e the forcing change per unit time, dt. The integration over time

begins when Earth is in near energy balance, e.g., in preindustrial time. The response function

yields an accurate estimate of global temperature change for any climate forcing history, with

nearly the same result as that of the global GCM that produced the response function (see Chart

15 of the Bjerknes presentation). 71 This approximation is expected to be good for any forcing

unless and until the forcing causes a fundamental reorganization of the global ocean circulation.

Fig. 4. (a) Global mean surface temperature response to instant CO 2 doubling and (b) normalized

response function (percent of final change). Thick lines in Figs. 4 and 5 are smoothed 73 results.

Ocean mixing is addressed by comparison of two versions of the GISS GCM: GISS (2014) 74 and

GISS (2020). 35 Both models 75 are described by Kelley et al. (2020). 34 Ocean mixing is markedly

improved in GISS (2020) by use of a high-order advection scheme, 76 finer upper-ocean vertical

resolution (40 layers), updated mesoscale eddy parameterization, and correction of errors in the

ocean modeling code. 34 The GISS (2020) model has improved internal variability, including the

Madden-Julian Oscillation (MJO), El Nino Southern Oscillation (ENSO) and Pacific Decadal

Oscillation (PDO), although the spectral signature of the ENSO-like variability is unrealistic and

its amplitude is excessive, as shown by the magnitude of oscillations in Fig. 4a. Ocean mixing in

GISS (2020) may still be a bit excessive in the North Atlantic, where the model’s simulated

Despite reduced ocean mixing, the response time of surface temperature in the GISS (2020)

model is no faster than the GISS(2014) model (Fig. 4b): it takes 100 years to reach within 1/e of

the equilibrium response. Slow response is partly explained by the larger ECS of the GISS

(2020) model, which is 3.5°C versus 2.7°C for the GISS (2014) model, but something more is

going on in the newer model, as exposed by the response function of Earth’s energy imbalance.

When Earth’s climate is perturbed by a forcing, the resulting Earth energy imbalance (EEI)

drives warming or cooling that tends to restore balance. Increasing GHGs and decreasing

aerosols at present cause a positive EEI – more energy coming in than going out – by about +1

W/m 2 averaged over several years. 78 Highest absolute accuracy of EEI is obtained by tracking

ocean warming – the primary repository for excess energy – and by adding the heat stored in

warming of continents and the heat used in net melting of ice. 78 Heat storage in air adds a small,

almost negligible, amount. Observations of radiation balance from Earth-orbiting satellites by

themselves cannot measure EEI to the needed accuracy, but, when calibrated with the in situ

data, satellite Earth radiation budget observations provide invaluable EEI data on finer temporal

Fig. 5. (a) Earth’s energy imbalance (EEI) for 2×CO 2 , and (b) EEI normalized response function.

After a step-function forcing is imposed, EEI and global surface temperature must each approach

a new equilibrium, but EEI does so more rapidly, especially for the GISS (2020) model (Fig. 5).

EEI in the GISS (2020) model needs only a decade to reach within 1/e of full response (Fig. 5b),

while global surface temperature requires a century (Fig. 4b). Rapid decline of EEI – to half the

forcing within 5 years (Fig. 5a) – has practical implications, if it is realistic. First, EEI defines the

rate that heat is pumped into the ocean, so if EEI is reduced, ocean surface temperature response

time increases. Second, rapid EEI decline – if it is realistic – implies that the assumption that

global warming and pumping of heat into the ocean can be stopped if humanity reduces climate

forcing by an amount equal to EEI may be wrong. Instead, the required reduction of forcing is

probably larger than EEI. In any scenario to stabilize climate, the difficulty in finding additional

reduction in climate forcing of even a few tenths of a W/m 2 is substantial. 54 Calculations that can

What is the physics behind the fast response of EEI? The 2×CO 2 forcing and initial EEI are both

nominally 4 W/m 2 . In the GISS (2014) model, the decline of EEI averaged over the first year is

0.5 W/m 2 (Fig. 5a), a moderate decline that might be largely caused by warming continents and

increased heat radiation to space. In contrast, EEI declines 1.3 W/m 2 in the GISS (2020) model

(Fig. 5a). Such a huge, immediate decline of EEI implies existence of an ultrafast climate

feedback. Climate feedbacks are the heart of climate change and warrant discussion.

Charney et al. 4 described climate feedbacks without discussing time scales. At the 1982 Ewing

Symposium, water vapor, clouds and sea ice were described as “fast” feedbacks 7 presumed to

change promptly in response to global temperature change, as opposed to “slow” feedbacks or

specified boundary conditions such as ice sheet size, vegetation cover, and atmospheric CO 2

amount, although it was noted that some specified boundary conditions, e.g., vegetation, in

Large response of EEI in one year (Fig. 5a) implies a third feedback time scale: ultrafast.

Ultrafast feedbacks are not a new concept. When atmospheric CO 2 is doubled, the added infrared

opacity causes the stratosphere to cool. Instantaneous EEI upon CO 2 doubling is only F i = +2.5

W/m 2 , but stratospheric cooling quickly increases EEI to +4 W/m 2 . 80 This quick adjustment led

to the choice of adjusted forcing, F a , as superior to F i as a measure of climate forcing.

Physics behind ultrafast change in the GISS model likely involves cloud change. Indeed, Kamae

et al. 81 review rapid cloud adjustments separate from surface temperature-mediated changes.

Clouds respond to radiative forcing, e.g., via effects on cloud particle phase, cloud cover, cloud

albedo and precipitation. 82 The GISS (2020) model alters glaciation in stratiform mixed-phase

clouds, which increases the amount of supercooled water in stratus clouds, especially over the

Southern Ocean [Fig. 1 in GISS (2020) GCM description 34 ]. The portion of supercooled cloud

water drops changes from too little in GISS (2014) to too much in GISS (2020). Although

neither model realistically simulates stratocumulus clouds – which are important for accurate

simulation of Earth’s albedo and climate sensitivity – that modeling deficiency does not affect

our assessment of climate sensitivity and it does not prevent use of the two GISS models to help

expose real-world physics affecting climate sensitivity and climate response times.

Cloud modeling is now a focus in GCM development. Several models in CMIP6 comparisons

find high ECS. 82 It would be informative if the models defined their temperature and EEI

response functions (Figs. 4 and 5). Failing that, model runs of even a decade could define the

most crucial portion of Figs. 4a and 5a. In addition, if many short (e.g., 2-year) 2×CO 2 climate

simulations were made with each run beginning at a different point in the model’s control run,

ultrafast feedbacks including cloud changes could be defined to an arbitrary accuracy by

averaging the responses and subtracting the same years in the control run. As noted in our

Supporting Material, definition of response functions for just a few forcings – say CO 2 , aerosols

and solar irradiance – would help assess the physical mechanisms causing ultrafast feedbacks

Zhu et al. 83 recently used the LGM climate to constrain the microphysics and ice nucleation

cloud parameterization in the Community Earth System Model CESM2. The paleo constraint

reduced the ECS of the model from >5°C to 4°C for 2×CO 2 . This independent study with a

model including cloud microphysics is consistent with our inferences and could be a vehicle to

evaluate EEI response with more realistic cloud physics. If the EEI response is much faster than

the temperature response, it implies that the climate forcing reduction required to stabilize

The ultrafast response of EEI in the GISS (2020) model also exists, although much smaller, in

the GISS (2014) model, as shown in our Supporting Material. The need for further study of

ultrafast feedbacks and the wide range of climate sensitivities among current GCMs does not

alter the high ECS that we infer from paleoclimate data, as that inference has little dependence

on GCMs. The main role of GCMs in the paleoclimate analyses is to define seasonal and

geographical climate patterns, which allows more accurate assessment of global temperature

Understanding clouds requires understanding aerosols, which are involved in cloud feedbacks.

Fig. 6. Observed global mean surface temperature (black line) and expected warming from

observed GHG changes with two alternative choices for ECS. The difference (blue area) is an

estimate of the cooling effect of the (unmeasured) aerosol forcing. The temperature peak in the

World War II era is in part an artifact of inhomogeneous ocean data during that period. 54

ECS near 4°C implies that expected warming for today’s GHGs far exceeds observed warming.

Expected GHG warming (Fig. 6) is calculated using equation (4) with the response function (Fig.

4b). 84 For ECS = 4°C, the expected GHG warming today (2.2°C) exceeds observed warming by

about 1°C. If ECS is 3.5°C, the gap is about 0.7°C. The indicated expected warming does not

include warming by slow feedbacks except for a small contribution in observed GHG amounts;

Human-made aerosols are the likely source of cooling that has partially offset GHG warming.

An alternative source of cooling is human-made increase of Earth’s surface albedo, which occurs

via deforestation, agriculture, road-building, and other human developments, partially offset by

decreased albedo due to deposition of soot on snow and ice surfaces. IPCC 13 (Chapter 7, Table

7.8) estimates the net forcing due to surface albedo change as –0.12 ± 0.1 W/m 2 , which is an

order of magnitude smaller than their estimated aerosol forcing. Thus, in our empirical

evaluation of human-made cooling, we associate almost the entire cooling with aerosols.

Aerosol cooling is described as a Faustian bargain. 85 Payment comes due as we reduce pollution

from shipping, vehicles, industry, and power plants, which we must do because ambient air

pollution causes millions of deaths per year, with particulates most responsible. 86

Aerosol climate forcing is difficult to measure because it occurs mainly via small induced cloud

changes. 13 The absence of significant global warming over the period 1850-1920 (Fig. SPM.1 of

the IPCC AR6 WG1 report) is a clue for the scale of aerosol forcing. GHG forcing increased

+0.54 W/m 2 in 1850-1920, which causes an expected warming ~0.3°C by 1920, based on the

climate response function with 3.5°C ECS (Fig. 4). Natural forcings – solar irradiance and

volcanic aerosols – could contribute to lack of warming, but we are unaware of a persuasive case

Fig. 7. Global mean surface temperature (left scale) and climate forcings (right). Scale factor

between temperature and forcings is 2.4°C per W/m 2 (see text). Antarctic (Vostok) temperature

change based on water isotopes 87,88 is multiplied by 0.75. Time scale is expanded post 1750.

Modern temperature is NASA GISS analysis. 89,90 Zero point for GHG forcing is the mean for

10-8 ky BP, a period expected to precede significant human effects. GHG + IPCC aerosol

forcing is indistinguishable from IPCC 13 all-anthropogenic forcing (Supporting Material).

for the required downward trends of those forcings. Aerosols from increasing industrialization

prior to most environmental protection laws are a more likely offset to GHG warming.

Paleoclimate data provide ways to assess aerosol climate forcing. Natural paleo aerosol changes

are fast feedbacks, as discussed above, but human-caused aerosols are a forcing – an imposed

perturbation of Earth’s energy balance. We will examine the continuity of modern climate data

with the paleoclimate record to show the magnitude of warming in the pipeline, if today’s level

of GHGs – or a greater amount – long persists. Then we use the relative stability of Holocene

global temperature to extract evidence that human-made aerosols were a significant climate

In the paper Target CO 2 67 the scale factor between equilibrium global temperature change and

GHG forcing is 1.5°C per W/m 2 of GHG forcing based on an assumed ECS of 3°C for 2×CO 2

(0.75°C per W/m 2 ) and an assumption that GHG and ice sheets contributed about equally to the

glacial-interglacial climate forcing (each 3 W/m 2 ). Our present assessment has ECS of 4°C (1°C

per W/m 2 ) and more precise LGM-to-Holocene GHG forcing (2.5 W/m 2 ) and ice sheet forcing

(3.5 W/m 2 ). Thus, the improved ΔT to F GHG scale factor in Fig. 7 is (F GHG + F Ice )/ F GHG × 1°C per

W/m 2 = 2.4°C per W/m 2 of GHG forcing. Temperature change in the paleo portion of Fig. 7 is

the full observed change, which includes slow feedbacks. Modern temperature (purple curve) has

not had time for the ocean to warm fully or for slow feedbacks to come fully into play.

Paleo GHG forcing in Fig. 7 is the first three terms in Eq. 2 with adjusted forcings for CO 2 , CH 4

and N 2 O from formulae in Supporting Material. The GHG forcings are a fit to radiative transfer

calculations in a GCM 31 and agree well with the net IPCC GHG forcing, as shown above. Fig. 7

Fig. 8. Global mean surface temperature change over the past 24 ky, reproduced from Fig. 2 of

also shows the GHG plus aerosol forcing using IPCC’s estimated aerosol forcing history. There

are gaps in the Vostok N 2 O record prior to 132 ky BP, so we approximate the earlier N 2 O forcing

by increasing the CO 2 + CH 4 forcing by 12 percent. Good accuracy of this approximation is

shown in Supporting Material for the past 132 ky, when N 2 O data are available.

Paleo GHG climate forcing and Antarctic temperature change are nearly congruent (Fig. 7). A

free parameter in Fig. 7 is the factor by which the Vostok temperature change is multiplied to

obtain approximate congruence with the climate forcing. With the climate forcings and climate

sensitivity assumed in Target CO 2 , close congruence of forcing and temperature was achieved if

Vostok temperature change was multiplied by 0.5, i.e., Southern Hemisphere polar amplification

of temperature was a factor of two. With GHG forcing of 2.5 W/m 2 and ECS of 1°C per W/m 2 ,

the factor by which Vostok temperature must be multiplied to achieve close congruence of

temperature and forcing is 0.75 (Fig. 7). Reduced Southern Hemisphere polar amplification is

consistent with recent estimates of LGM-Holocene global temperature change. 23,24,26

Target CO 2 and Fig. 7 use the Vostok temperature derived from water isotopes. 87,88 Recent

analysis 92 of LGM Antarctic cooling based on borehole thermometry and firn properties reveal

that glacial cooling at the ice sheet surface is less than suggested by water isotopes, implying that

the scale factor between Vostok and global temperature may be different from the value (0.75) in

Fig. 7. Neither this specific scale factor for Vostok (where the temperature depends on changing

ice surface height and other local factors) nor ice age polar amplification in general are important

for our present paper. Here we only want to explain clearly the contents of Fig. 7.

A stunning result in Fig. 7 is that equilibrium global warming for today’s GHG level is 10°C.

Aerosols, at their maximum level in the early 21 st century, reduce equilibrium warming to 7°C,

but the aerosol amount is in decline The paleo temperature changes occurred over millennia, on

the time scale of the climate forcing. Today’s GHG forcing is rising faster than any known paleo

case. In a following paper 93 we will use paleoclimate data, climate modeling and modern

observations to assert that a large ice sheet response and several meters of sea level rise are likely

on the century time scale in response to continued extraordinary human-made climate forcing.

Fig. 9. GHG climate forcing in past 20 ky with vertical scale expanded for the past 10 ky on the

right. GHG amounts are from Schilt et al. 56 and formulae for forcing are in Supporting Material.

We focus here on the two forcings – human-caused changes of GHGs and aerosols – that are the

dominant causes of ongoing climate change. Volcanic and solar irradiance forcings have notable

short-term variation, but no significant long-term trend. Other human-made forcings estimated

by IPCC have little net effect on total forcing history (see Supporting Material).

Several proxy-based analyses (e.g.) 94,95 found global cooling in the second half of the Holocene,

but a recent analysis 24 that uses GCMs to overcome spatial and temporal biases in proxy data

finds a rising global temperature in the first half of the Holocene followed by nearly constant

global temperature in the last 6000 years until the past few centuries (Fig. 8, extracted from Fig.

2 of Osman et al. 24 ). The deep ocean, tropical sea surface, and Antarctica all had stable

The final 6000 years of the Holocene are unusual. GHG forcing (Fig. 9) increased by 0.5 W/m 2 ,

yet global temperature was stable, if not declining. Even the Osman et al. 24 analysis (Fig. 8),

which shows Holocene warming over the last 9000 years, has no warming in the last 6000 years.

Six thousand years is sufficient time for slow feedbacks to operate, as well as fast feedbacks.

Global warming of about 1°C would be expected, based on climate sensitivity implied by Fig. 7.

How can we interpret the absence of warming? Was another climate forcing at work?

Ruddiman hypothesized 52 that humanity began to influence climate with the advent of land-

clearing and agriculture. In a review 96 of his hypothesis, Ruddiman places the beginning of

significant deforestation at 6500 yr BP and rice irrigation at 5000 yr BP, causing respective

increases of atmospheric CO 2 and CH 4 . In his analysis, Ruddiman seeks human-made sources of

CO 2 and CH 4 of sufficient magnitude to compensate for large declines of those gases in the latter

parts of prior interglacial periods. While we support Ruddiman’s assertion that humans began to

affect climate prior to the industrial revolution, we note that such large sources are unnecessary

to account for Holocene GHG levels. Our principal interest is in preindustrial aerosols, but first

Fig. 10. Sea level since the last glacial period relative to present. Credit: Robert Rohde 97

GHG decreases during a typical interglacial period are slow feedbacks that occur in concert with

global cooling. However, global cooling did not occur in the past 6000 years, so the feedbacks

did not occur. A principal mechanism in glacial-interglacial swings of atmospheric CO 2 and

global temperature (Fig. 2) is the net rate of uptake of carbon by the deep ocean. Carbon

sequestration in the deep ocean increases if ocean overturning slows because the rate of carbon

return to the atmosphere is reduced. Maximum insolation at 60°S was in late-spring (mid-

November) 6000 years ago; since then, the date of maximum insolation at 60°S slowly advanced

through the year, recently passing mid-summer (Fig. 26b of Hansen et al. 14 ). Maximum

insolation from late-spring through mid-summer is optimum for direct warming of the Southern

Ocean and for promoting early warm-season ice melt, which reduces surface albedo and

magnifies regional warming. 48 Thus, Earth orbital parameters were optimum for keeping the

Southern Ocean warm as needed to maintain a strong overturning ocean circulation.

GHG forcing decreased 0.2 W/m 2 between 10 and 6 ky BP, but the decrease was exceeded by a

forcing due to shrinkage of ice sheets. Sea level 10 ky ago was 40 m below today (Fig. 10); loss

of that ice causes a climate forcing of just over +1 W/m 2 , as shown in Supporting Material of the

Target CO 2 paper. 67 Error! Bookmark not defined. The net forcing was enough to produce the

global warming of less than or about 1°C deduced from paleo data for the period 10-6 ky BP

(Fig. 8). The mystery is the past 6000 years, when sea level and thus ice sheet volume were

static. The 0.5 W/m 2 rise of GHG forcing over 6000 years must have been counteracted by a

comparable negative forcing to yield the near constant global temperature deduced by Osman et

al. 24 An even larger negative forcing is required, if there was global cooling in the past 6000

Hansen et al. 48 suggested that human-made aerosol cooling offset or exceeded GHG warming in

the past 6000 years. Growth of population, agriculture and land clearance 96 produced aerosols as

well as CO 2 . Wood was the principal fuel for cooking and heating. As today, the largest aerosol

forcing would be via effects on cloud cover and cloud brightness. This aerosol indirect effect

tends to saturate as aerosol amount increases, so aerosol effectiveness per aerosol amount was

greatest as civilization developed. Thus, it is unsurprising that human-made global aerosol

Fig. 11. (a) Estimated greenhouse gas and aerosol climate forcings relative to 1750 values. (b) Aerosol

climate forcing as percent of GHG forcing that it offsets. Aerosol and GHG forcings for dark blue area

are relative to 1750 values. Light blue area is added when the GHG and aerosol forcings are defined

relative to their values 6000 years ago, with GHG and aerosol forcings both reaching 0.5 W/m 2 by 1750.

forcing approximately offset human-made CO 2 forcing, as required to explain the absence of

Hemispheric differences provide a consistency check. GHG (mainly CO 2 ) forcing is global,

while the aerosol forcing was mainly in the Northern Hemisphere. Global offset of the two

forcings implies a net negative forcing in the Northern Hemisphere and positive forcing in the

Southern Hemisphere. Therefore, human-made aerosols were likely a contributor to observed

Northern Hemisphere cooling, which occurred on at least a regional scale, while GHG warming

Scientific advances often face early resistance from other scientists. 98 Examples are the snowball

Earth hypothesis 99 and the role of an asteroid impact on extinction of non-avian dinosaurs, 100

which initially were highly controversial but are now more widely accepted. Ruddiman’s

hypothesis, right or wrong, is still controversial. Thus, we minimize this issue by showing

Global aerosol properties have not been monitored with the detail and accuracy needed to define

the aerosol climate forcing, 101 but IPCC 13 estimates the aerosol forcing (Fig. 11a) based on

assumed aerosol precursor emissions – mainly related to fossil fuel use – and aerosol models that

are tested against a range of observations in recent decades. The task is difficult because of the

multitude of aerosol types and the complex effects of aerosols on clouds. Uncertainty in aerosol

forcing is at least 50 percent of the estimate in Fig. 11a 13 and probably is constrained more by

observed global temperature change than by measurements of aerosols or precursor gases.

Using IPCC’s best estimate aerosol forcing history (Fig. 11a) and the accurately known GHG

history, we calculate the percent of GHG climate forcing offset by aerosol cooling – the dark

blue area in Fig. 11b, which is simply the ratio of aerosol and GHG forcings. However, if

human-made aerosol forcing was – 0.5 W/m 2 by 1750, offsetting the known +0.5 W/m 2 GHG

forcing in 1750 (regardless of whether or not the GHG forcing was human-made), that aerosol

forcing should be included in the total aerosol forcing and offset a 1750 GHG forcing of 0.5

W/m 2 (Fig. 9). This 1750 aerosol forcing – largely cloud effects of aerosols from land use,

human-caused fires, and burning of biomass – is assumed to continue until today. The picture of

the aerosol role in climate change, shown by both shades of blue in Fig. 11b, thus changes when

Fig. 12. Observed global surface temperature change relative to 1880-1920 based on GISS

analysis. 89,90 Data for 2022 is January-October mean. Monthly updates are available. 102

the full effect of aerosols from land-use and human-caused fires is included. Aerosol offset of

GHG warming was dominant until about 1970. The relative decline of aerosol forcing after 1970

Fig. 11b, with and without the (light blue) preindustrial aerosols, encapsulates two alternative

views of the historical role of human-caused aerosols. IPCC’s aerosol history, with aerosol

forcing gradually becoming important relative to GHG forcing, derives from aerosol simulations

driven mainly by fossil fuel emissions. In the alternative view, civilization always produced

aerosols as well as GHGs. Organized societies and rapid population growth began on coasts as

stabilizing sea level increased coastal margin biologic productivity 103 and inland as agriculture

developed. Wood was the main fuel; it would be surprising if the growing human population did

not produce aerosols that affected clouds of a prior virgin atmosphere. Aerosols travel great

distances, as shown by the presence of Asian aerosols in the United States and by satellite

tracking of fire-produced aerosols. Small aerosol amounts in otherwise pristine marine air can

produce a significant climate forcing. In our view, humans likely contributed to both rising GHG

and aerosol climate forcings in the past 6000 years. No persuasive alternative explanation has

been proffered for the absence of global warming in that period of increasing GHG amounts.

At face value, the steep decline in the aerosol offset of GHG forcing beginning in the early 1970s

(Fig. 11b) – a result of stabilization of global aerosol forcing (Fig. 11a) – is a triumph of aerosol

modeling, providing a partial explanation for the steep global temperature rise that began at that

time (Fig. 12). However, we must bear in mind that the temperature record was known when the

aerosol scenarios were developed. It is not a case of prediction and observational confirmation.

One implication of our alternative view of aerosol forcing history is that the human-caused

negative aerosol climate forcing is 0.5 W/m 2 larger than obtained from models that deal with

aerosol change only in the past century or two. Thus, the Faustian payment that will eventually

Global warming in the past 100 years (Fig. 12) is commonly used to estimate climate sensitivity,

but by itself it is ill-suited for that purpose. Global warming does not yield a unique climate

sensitivity because the warming depends on three major unknowns with only two fundamental

constraints. 15 Unknowns are: ECS, net climate forcing (uncertain because aerosol forcing is

unmeasured), and ocean mixing (uncertain based on evidence that many ocean models are too

diffusive). Constraints are observed global temperature change (Fig. 12) and EEI. 78 Accurate

knowledge of EEI began with the Argo float program, 104 which initiated well-calibrated

measurements of ocean heat content globally in the first decade of the 21 st century.

In an analysis 105 using early Argo data, we reduced unknowns to two by assuming ECS = 3°C.

From EEI ~ 0.58 W/m 2 for the 2006-2010 solar minimum, we inferred a solar cycle mean EEI ~

0.75 W/m 2 . Our aerosol forcing versus time was from aerosol modeling of Koch 106 that

incorporated changing technology factors defined by Novakov. 107 We solved for aerosol forcing

in 2010, obtaining –1.63 ± 0.3 W/m 2 relative to 1880 – in the range estimated in the radiative

forcing chapter of IPCC reports, but more negative than aerosol forcing used in most GCMs in

CMIP and IPCC reports. Our interpretation – in agreement with Knutti 108 – was that most

climate models compensate for excessive ocean mixing (which reduces surface warming) by

using aerosol forcing less negative than the real world to achieve realistic global warming.

With ECS uncertain, we are back to an underdetermined system. Moreover, response functions

for the GISS GCMs (Figs. 4 and 5) imply that climate response depends on more than ECS and

ocean mixing; atmospheric processes also affect climate response time. We infer – for want of a

likely alternative – that cloud changes are probably involved in the ultrafast feedbacks. Thus, a

comprehensive analysis must include analysis of the role of cloud feedbacks in global climate

change. A first step is to assess the mechanisms in ultrafast feedbacks, which can be done via

many short (~2 year) GCM runs for 2×CO 2 , as described above. If CMIP included such short

simulations, it may identify observable feedbacks that produce high ECS in several GCMs. 82 A

second step is to define response functions individually for CO 2 , aerosols and solar irradiance.

Aerosols, located mainly beneath clouds, may have a different response function than GHGs.

The solar variability response is needed because the period with accurate EEI data is limited;

Such an analysis would require many GCM runs and is beyond the scope of this paper. Instead,

for the purpose of estimating today’s aerosol climate forcing, we make the usual assumption that

effective forcings defined by a primitive GCM 31 allow all forcings to be treated as equivalent and

thus we use of a single response function in Eq. (4). This assumption is called into question by

the indirect evidence that cloud feedbacks may be more complex and have a larger effect than in

early GCMs, yet we may be able to draw some general conclusions with appropriate caveats.

Many combinations of climate sensitivity and aerosol forcing are consistent with observed global

warming. 15 The response function for the GISS (2014) model yields a result (Fig. 13) typical of

models in CMIP and IPCC reports: AR6 aerosol forcing yields good agreement with observed

warming in the last half century – the period when human-made climate forcing overwhelms

Fig. 13. Global temperature change T G due to aerosols + GHGs calculated with Green’s function

Eq (4) using GISS (2014) and (2020) response functions (Fig. 4). Observed temperature is the

natural forcings, unforced climate variability, and flaws in observed warming due to inadequate

data. However, agreement with observed temperature also can be achieved by climate models

with high ECS. The GISS (2020) model, with ECS = 3.5°C, yields greater warming than

observed if IPCC aerosol forcing is used (Fig. 13), but less warming than observed for the

aerosol scenario inferred in our 2011 EEI study. 105 This latter aerosol scenario yields close

agreement with observed warming if ECS ~ 4°C (green curve in Fig. 13). 109 Agreement also can

likely be achieved with a still higher ECS by use of a larger (more negative) aerosol forcing.

IPCC AR6 WG1 best estimate aerosol forcing (Table AIII.3) 13 is near maximum (negative) value

by 1975, then nearly constant until the 21 st century, when it rises to –1.09 W/m 2 in 2019 (Fig.

14). This AR6 aerosol forcing is in the upper (less negative) portion of the range estimated in the

radiative forcing chapter of AR6. 110 In contrast, some aerosol models, including those of Koch 106

The issue we raise is the magnitude of aerosol forcing. Aerosol forcing is unmeasured and hard

to model because it involves complex aerosol effects on clouds. The result is wide latitude in the

aerosol forcing used in climate simulations. Two GCM problems cause a tendency to minimize

aerosol forcing: (1) excessive mixing of heat into the deep ocean, and (2) a too-small ECS, likely

due to GCM failure to fully capture cloud feedbacks. An analysis based on observed EEI and

global temperature, largely independent of GCMs, concluded 105 that aerosol forcing was about –

1.6 W/m 2 in 2010 relative to 1880. Our present finding of ECS ~ 4°C favors a slightly larger

aerosol forcing (Fig. 13) for consistency with observed global warming. 112 We conclude that the

peak human-made aerosol forcing was likely in the (negative) range 1.5-2 W/m 2 relative to

Declining aerosol amount implies acceleration of global warming above the 1970-2010 rate

(0.18°C per decade). Just as 1970 was a hinge point for global warming (Fig. 12), so 2010 is

another hinge point as it marks transition to declining aerosols from China (a decline not

captured well in most CMIP6 models 113 ), where emissions of SO 2 (a principal precursor of

aerosols) declined 70 percent between 2006 and 2017, 114 and strengthening of regional and

global regulations on ship emissions in 2015 and 2020 by the International Maritime

Organization (IMO). 115 A review 116 of aerosol forcing estimates for 2000-2019 found a

consensus that aerosol forcing decreased (became less negative), but the decrease among the

estimates ranged from about 0.1 W/m 2 to about 0.3 W/m 2 for the 20-year period. Uncertainties in

each case are substantial, but together they make a persuasive case that the change of aerosol

forcing in that period is of the opposite sign from that in the 20 th century, i.e., aerosol change no

longer partly offsets growth of GHG climate forcing, rather it adds to the GHG forcing growth.

Expectation of continued decline in aerosol amount is based in part on growing concern about

health effects of aerosol pollution that causes millions of deaths per year 86 and global trends

toward renewable energy, nuclear power, and replacement of coal by gas. Reduction of human-

made aerosols by half between 2010 and 2050 would contribute a climate forcing about +0.2

W/m 2 per decade. Although an aerosol reduction that rapid is uncertain, we expect some

reduction and a forcing increase of at least +0.1 W/m 2 per decade for the reasons given here.

Average GHG climate forcing increase since 1970 is about 0.45 W/m 2 per decade, but about 0.5

W/m 2 in the past decade (Fig. 15). This graph includes all well-mixed GHGs and changes of O 3

and stratospheric H 2 O associated with CH 4 change. It thus misses only about half of the O 3

change, which is estimated to cause a forcing of about 0.2 W/m 2 over 1750-2021, about 5% of

the total GHG forcing (see Supporting Material). GHG climate forcing growth has continued

unabated since the 1997 Kyoto Protocol and subsequent agreements of the Conference of the

Parties (COP) meetings, such as the Paris Agreement of 2015, and the growth rate has even

increased. For reasons given in the Discussion section below, we expect a similar growth rate to

continue for the next few decades, at least as long as polices are based on goals and targets.

During 1970-2010 the drive for global warming increased ~0.3 W/m 2 per decade (+0.45 W/m 2

for GHGs, –0.15 W/m 2 for aerosols). Going forward from 2010 for a few decades, we expect this

drive to be at least 0.5-0.6 W/m 2 per decade, for reasons given above. Global temperature

responds reliably to climate forcing on decadal time scales, with about 50% of the response in

the first decade, with about 15% more in the next 100 years (Fig. 4b). Thus, the rate of global

Fig. 15. Update 41 of annual growth of climate forcing by GHGs, including part of O 3 forcing via

the efficacy of CH 4 forcing (cf. Supporting Material). MPTG and OTG are Montreal Protocol

and Other Trace Gases. RCP2.6 is a scenario designed to keep global warming below 2°C.

temperature change in 2010-2040 should be nearly proportional to the multi-decadal growth rate

of the climate forcing. Therefore, we estimate that the global warming rate in 2010-2040 will be

Interpretation of past climate change and prediction of the future could be more precise if aerosol

climate forcing were monitored. Aerosol monitoring could be analogous to the method used for

GHGs. The GHG forcing is obtained by measuring GHG changes accurately and making a

radiative transfer calculation with a realistic global distribution of clouds. A similar approach for

aerosols requires global monitoring of changes of aerosol and cloud particle microphysics with

precision adequate to define their climate forcing. 117,118 In the absence of such Keeling-like

global monitoring, progress has been made via limited satellite measurements of aerosol and

cloud properties, field studies, and aerosol and cloud modeling. These data provide an estimate

of the small direct aerosol forcing in cloud-free regions. The indirect forcing caused by aerosol-

induced cloud changes must be extracted from natural (unforced) cloud variability and cloud

changes caused by ongoing global warming. In the absence of precise global monitoring of

aerosol and cloud particle microphysics, progress has relied heavily on modeling of aerosol-

Sulfate aerosols are a cloud condensation nuclei (CCN), so ship emissions result in a larger

number of smaller cloud particles, affecting cloud albedo and cloud lifetime. 119 Ships provide a

large percentage of sulfates in the North Pacific and North Atlantic regions (Fig. 16). Ship-tracks

(cloud trails) produced by aerosol CCN are apparent, but it has been suggested that cooling by

these clouds is overestimated because of cloud liquid water adjustments. 120 On the other hand,

Manshausen et al. 121 present evidence that liquid water path (LWP) effects are mis-estimated,

including regions without visible ship-tracks; they estimate a LWP forcing – 0.76 ± 0.27 W/m 2 ,

in stark contrast with the IPCC estimate of + 0.2 ± 0.2 W/m 2 . Wall et al. 122 use satellite

observations to quantify relationships between sulfates and low-level clouds; with help of the

“opportunistic experiment” provided by decrease of air pollution downwind of eastern North

Fig. 16. Total sulfate (parts per trillion by volume) and percentage of total sulfate provided by

shipping in simulations of Jin et al. 123 prior to IMO regulations on sulfur content of fuels.

America, they infer a sulfate indirect aerosol forcing of – 1.11 ± 0.43 W/m 2 over the global

ocean, consistent with our inference of a large (negative) total aerosol forcing.

IMO emission regulations provide a great opportunity for insight into aerosol climate forcing.

Sulfur content of fuels was limited to 1% in 2010 near the coasts of North America and in the

North Sea, Baltic Sea and English Channel, and further restricted there to 0.1% in 2015. 115 In

2020 a limit of 0.5% was imposed worldwide. The 1% limit did not have a noticeable effect on

ship-tracks, but a striking reduction of ship-tracks was found after the 2015 IMO regulations,

especially in the regions near land where emissions were specifically limited. 124 Following the

additional 2020 regulations, 125 global ship-tracks were reduced more than 50%. 126

Earth’s albedo (reflectivity) measured by CERES (Clouds and Earth’s Radiant Energy System)

satellite-borne instruments 79 over the 22-years March 2000 to March 2022 reveal a decrease of

albedo and thus an increase of absorbed solar energy coinciding with the 2015 change of IMO

emission regulations. Global absorbed solar energy is +1.01 W/m 2 in the period January 2015

through March 2022 relative to the mean for the first 10 years of data (Fig. 17). This increase is

4.7 times greater than the standard deviation (0.22 W/m 2 ) of annual absorbed solar energy in the

Fig. 18. Absorbed solar radiation for indicated regions relative to first 120 months of CERES

data. Southern Hemisphere 20-60°S is 89% ocean. North Atlantic is (20-60°N, 0-60°W) and

North Pacific is (20-60°N, 120-220°W). Data source: http://ceres.larc.nasa.gov/order_data.php

first 10 years of data and 4.5 times greater than the standard deviation (0.23 W/m 2 ) of CERES

data through December 2014. The increase of absorbed solar energy is notably larger than

estimated potential CERES instrument drift, which is <0.085 W/m 2 per decade. 79 Increased solar

energy absorption occurred despite 2015-2020 being the declining phase of the ~11-year solar

irradiance cycle. 127 Nor can increased absorption be attributed to correlation of Earth’s albedo

(and absorbed solar energy) with the Pacific Decadal Oscillation (PDO): the PDO did shift to the

positive phase in 2014-2017, but it returned to the negative phase in 2017-2022. 128

Given the large magnitude of the solar energy increase, cloud changes are likely the main cause.

Quantitative analysis 128 of contributions to the 20-year trend of absorbed solar energy show that

clouds provide most of the change. Surface albedo decrease due to sea ice decline contributes to

the 20-year trend in the Northern Hemisphere, but that sea ice decline occurred especially in

2007, with minimum sea ice cover reached in 2012; over the past decade as global and

hemispheric albedos declined, sea ice had little trend. 129 Potential causes of the cloud changes

include: 1) reduced aerosol forcing, 2) cloud feedbacks to global warming, 3) natural

variability. 130 Absorbed solar energy was 0.78 W/m 2 greater in 2015-2021 than in the first

decade of CERES data at latitudes 20-60°S (Fig. 18), a region of relatively little ship traffic. This

change is an order of magnitude larger than the estimate of potential detector degradation. 79

Climate models predict a reduction of cloud albedo in this region as a feedback effect driven by

global warming. 131 Continued monitoring of absorbed energy can confirm the reality of the

change, but without global monitoring of detailed physical properties of aerosols and clouds, 101 it

The North Pacific and North Atlantic regions of heavy ship traffic are ripe for more detailed

study of cloud changes and their causes, although unforced cloud variability is large in such sub-

global regions. North Pacific and North Atlantic regions both have increased absorption of solar

radiation after 2015 (Fig. 18). The 2014-2017 maximum absorption in the North Pacific is likely

enhanced by reduced cloud cover during the positive PDO, but the more recent high absorption

is during the negative PDO phase. In the North Atlantic. The persistence of increased absorption

for the past several years exceeds prior variability, but longer records plus aerosol and cloud

The equilibrium climate sensitivity (ECS) estimated by the 1979 Charney study – 3°C for 2×CO 2

– stood for more than 40 years. The current IPCC report 13 concludes that 3°C is their best

Earth’s climate history provides reliable assessment of ECS because it allows comparison of

equilibrium climate states. All feedbacks are included in comparisons of real-world equilibrium

climate states. In contrast, transient climate change such as that occurring today is affected by

innumerable, partially complete, climate feedback processes that occur on a range of time scales.

The most accurate evaluation of ECS is from comparison of recent glacial and interglacial

The fundamental constraint that forces a conclusion that ECS is at least approximately 4°C is the

requirement that Earth was in energy balance during the ice ages. It was recognized almost 40

years ago that CLIMAP 21 (and later MARGO 49 ) boundary conditions for the last ice age (LGM)

implied 7 the impossible conclusion that Earth was out of energy balance during the LGM by

about 2 W/m 2 , equivalent to a (negative) forcing half that of 2×CO 2 . However, at that time, a

conclusion of high climate sensitivity needed to challenge the extensive, respected, CLIMAP

analysis of climate conditions during the LGM. Resolution of that matter lay dormant until

Our assessment is that recently developed techniques of Tierney at al. 23 and Seltzer et al. 26 allow

a firm conclusion that LGM global cooling relative to preindustrial conditions was much greater

than the ~3.5°C implied by CLIMAP and MARGO. This leads to a 2×CO 2 equilibrium climate

sensitivity (ECS) at least near 4°C. The Eemian/PGM (prior glacial maximum) climate change

provides a valuable check because of the complex Holocene GHG history and the possibility that

human-made aerosols contributed a negative climate forcing in the second half of the Holocene.

The Eemian/PGM case confirms and strengthens the conclusion that ECS is at least near 4°C for

2×CO 2 and by itself would favor a best estimate near 5°C. ECS assessment could be tightened

via a climate model intercomparison project to define accurately the surface climate forcing

ECS is the climate sensitivity defined by the Charney study, with ice sheets and GHGs fixed.

Earth system sensitivity (ESS) 16,17,18 is the complete climate sensitivity in which the climate

system – including GHGs and ice sheets, which we refer to (somewhat misleadingly) as slow

feedbacks – responds to the imposed forcing. During the past 800,000 years, CO 2 provided

~80% of GHG climate forcing, i.e., the total GHG forcing is 25% larger than the CO 2 forcing.

Thus, the climate sensitivity in which non-CO 2 GHG feedbacks are allowed to change increases

from ~4°C to ~5°C. When all feedbacks, including ice sheets, are allowed to respond to the

climate forcing, the equilibrium response is approximately doubled, i.e., ESS is ~ 10°C.

Earth warmed 1.2°C in the past century (Fig. 12) and is out of energy balance by about +1 W/m 2 ,

so climate is headed toward a warmer state than existed in the period covered by ice cores. The

Pliocene provides the best chance for empirical assessment of climate sensitivity for a warmer

Earth that still has ice sheets. The chief difficulty is uncertainty about Pliocene CO 2 amount,

which is estimated to have been 300-450 ppm based on sediment-derived CO 2 proxies, 132 but

CO 2 amounts from boron and alkenone proxies diverge strongly in the Pliocene. 133 Unless and

until knowledge of Pliocene CO 2 is reliably sharpened, we can only conclude that the Pliocene

The Paleocene-Eocene Thermal Maximum (PETM) provides an opportunity to assess climate

sensitivity in the absence of large ice sheets. PETM warming of about 5-6°C was driven by an

approximate doubling of CO 2 , which occurred over a period of 3,000-10,000 years. 19,20 Because

there were no large ice sheets, this empirical estimate of ESS is converted to ECS by accounting

for forcing by changes of non-CO 2 GHGs. If we assume that forcing by non-CO 2 GHGs

increased in the same proportion (~25%) to CO 2 forcing as in the period covered by ice core

data, we obtain an ECS estimate of 4-5°C for 2×CO 2 . One review 19 of PETM data infers ECS

~3.6°C with 66% confidence that ECS is in the range 2.3-4.7°C, but a recent PETM study, 134

aided by GCM-guided analysis of the sparse data, estimates the PETM CO 2 increase as 86% of a

CO 2 doubling, thus finding ECS ~6.5°C, if non-CO 2 gases are fixed. With the assumption that

non-CO 2 gases amplified the CO 2 forcing by 25%, we obtain ECS ~ 5°C. High ECS in a warm

ice-free climate may be a result of amplifying cloud feedbacks, 18,135 and a rising tropopause may

Other studies of ECS based on paleoclimate change yield a wide range of estimates, mainly

because of the large uncertainty in CO 2 amount. Present knowledge of long-term CO 2 change is

analogous to knowledge of LGM SST 40 years ago: at least some of the estimates of paleo CO 2

must be flawed. Sophisticated statistical treatments of the data do not eliminate effects of wrong

data. Until accurate knowledge of long-term CO 2 history is achieved, the best analysis of ECS is

High climate sensitivity implies that there is more global warming in the pipeline – and greater

climate impacts – than has been widely recognized. The poster child for warming in the pipeline

is Fig. 7, showing that equilibrium warming for today’s GHG level, including slow feedbacks, is

about 10°C. Today’s level of particulate air pollution reduces equilibrium warming to about 7°C.

How can the equilibrium warming be so large with today’s CO 2 level of “only” 415 ppm, a level

that might have been reached in the Pliocene? Many of today’s GHGs, such as CFCs, did not

exist in paleoclimate and others have increased by extraordinary amounts. With all trace gases

included, the increase of GHG effective forcing between 1750 and 2021 is 4.09 W/m 2 , which is

equivalent to increasing the 1750 CO 2 amount (278 ppm) to 561 ppm (formulae in Supporting

Material). We have already reached the GHG climate forcing level of doubled CO 2 .

The 7-10°C global warming is the eventual response if today’s level of GHGs is fixed and the

aerosol amount is somewhere between its year 2000 amount and preindustrial amount. Given the

time required for the ocean to warm and ice sheets to shrink to new equilibria, this is not a

warming that will be experienced by today’s public, but it is an indication of the path upon which

we have set our planet. Moreover, we are in the process of setting the planet upon an even more

extreme course as the net human-made climate forcing and global temperature are continuing to

rise, even at accelerating growth rates. As long as there is such a large gap between the present

climate and the equilibrium climate, the climate system will drive hard toward hotter climate.

Doubled CO 2 is already a huge climate forcing that will have large impacts, if left in play for

long. The large global warming in the pipeline today is not widely appreciated. Civilization and

its infrastructure are not set up for a 2×CO 2 world. We need to reduce human-made climate

forcing before it exerts its full influence on the climate system. It will take time to halt and

reverse growth of GHGs, so it is important to understand response times of the climate system.

We expected climate response time – as simulated by an atmosphere-ocean GCM with fixed ice

sheets – to become faster as ocean models reduced excessive downward mixing of heat. 71 This

expectation was not met when we compared two generations of the GISS GCM. The GISS

(2020) GCM is demonstrably improved 34,35 in its ocean simulation over the GISS (2014) GCM

as a result of higher vertical and horizontal resolution, more realistic parameterization of sub-grid

scale motions, and correction of errors in the ocean computer program. 34 Yet the time required

for the model to achieve 63% of its equilibrium response remains about 100 years. There are two

The surface in the newer model warms as fast as in the older model, but it must achieve greater

warming to reach 63% of equilibrium because its ECS is higher, which is one reason that the

response time stays long. The other reason is that Earth’s energy imbalance (EEI) in the newer

model decreases rapidly. EEI defines the rate that heat is pumped into the ocean, so a smaller

EEI implies a longer time for the ocean to reach its new equilibrium temperature. Quick drop of

EEI – in the first year after introduction of the forcing – implies existence of an ultrafast

feedback in the GISS (2020) model. For want of an alternative with such a large effect on Earth’s

energy budget, we suspect a rapid cloud feedback and we suggested a set of brief GCM runs that

could define cloud changes and other diagnostic quantities to an arbitrary accuracy.

The Charney report 4 recognized that clouds were a main cause of a wide range in ECS estimates.

Today, clouds still cast uncertainty on climate predictions. Several CMIP6 33 GCMs have ECS of

~ 4-6°C for 2×CO 2 , 137,138 with the high sensitivity caused by cloud feedbacks. 82 Even if the most

extreme cloud feedbacks are exaggerated, these models draw attention to this large uncertainty in

climate models. As cloud modeling progresses, it will aid understanding if climate models report

Fast EEI response – faster than global temperature response – has a practical effect: observed

EEI understates the reduction of climate forcing required to stabilize climate. Although the

magnitude of this effect is uncertain (see Supporting Material), it makes the task of restoring a

hospitable climate and saving coastal cities more challenging. On the other hand, long climate

response time implies a potential for educated policies to affect the climate outcome before

undesirable consequences have occurred. For the purpose of avoiding unacceptable outcome, we

Fig. 19. Accelerated warming rate post-2010 (yellow area) if aerosol reductions approximately

double the net (GHG + aerosol) climate forcing. Upper and lower edges of the yellow area are

Aerosol climate forcing is larger than the recent (AR6) IPCC estimate. Aerosols probably

provided a significant climate forcing prior to the industrial revolution. We know of no other

persuasive explanation for the absence of significant global warming during the past 6000 years

(Fig. 8), a period in which the GHG forcing increased 0.5 W/m 2 . Climate models that do not

incorporate a growing negative aerosol forcing yield significant warming in that period, 139 a

warming that, in fact, did not occur. Negative aerosol forcing, increasing as civilization

developed and population grew, should be expected. As humans burned fuels at a growing rate –

wood and other biomass for millennia and fossil fuels in the industrial era – aerosols as well as

GHGs were an abundant, growing, biproduct. The wood-burning aerosol source has continued in

modern times. 140 GHGs are long-lived and accumulate, so their forcing eventually dominates

We conclude that peak aerosol climate forcing – in the first decade of this century – was of a

(negative) magnitude of at least 1.5-2 W/m 2 . We estimate that the GHG plus aerosol climate

forcing during the period 1970-2010 grew +0.3 W/m 2 per decade (+0.45 from GHG, – 0.15 from

aerosols), which produced observed warming of 0.18°C per decade. With current policies, we

expect climate forcing for a few decades post-2010 to increase 0.5-0.6 W/m 2 per decade and

produce global warming at a rate at least +0.27°C per decade. In that case, global warming

GHG climate forcing imagined by Charney, Tyndall and other greenhouse giants 1 is no longer

imaginary: the GHG climate forcing increase since 1750 (~4.1 W/m 2 ) is equivalent to 2×CO 2 .

GHG forcing is now growing by almost 0.5 W/m 2 per decade (Fig. 15) and it is no longer partly

offset by increasing aerosols. The situation that E.E. David warned about is now coming into

play. As David noted, a system with long delay and amplifying feedbacks can break down,

unless “anticipation is built into the loop.” Required anticipation was development of energies

that produce no GHGs. Instead, the fossil fuel industry, subsidized by the government, developed

Climate’s delayed response allowed policy procrastination, as the impacts of climate change

were not glaringly apparent to the public. The designated scientific authority (IPCC), relying

primarily on climate models, continued for decades to report a broad range for estimated global

climate sensitivity: 1.5-4.5°C for 2×CO 2 , with non-negligible possibility that it was less than

1.5°C. Meanwhile, Earth’s paleoclimate history tells a clearer story: climate sensitivity is near

the high end of that long-time estimated range. Another excuse for inaction was hope that large

climate impacts will be delayed until humanity is wealthier and able to mitigate problems. Hope

of lethargic climate was based in part on the millennial time scale of large paleoclimate changes

(Fig. 2). However, the timescale of those paleoclimate changes results more from the timescale

of the forcings, rather than an inherent lethargy of the climate system. Our second perspective

article – Sea Level Rise in the Pipeline 93 – concludes, as outlined already, 15 that exponential

increase of sea level rise to at least several meters is likely if high fossil fuel emissions continue.

Specifically, it is concluded that the time scale for loss of the West Antarctic ice sheet and

multimeter sea level rise would be of the order of a century, not a millennium. Eventual impacts

would include loss of coastal cities and flooding of regions such as Bangladesh, the Netherlands,

a substantial portion of China, and the state of Florida in the United States. For practical

purposes, the losses would be permanent. Such outcome could be locked in soon, which creates

an urgency to understand the physical system better and to take major steps to reduce the human-

Rapid human-made global warming that began in about 1970 (Fig. 12) has taken climate far out

of the Holocene range, the climate to which civilization is adapted, but ocean and ice sheet

inertia still allow reasoned policy response that may preserve a bright future for young people

and future generations. The basic requirement to preserve shorelines is return to a climate no

warmer than the mid-20 th century, possibly a bit cooler. This cooler climate will also address

other problems such as overheating of low latitudes and increasing regional climate extremes.

Climate science has exposed a crisis that the world is loath to fully appreciate. Delayed response

of the climate system has allowed huge global warming to build up in the pipeline. Humanity is

now entering the period of consequences. Scientists – as informed witnesses of ongoing efforts

of the world to deal with climate change via the Framework Convention and IPCC processes –

have the opportunity, indeed, the obligation, to assess the present course of those efforts.

Fig. 20. Global energy consumption and CO 2 emissions (Hefner et al. 141 and BP 142 ; see text).

Fossil fuels powered the industrial revolution as the energy source that raised living standards in

much of the world. Fossil fuels continue to provide most of the world’s energy (Fig. 20a) and

produce most of the CO 2 emissions (Fig. 20b). Fossil fuel reserves and recoverable resources are

more than enough to provide most of the world’s energy for the rest of this century, 143 and

barring new effective policies they will continue to do so – and they will continue to drive global

warming at an unprecedented rate that we now understand is exceedingly dangerous.

Climate policy under the Framework Convention is demonstrably flawed. Empirical data (Fig.

20) confirm that as long as fossil fuel pollution can be dumped in the air free of charge,

agreements such as the 1997 Kyoto Protocol and 2015 Paris Agreement have little effect on

global emissions. 144 The reason is clear: much of the world is in early or middle stages of

economic development. Energy is required to raise living standards, and fossil fuels continue to

be the most convenient, affordable source for that energy. Thus growth of emissions is occurring

in the developing world (Fig. 21a), while the developed world is still the larger source of the

cumulative emissions (Fig. 21b) that drive climate change. 145,146 Thus, exhortations at annual

COP meetings, imploring each nation to set tighter emission targets, have little effect on global

emissions, as is blatantly apparent in the yawning gap between reality and the RCP2.6 scenario

Moreover, a 2°C (or 1.5°C) target is more politically-based or practically-based than science-

based. The science-based target, we assert, should be return to Holocene climate, the climate in

which civilization developed and is adapted to. We must avoid passing points-of-no-return with

unacceptable consequences, such as loss of the West Antarctic ice sheet and thus loss of global

cities. That goal should also suffice for other unacceptable consequences, such as avoiding

unlivable conditions in the tropics and subtropics. A crucial issue that science must address is the

magnitude and duration of Holocene-overshoot that can occur without passing points-of-no-

return. We are already into overshoot territory and greater overshoot is in the offing.

Given the situation that we have allowed to develop, three actions now seem to be essential.

First, a rising global price on GHG emissions must underly energy and climate policies, with

enforcement by border duties on products from countries that do not have an internal carbon fee

or tax. Public buy-in and maximum effectiveness require the collected funds to be distributed to

Fig. 21. Fossil fuel CO 2 emissions by nation or region as a fraction of global emissions. Data

sources: Heffner et al. 141 for 1751-2017 concatenated with BP 142 for 2018-2020.

the public, an approach that helps address global wealth disparities. Economists in the U.S.

overwhelming support such a carbon fee-and-dividend; 147 college and high school students, who

have much at stake, join in advocacy. 148 The science rationale for a rising carbon price with a

level playing field for energy efficiency, renewable energies, nuclear power, and all innovations

has long been understood, but not achieved. Instead, fossil fuels and renewable energy are

heavily subsidized, including use of “renewable portfolio standards” that allow utilities to pass

added costs to consumers. Nuclear energy has thus been relatively disadvantaged and excluded

as a “clean development mechanism” under the Kyoto Protocol, based in part on myths about

damage caused by nuclear energy that are not supported by scientific facts. 149 A rising carbon fee

is an underlying strategy to develop a mix of electricity sources needed to achieve rapid

Second, human-made geoengineering of Earth’s climate must be rapidly phased out. The best

measure of geoengineering is Earth’s energy imbalance (EEI), which has reached a level of at

least 0.9 W/m 2 averaged over the solar cycle (equivalent to the energy of 600,000 Hiroshima

atomic bombs per day) with most of the excess energy heating the ocean and melting ice.

Realistic phasedown of emissions alone cannot remove this imbalance in less than several

decades (cf. Fig. 15 and discussion thereof), which may be too slow to avoid locking in loss of

the West Antarctic ice sheet and sea level rise of several meters. Removal of human-made GHGs

from the air will be spurred by a carbon price, but GHG removal sufficient to reduce EEI to zero

may require decades, if it is even feasible. Given that GHG forcing is still rising rapidly, highest

priority must be given to phasing down emissions. However, given that GHG forcing is already 4

W/m 2 , it may be necessary to temporarily affect EEI via solar radiation management (SRM), if

the world is to avoid disastrous consequences, including large sea level rise. Risks of such

intervention must be defined, as well as risks of no intervention. Thus the U.S. National

Academy of Sciences recommends research on SRM. 151 An example of SRM is injection of

atmospheric aerosols at high southern latitudes, which global simulations suggest would cool the

Southern Ocean at depth and limit melting of Antarctic ice shelves. 15,152 The most innocuous

aerosols may be salt or fine salty droplets extracted from the ocean and sprayed into the air by

autonomous sailboats. 153 This approach has been discussed for potential use on a global scale, 154

but even use limited to high latitudes in the Southern Hemisphere will require extensive research

and forethought to avoid unintended adverse effects. 155 The present decade may be the last

opportunity to develop the knowledge, technical capability, and political will for the actions that

Third, effective global cooperation is needed to achieve the required reduction of GHG climate

forcing over the next several decades. Higher income countries – most of them in the West – are

responsible for most of the cumulative fossil fuel CO 2 emissions (Fig. 21b and Fig. 22), which

are the main drive for global warming, 145,146 even though the West constitutes a small fraction of

global population. De facto cooperation between the West and China drove down the price of

renewable energy, especially solar power, and further cooperation is needed to develop emission-

free technologies for the rest of the world that will be the source of most future GHG emissions

(Fig. 21a). Given China’s huge demand for carbon-free energy if its coal use is to be displaced,

China-U.S. cooperation in development of modern nuclear power was proposed, but then

stymied by U.S. prohibition of technology transfer. 156 Competition is normal, but if there is a

will it can be managed, reaping the benefits of cooperation over confrontation. 157 Of late, priority

seems to be given to economic and military hegemony, despite recognition of the long-term

existential threat posed by climate change. It is important to not foreclose the possibility of return

to a more ecumenical perspective of our shared future. In this situation, scientists can improve

global prospects by maintaining and expanding international cooperation. Public and political

awareness of the gathering climate storm will grow this decade as climate anomalies increase, so

it is important to improve scientific understanding and lay groundwork for effective actions.

Fig. S1. GHG forcings. IPCC from AR6 Table AIII.3. Our forcings from Eq. (3) and Table 1.

Formulae (Table 1) for adjusted forcing, F a , were numerical fits 37 to 1-D calculations with the GISS GCM

radiation code using the correlated k-distribution method. 38 Gas absorption data were from high spectral

resolution laboratory data. 39 These F a were converted to F e via GCM calculations that include 3-D effects,

as summarized in Eq. (3), where the coefficients are from Table 1 of Efficacy. 31 The factor 1.45 for CH 4

includes the effect of CH 4 change on stratospheric H 2 O and tropospheric O 3 . We assume that CH 4 is

responsible for 45% of the O 3 change. 40 The remaining 55% of the O 3 forcing is obtained by multiplying

the IPCC AR6 O 3 forcing (0.47 W/m 2 in 2019) by 0.55 and by 0.82, where the latter factor is the efficacy

that converts F a to F e . The non-CH 4 portion of the O 3 forcing is thus 0.21 W/m 2 in 2019. The time-

dependence of this portion of the O 3 forcing is from Table AIII.3 in IPCC AR6. MPTGs and OTGs are

Montreal Protocol Trace Gases and Other Trace Gases. 41 An updated list of these gases and a table of

Fig. S2. Test of accuracy of 2-term approximation for forcing by the three gases.

CO 2 and CH 4 are well-preserved in ice cores. However, the N 2 O record is corrupted in some time

intervals by chemical reactions with dust particles in the ice core. For such intervals we

approximate the N 2 O forcing by increasing the sum of CO 2 and CH 4 forcings by 12%, i.e., we

approximate the forcing for all three gases as 1.12×[F(CO 2 ) + F(CH 4 )]. The accuracy of this

approximation is checked in Fig. S2 via computations for the past 132 ky, when data are

Fig. S3. Climate forcings provided in current IPCC report 13 for GHGs plus aerosols and for all

IPCC all human-made forcings include land-use effects and contrails, which have large relative

uncertainties. The forcings in Fig. S3 are those provided by IPCC (cf. Annex III of the current

Fig. S4. Ratio of warming in the pipeline to EEI, (Teq – T)/EEI, for the first 300 years after

Global warming in the pipeline (∆Tpl) after a CO 2 doubling is the portion of the equilibrium

response (Teq) that remains to occur at time t, i.e., ∆Tpl = Teq – T(t). If EEI were equivalent to a

climate forcing, warming in the pipeline would be the product of EEI and climate sensitivity (°C

per W/m 2 ), i.e., warming in the pipeline would be EEI ×ECS/4, where we have approximated the

Fig. S4 shows the 2×CO 2 results for the GISS (2014) and GISS (2020) GCMs. EEI is not a good

measure of the warming in the pipeline, especially for the newer GISS model. The warming in

the pipeline for the GISS (2014) model is typically ~30% larger than implied by EEI and ~90%

larger in the GISS (2020) model. If these results are realistic, they suggest that reduction of the

human-made climate forcing by an amount equal to EEI will leave a planet that is still pumping

Real-world climate forcing is added year-by-year with much of the GHG growth in recent years,

which Fig. 4 suggests will limit the discrepancy between actual warming in the pipeline and that

inferred from EEI. Thus, we also make Green’s function calculations of global temperature and

EEI for 1750-2019 for GHG plus IPCC aerosol forcings. Green’s function calculations are

useful, with a caveat noted below, for quantities for which the response is proportional to the

where R EEI (Fig. 5b) is the EEI response function (% of equilibrium response) and dF is forcing

change per unit time. Integrations begin in 1750, when we assume Earth was in energy balance.

The results (Fig. S5) show that the excess warming in the pipeline (excess over expectations

based on EEI) is reduced to 15-20% for the GISS (2014) model, but it is still 70-80% for the

GISS (2020) model. This topic thus seems to warrant further examination, but it is beyond the

Fig. S5. Ratio of warming in the pipeline to EEI, (Teq – T G )/EEI G , in response to GHG and

IPCC aerosol forcing for the period 1750-2019 using the response functions for the GISS (2014)

The first matter to investigate is the cause of the ultrafast response of EEI (Fig. 5 of the main

paper), which could be done via the model diagnostics discussed in that section of our paper. If

the large difference between the EEI response functions of the two GISS models is related to

supercooled cloud water, Fig. 1 of the Kelley et al. paper 34 suggests that the real-world effect

may fall between that of the two models. If the higher climate sensitivity of the GISS (2020)

model is related to this cloud water phase problem, more realistic treatment of the latter may

If real world climate sensitivity for 2×CO 2 is near 4°C or higher, as we have concluded, the total

cloud feedback is likely to be even higher than that of the GISS (2020) model. We suggest that it

would be useful to calculate response functions for other models, especially models with high

climate sensitivity, to help analyze feedbacks and to allow inexpensive climate simulations for

arbitrary forcing scenarios. One major caveat: we have used a single response function calculated

for 2×CO 2 . Especially in view of cloud feedbacks, it seems likely that the response function for

aerosol forcing is different from that for CO 2 forcing, because most tropospheric aerosols exist

well below the clouds. Much might be learned from calculating response functions for GHGs,

tropospheric aerosols, stratospheric aerosols, and solar irradiance, for example.

The response functions for global temperature and EEI, for both the 2014 and 2020 models,

"The data used to create the figures in this paper are available in the Zenodo repository,

We thank Eelco Rohling for inviting JEH to describe our perspective on global climate response

to human-made forcing. JEH began to write a review of past work, but a paper on the LGM by

Jessica Tierney et al. 23 and data on changing ship emissions provided by Leon Simons led to the

need for new analyses and division of the paper into two parts. We thank Jessica also for

pointing out the paper by Zhu et al. 83 , which independently identifies cloud drop microphysics at

cloud tops as key physics affecting climate sensitivity that can be constrained by paleoclimate

data. JEH designed the study and carried out the research with help of Makiko Sato; Larissa

Nazarenko provided data from GISS models and helped with analysis; Leon Simons provided

ship emission information and aided interpretations; Norman Loeb and Karina von Schuckmann

provided EEI data and insight about implications; Matthew Osman provided paleoclimate data

and an insightful review of the entire paper; Qinjian Jin provided simulations of atmospheric

sulfate and interpretations; all authors contributed to our prior research incorporated in the paper

All authors declare that they have no conflicts of interest. Climate Science, Awareness and

Solutions, which is directed by JEH and supports MS and PK is a 501(C3) non-profit supported

100% by public donations. Principal supporters in the past few years have been the Grantham

Foundation, Carl Page, Frank Batten, James and Krisann Miller, Peter Joseph, Ian Cumming,

Eric Lemelson, Gary and Claire Russell, Donald and Jeanne Keith Ferris, Aleksandar Totic,

Chris Arndt, Jeffrey Miller, Morris Bradley and about 150 more contributors to annual appeals.

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Global warming in the pipeline

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Global warming in the pipeline

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