“A Miracle Will Occur” Is Not Sensible Climate Policy
07 December 2023
James Hansen, Pushker Kharecha, Makiko Sato
The COP28 Chairman and the United Nations Secretary General say that the goal to
keep global warming below 1.5°C is alive, albeit barely, implying that the looser goal
of the 2015 Paris Agreement (to keep warming well below 2°C) is still viable. We find
that even the 2°C goal is dead if policy is limited to emission reductions and plausible
CO2 removal. IPCC (the Intergovernmental Panel on Climate Change, which advises
the UN) has understated global warming in the pipeline and understated fossil fuel
emissions in the pipeline via lack of realism in the Integrated Assessment Models that
IPCC uses for climate projections. Wishful thinking as a policy approach must be
replaced by transparent climate analysis, knowledge of the forcings that drive climate
change, and realistic assessment of policy options. The next several years provide a
narrow window of time to define actions that could still achieve a bright future for
today’s young people. We owe young people the knowledge and the tools to
continually assess the situation and devise and adjust the course of action.
Our approach to analysis of global climate change, as described in Global Warming in the
Pipeline,1 puts comparable emphasis on (1) Earth’s paleoclimate history, (2) global climate
models (GCMs), (3) modern observations of climate processes and climate change. One
purpose of the Pipeline paper was to distinguish between this approach and that of IPCC,
which puts principal emphasis on GCMs. GCMs are an essential tool, but the models must be
consistent with Earth’s history and the projections of future climate must employ plausible
scenarios for energy use and for the climate forcings that drive climate change.
Policy implications of climate science can be grasped from a basic understanding of the
human-made forcings that are driving Earth’s climate away from the relatively stable climate
of the Holocene (approximately the past 10,000 years). Our task is to provide understandable
quantification of climate forcings and changes that will be needed to maintain a hospitable
climate. Concerned public, including policymakers, must learn to appreciate basic graphs that
summarize real-world data, because these must provide the basis for policy discussion.
1. CLIMATE SCIENCE
There are two major climate forcings: human-made greenhouse gases (GHGs) and aerosols
(fine airborne particles). GHGs reduce Earth’s thermal (heat) radiation to space and are the
main cause of global warming. Aerosols reflect sunlight to space, mainly via their effect as
condensation nuclei for clouds; more nuclei lead to smaller cloud drops and brighter, longer-
lived, clouds. Aerosols thus cause a global cooling that partially offsets GHG warming.
Greenhouse Gases. We begin with a graph that describes continuing growth of GHG climate
forcing. This graph provides a proper comparison of the relative importance of different gases
in driving global warming. Fig. 1 shows the annual change of the GHG climate forcing,
which is increasing about 0.05 W/m2 per year, a rate that has increased since the early 1990s.
Maximum growth rate occurred about 1980, as chlorofluorocarbons (CFCs) were increasing
rapidly until being curtailed by the Montreal Protocol.2 We show the 5-year running mean in
Fig. 1 because the “noise” in the annual mean hides trends that we wish to understand.3 The
2022 point in the graph is a 1-year mean and the 2021 point is a 3-year mean; thus these are
provisional and will change as later data are added. The GHG climate forcing today (the sum
of annual increments from 1750 through 2023) is 4.2 W/m2. Our calculated forcing agrees
Fig. 1. Annual growth of climate forcing by GHGs4 including the part of O3 forcing not
included in the CH4 forcing.1 MPTG and OTG are Montreal Protocol and Other Trace Gases.
closely with the IPCC5 calculated forcing. This is a huge climate forcing,6 slightly exceeding
the ~ 4 W/m2 forcing for doubled CO2 (2×CO2). If a forcing this large or larger is left in place
indefinitely, it will transform the planet.
IPCC’s best estimate for equilibrium climate sensitivity (ECS) is 3°C, but we show from
paleoclimate data that ECS = 4.8°C ± 1.2°C, which excludes 3°C with > 99% certainty.1 ECS
includes only “fast” feedbacks that occur in response to climate change, the most important
being changes of atmospheric water vapor, clouds, and sea ice.; it excludes “slow” feedbacks
such as change of ice sheet size and methane (CH4) released from melting permafrost or
methane hydrates. Nevertheless, ECS is the proper sensitivity to employ in analysis of
human-made climate change to date, because the ice sheets have not yet changed much in
size and any slow feedback release of GHGs is accounted for (treated as a climate forcing) in
GCM simulations based on measured GHG changes.
Real-world GHG forcing is also compared in Fig. 1 with scenarios defined a decade ago and
used by IPCC. Note that the real world is closer to the extreme scenario RCP8.5 than it is to
scenario RCP2.6. [Numerals after RCP are the scenario’s GHG forcing in year 2100.] RCP2.6
was defined such as to provide a 66 percent chance of keeping global warming under 2°C.
Scenario RCP2.6 is a product of an Integrated Assessment Model (IAM). Assumptions in
IAMs can yield implausible results. RCP2.6, e.g., assumes deployment of a vast array of
powerplants that burn biofuels, capture the CO2 emissions, and permanently bury the CO2
(BECCS = bioenergy with carbon capture and storage). BECCS on such a massive scale
would be nature-ravaging and food-security-threatening.7 In Pipeline and elsewhere we note
that the gap between RCP2.6 and reality could be closed via direct-air CCS, but the annual
cost with present CCS is $3.4-7 trillion and growing; also, transporting and storing that much
CO2 underground would likely generate public opposition.
Additional concerns about unrealism in IAMs are discussed below. However, note first that
global warming through 2022 is only ~ 1.2°C, much less than expected equilibrium warming.
There are two reasons for the “small” magnitude of observed warming. First, the ocean’s
great thermal inertia slows the response to climate forcing. [Even after 100 years the expected
surface temperature response to a forcing is only about 60 percent (Fig. 4, Pipeline).] Second,
human-made aerosol forcing causes cooling that partly offsets GHG warming. Thus, we must
discuss aerosol climate forcing before we can address IAMs in more detail.
Aerosol climate forcing and cloud feedbacks. Measurement of aerosol climate forcing
requires precise global monitoring of aerosol and cloud particle microphysics,8 which has not
3
been achieved. Thus, aerosol forcing has de facto been a free parameter, which allows any
ECS to match observed global warming of the past century. A climate model with high ECS
must use large aerosol cooling to match observed warming. There is nothing nefarious here –
the shotgun marriage of climate sensitivity and aerosol forcing is due to the status quo of
scientific knowledge of aerosol forcing.
Given the absence of direct measurement of aerosol forcing, we obtain useful indications
about aerosol forcing from both paleoclimate and from modern observations of a remarkable,
inadvertent, global aerosol “experiment” now playing out. Let’s start with the paleoclimate.
In Pipeline, we describe resolution of a 40-year-old mystery about global temperature in the
last glacial maximum (LGM).9 The answer, noted above, is that the LGM at the time of peak
cooling (18-21 kyBP, kiloyears before present) was 7°C ± 1°C colder than the Holocene. This
large LGM cooling leads to ECS = 4.8°C ± 1.2°C for 2×CO2. If LGM cooling were only
3.5°C, as was widely assumed 40 years ago, ECS would be only ~ 2.4°C for 2×CO2. GCMs
alone could never have resolved this mystery, because GCM results depend on all climate
feedbacks, which are still poorly understood. In contrast, real-world climate change between
LGM and Holocene equilibria exactly includes all real-world feedbacks, thus permitting
precise evaluation of ECS once the LGM and Holocene climate states are well-defined.
Given that aerosol climate forcing occurs mainly via alteration of cloud properties, we must
consider cloud feedbacks at the same time as aerosol forcing. Cloud feedbacks do not make it
impossible to evaluate aerosol forcing from observations, but they make the task difficult.
Cloud feedbacks cause GCMs to yield a wide range of ECS. GCMs with fixed clouds, i.e.,
with a neutral cloud feedback, yield ECS ~ 2.4°C for 2×CO2. However, GCMs that yield a
small decrease of cloud cover as Earth warms can readily yield an ECS of 4.8°C or greater. In
recent years, a number of GCMs have yielded ECS ~ 4-6°C for 2×CO2, especially among
GCM groups that attempt to model complex cloud microphysics. With a hint of denigration,
these high-ECS models have been described as the “wolf-pack,” to contrast them with the
“pack,” the mainstream climate models with ECS nearer 3°C. IPCC, in taking 3°C as their
best estimate for ECS, in effect, endorses “mainstream” climate models. At a webinar10 on
our Pipeline paper, George Tselioudis showed figures of Zelinka et al.11 and Jiang et al.12
revealing that the high sensitivity models are in much better agreement with satellite
observations of seasonal and latitudinal cloud changes. Cloud modeling is primitive, but these
models suggest that cloud feedbacks are a major cause of high climate sensitivity. One
implication is that interpretation of satellite observations of Earth’s energy balance must
distinguish between cloud feedbacks and aerosol forcing of cloud changes.
Separating cloud feedbacks from aerosol induced cloud changes might be a Sisyphean task, if
not for the “experiment” initiated by the International Maritime Organization (IMO) when it
placed a constraint on sulfur content of ship fuels beginning January 2015 and tightened it in
January 2020.13 This experiment, although the change occurs in two steps, has a reasonably
sharp beginning14 and ship traffic has a known geographical distribution (Fig. 20 in Pipeline,
from Jin et al.15). Prior to the IMO regulations, ships were the main source of sulfur aerosols
in the North Atlantic and North Pacific regions. In contrast, the largest cloud feedback is
probably in the southern half of the Southern Hemisphere (south of 30°S).11
The IMO experiment and implications. The most informative diagnostic for interpretation
of the IMO aerosol experiment is change of absorbed solar radiation. Earth radiation budget
data are acquired by CERES16 (Clouds and Earth’s Radiant Energy System) launched early
this century. CERES measures solar radiation reflected by Earth and thermal (heat) radiation
emitted by Earth. Reflected solar radiation declines coincident with imposition of the IMO
sulfur rules. We graph the increase of absorbed solar radiation (Fig. 2); it reveals a decrease
of Earth’ albedo (reflectivity) of 0.4% (1.37/340).17 This reduced albedo is a BFD (a big
deal).10 It is equivalent to a sudden increase of atmospheric CO2 from 420 ppm to 525 ppm.
This large change of Earth’s albedo accelerates global warming. We will infer that most of the
increased absorption of solar energy following the IMO rule change is aerosol forcing. This
added forcing also spurs “fast” feedbacks, which come into play not in immediate and direct
response to the forcing, but in response to global temperature change, which lags the forcing.
We estimate the net effect of the aerosol forcing from the climate response function (Fig. 3).1
Warming depends little on ECS in the first few years after a forcing is introduced; early
warming is ~ 1°C for 2×CO2, thus ~ 0.25°C for 1 W/m2 forcing. Fast feedbacks amplify the
warming by a factor 1.5-2 by year 10 after the forcing is imposed (2015 and 2020 forcings
have now existed 9 and 4 years). The fast feedbacks are water vapor, clouds and sea ice.
Cloud and sea ice feedbacks operate by reducing Earth’s albedo, i.e., by increasing absorbed
solar radiation. Recent increase of absorbed solar radiation, reaching 2 W/m2 on 12-month
running mean (Fig. 2), in part reflects growing fast feedbacks, but it is consistent with an
increase of aerosol forcing of the order of 1 W/m2. Increased absorption is especially large in
the North Pacific and North Atlantic (Pipeline Fig. 22). This increased regional absorption of
solar energy and doubling of Earth’s energy imbalance are likely to affect Arctic sea ice.
Thus, we expect Arctic sea ice cover to soon fall below the 2012 minimum
Accelerated global warming. We projected that decreased aerosols will increase the global
warming rate, which was 0.18°C per decade in 1970-2010, by 50-100 percent, i.e., to 0.27 to
0.36°C per decade (Fig. 4).1 Natural climate variability, due mainly to El Nino/La Nina,
makes it difficult to measure change, but Fig. 5 provides insight. Deviations of global
warming from its post- 1970 trend are well correlated with Nino3.4, despite disruptions such
as global cooling after the 1991 Pinatubo volcanic eruption. Fig. 5 reveals recent excess
global warming of ~ 0.2°C, consistent with most of the 1 W/m2 absorbed solar radiation
anomaly being aerosol forcing (the remainder presumably being fast feedbacks).
Our Pipeline projection of peak warming 1.6-1.7°C in mid-2024 is based on the observed
doubling of Earth’s energy imbalance. An alternative derivation,20 based on aerosol forcing of
1 W/m2 produces 0.3-0.4°C global warming by now (Fig. 3). With El Nino raising global
temperature 0.1-0.2°C above the trend line and aerosol forcing adding 0.3-0.4°C to global
warming, we would project peak 12-month-mean warming 0.4-0.6°C above the trend line in
May 2024, thus including the possibility of warming slightly above the 1.6-1.7°C range
estimated in Pipeline (pink region, Fig. 4). Regardless, if global warming exceeds the 1.6°C
level, it will provide strong confirmation of global warming acceleration.
Decline of global temperature after the El Nino should be limited as “fast” feedbacks come
into play more. Recent rise of the absorbed solar radiation 12-month running-mean (Fig. 2) is
likely due to continued growth of cloud and sea ice feedbacks as well as the current solar
cycle maximum (see below). Amplifying cloud feedback grows in proportion to temperature
https://ceres.larc.nasa.gov/data/
rise, which is especially delayed in the Southern Hemisphere by the vast ocean. Increase of
Earth’s energy imbalance in the past decade speeds ocean warming and growth of the cloud
feedback. Thus, the long-anticipated decrease of Southern Hemisphere sea ice, which began
this year, will increase absorbed solar radiation and tend to limit global temperature decline.
Cause of current extreme global temperatures. Several ideas are eliminated by Earth’s
radiation budget data. Water vapor from the Hunga Tonga volcanic eruption in early 2022 and
the moderate increase of the GHG growth rate in recent years (Fig. 1) have a warming effect,
but they do so by decreasing thermal radiation to space. As Fig. 6 shows, infrared radiation to
space has increased. The energy source driving extreme warming, instead, is a large increase
(1.37 W/m2) of absorbed solar radiation (Fig. 2). Increased emission to space (0.54 W/m2) is
less than half of the increased solar radiation absorption. This is the expected portion (Fig. 3)
of the 1.37 W/m2 “forcing” that appears as “realized warming” in less than 10 years. The
remaining portion adds to Earth’s energy imbalance (EEI), almost doubling the pre-2015
imbalance (Fig. 7). Increased EEI is the proximate drive of global warming acceleration.
Solar irradiance is near a maximum of the current solar cycle. Thus, the Sun adds a bit to
recent high absorbed radiation (Fig. 2), but the full amplitude of the solar cycle is only about
0.2 W/m2 (Fig. 7 of “Acceleration” communication21). Solar radiance declined in 2015-2020
when absorbed solar radiation rose rapidly, so solar variability is not the cause of accelerated
global warming and it is only a minor contributor to current extreme global temperatures.
The current El Nino strengthened in recent weeks as a burst of westerly winds in the mid-
Pacific equatorial region pushed warmer West Pacific surface water toward South America,22
putting this El Nino in the “super El Nino” class of the 1997-98 and 2015-16 El Ninos as
Measured by the Nino3.4 index. The El Nino causes the steep rise of global temperature, but,
as shown by Fig. 5, additional post-2015 warming (coinciding with increased absorption of
solar radiation, Fig. 2) makes the current global temperature unusually extreme.
Implications of global warming acceleration. Accelerated global warming will cause the
12-month running-mean global temperature to exceed 1.5°C within the next few months and
reach a level far above 1.5C by May 2024. Global temperature should fall back below 1.5C
with the next La Nina, but the decline likely will be limited and the El Nino/La Nina mean of
1.5°C will have been reached. Subsequently, global temperature will go even higher; that’s
assured by Earth’s huge energy imbalance, which makes it unnecessary to wait a decade to
declare that the 1.5°C limit has been breached.
We conclude that the increase of aerosol forcing since 2015 is of the order of 1 W/m2, i.e.,
O(1 W/m2). The continued rise of absorbed solar radiation is caused by growth of cloud and
sea ice fast feedbacks, which rise in proportion to the accelerated global warming. Some
studies, e.g., Diamond,25 suggest that the global IMO-induced aerosol forcing is only O(0.1
W/m2). Such a small aerosol-cloud forcing would be consistent with the IPCC best estimate
for the total indirect (aerosol-cloud) forcing of only ~ 0.5 W/m2 (Fig. 8). However, the
observed response to the IMO “experiment” rules out such small estimates for the aerosol-
cloud forcing. On the contrary, we find1 that a single aerosol type (sulfate from ships) yields a
forcing at least that large, that preindustrial humanity was already producing an additional
aerosol forcing of at least 0.5 W/m2 from wood and biomass fuel burning that continues
today,1 and thus that the peak human-made aerosol forcing is at least ~2 W/m2.
A comprehensive review by Bellouin et al. reveals how IPCC was led to their unrealistically
small aerosol forcing, as summarized in Fig. 8. Review of aerosol physics by Bellouin yields
a range of estimated aerosol forcing as large as 3.6 W/m2 (blue bars in Fig. 8), but forcing
exceeding 1.6 W/m2 (red region in Fig. 8) was ruled out because greater aerosol forcing and
“mainstream” climate sensitivity would not yield global warming as large as observed. The
8
aerosol forcing that IPCC defines as highest probability is that which yields best agreement
with mainstream GCMs. IPCC describes this substitute for measurement of aerosol forcing as
an “emergent constraint” on ECS, a sophisticated pseudonym for the artifice. However, given
the larger ECS dictated by real-world paleoclimate data (4.8°C±1.2°C for 2×CO2), that
“constraint” instead implies a large aerosol forcing.
In summary, global warming acceleration is a result of high climate sensitivity (proven by
paleoclimate data) and large (negative) aerosol forcing (implied by high climate sensitivity
and supported by the IMO “experiment”). Observed doubling of Earth’s energy imbalance
and the rising anomaly of absorbed solar radiation assure that an accelerated global warming
rate will continue for at least a decade. Thus, the 2°C global warming limit will also be
breached, unless purposeful actions are taken to reduce our present extraordinary planetary
energy imbalance. In other words, if we wish young people to inherit a planet comparable to
the one that has existed for the past 10,000 years, it will be necessary to reduce the enormous
geoengineering of the planet that our human-made emissions have engendered.
2. CLIMATE POLICY
Climate change has become a great threat because of climate’s delayed response to a forcing
such as a human-made change of the atmosphere. Delayed response means that by the time
human-made climate change is obvious and effects are widely understood to be detrimental,
there is much more climate change “in the pipeline” that will be difficult to avoid. This
delayed response makes climate change an intergenerational issue.
Climate change is a difficult problem for another reason: the principal source of greenhouse
gases (GHGs) is fossil fuels, which are extremely beneficial to humanity. Fossil fuels have
raised living standards almost worldwide and they still provide almost 80 percent of the
world’s energy. Fossil fuels are readily available, so the world will not give up their benefits
without equal or better alternatives.
These characteristics make the climate problem difficult to solve. The policy actions must
penetrate to the deep roots of fundamental matters, but the United Nations has not even
attempted to address the basic problems, as we will discuss. Thus, the world has already
entered a period of consequences, and, because of climate’s delayed response, we are near a
point of no return, a point at which extreme consequences spiral out of humanity’s control.
There remains but a narrow window of time to define and take actions to avoid that result.
We begin this policy discussion with a subsection in which the first author describes a few
personal experiences that help clarify the basic policy requirements. Next, again using
personal examples, we try to explain the reluctance of the scientific community to reveal the
stark reality of the climate situation to the public. With that background, we discuss the three
actions that are required to successfully address climate change and achieve a bright future
for today’s young people and their children. Then we discuss why we believe that a happy
ending is still possible and how young people should be invigorated, not depressed, by the
current situation. Finally, we respond briefly to commentaries on our Pipeline paper.
2.1 A Personal Education in Policy
In 2000, after 25 years of research in climate science, I was concerned that all IPCC scenarios
led to dramatic, undesirable, changes of the planet to which civilization was adapted. Thus,
with help of several colleagues, I suggested an “alternative scenario,”26 a slow phasedown of
CO2 emissions in the first half of the 21st century that we argued was possible with increased
emphasis on energy efficiency and development of clean energies. We included a focus on
pollution that affects human-health (black soot, ozone, and methane, which affects low level
9
ozone). Our rationale was that this would unite the interests of developed and developing
countries. We had no experience in policy, but Gerry Lenfest responded to our plea and
provided funding that allowed us to hold large 5-day workshops at the East-West Center in
Hawaii, in 2002 and 2005, in which we included climate, energy, health and policy scientists
from China, India, the United States, and Europe.27 It was a beginning.
Expanded opportunities arose after I gave a talk28 criticizing the Bush Administration’s
energy policy shortly before the 2004 Presidential election and an updated version29 of the
talk at the 2005 American Geophysical Union meeting. Calls from the White House to NASA
requesting that I be silenced, as documented by Mark Bowen in a book30 and on his website,
resulted in unconstitutional restrictions on my ability to speak to the media. Hullabaloo that
followed led to opportunities for extensive policy-relevant interactions with politicians, utility
CEOs and their staffs, oil and coal executives, and environmentalists in at least a dozen
countries.31 A few highlights help expose major policy failures that continue today.
Carbon fee. It was clear (from graphs of emissions) that the wishful-thinking policy
approach of the Kyoto Protocol, in which each nation is asked to please reduce emissions, is
ineffectual. An invitation to give a talk on the Kyoto Protocol on July 4, 2008, at the United
Nations University in Tokyo, coincident with a G8 meeting hosted by Japan, was a chance to
deliver a letter to Prime Minister Fukuda of Japan.32 I had concluded that the essential policy
is what I first called “carbon tax and 100% dividend,” a tax collected from fossil fuel
companies at domestic mines or ports of entry, with the funds distributed to the public as a
dividend (I changed the name to “fee and dividend” in 2009). We also noted the need for “an
import duty on products produced in other countries that do not impose a comparable carbon
tax,” as an incentive designed to make the carbon fee near-global.
A near-global carbon tax or fee is the basic policy action needed to limit and then bring down
human-made climate change. It’s the sine qua non: a carbon fee is essential to address the
“tragedy of the commons,” the fact that waste products from fossil fuels can be dumped in
the atmosphere freely. At present, without a near-global carbon fee, reduced emissions in
some nations serve to reduce demand, make fossil fuels cheaper, so the fuels will be burned
somewhere. Economists now widely agree on the need for carbon fee and dividend.33 Its
simplicity is essential, avoiding the loopholes in cap-and-trade schemes and offsets, which
are all designed to allow payoffs to special interests.31
In retrospect, my letter probably focused too much on the need to phase out coal and leave
unconventional fossil fuels in the ground. Climate got little attention at the G8. The rushed
trip to Japan seemed to provide only an opportunity for pertinent sarcasm.34
Corollary to carbon fee. Discussions with CEOs and staffs of three utilities (Duke, PSE&G,
and Florida Light and Power) in 2008 raised an issue. They did not dispute the merits of a
rising carbon fee, but they still planned to use fossil fuels to provide baseload dispatchable
power to complement intermittent renewable energy, especially in the Eastern United States,
where hydropower is limited. If we want carbon-free electricity, they said, the government
should support nuclear power the way it supports renewables via the subsidy of renewable
portfolio standards. Without “clean energy portfolio standards” and government policies more
supportive of nuclear power, utilities will continue to burn fossil fuels for many decades.
Thus, the corollary to carbon fee & dividend, as I described in a letter to President-elect
Obama35 in late 2008, was the need for support of modern nuclear power.
East-West cooperation. The second major policy requirement, in addition to a carbon price,
is the need for the West to cooperate with nations with emerging and developing economies.
Fossil fuel additions to atmospheric CO2 are long-lived, so global warming caused by human-
made emissions is proportional to cumulative (historical) CO2 emissions.36,37 Developed
nations in the West, as of today, are responsible for more than half of cumulative emissions
(Fig. 9b), but current emissions are more from China and other emerging and developing
economies. (Fig. 9a). On a per capita basis, the responsibility of the West for climate change
is still clearer (Fig. 10). Fossil fuels have been a boon, helping to raise living standards, but it
is now clear that further CO2 emissions must be limited to obtain a bright future for all
humanity. Little is gained by arguing about responsibilities for emissions, but much can be
gained for everyone by East-West cooperation to reduce future emissions.
Cooperation seemed possible a decade ago, when the Kissinger Institute on China and the
United States invited me to a symposium on U.S.-China relations in Beijing – titled “New
Type of Great Power Relationship” – focused on climate and public health. This was during
the Obama administration and seemed promising. Tom Frieden,40 Director of the Center for
Disease Control, was the other invited scientist on the American side. The Chinese scientists
were the thinktank of China’s State Council. My presentation41 was a summary of the climate
threat and the opportunity for U.S.-China cooperation to address that threat
The clear need is to replace the world’s huge fossil fuel energy system with clean energies,
which likely would include a combination of “renewables” and nuclear power. Even if the
renewables provide most of the energy, engineering and economic analyses indicate that
global nuclear power probably needs to increase by a factor of 2-4 to provide baseload power
to complement intermittent renewable energy, especially given growing demands of China,
India and other emerging economies. The scale of China’s energy needs makes it feasible to
drive down the costs of renewables and nuclear power below the cost of fossil fuels.
The Chinese hosts gave us tours of large, new, facilities making solar panels and windmills,
and Chinese mayors described plans to expand renewable energy use – yet their coal use was
growing rapidly. When I asked about the absence of nuclear power from the agenda, the
response was that the U.S. government seemed to have little interest in nuclear power. Thus,
immediately following the Beijing symposium, I worked with American nuclear experts and
Chinese colleagues to organize a workshop to define potential China-U.S. cooperation to
advance nuclear power, intending it to be held at the East-West Center in Hawaii. When I
struggled to find funding, Junji Cao offered to host the workshop in Hainan, China.
We published a workshop summary in Science.43 We suggested that a large reduction of cost
and construction time was possible via mass manufacturing, analogous to ship and aircraft
construction, with product-type licensing that avoids long delay and cost associated with
case-by-case approval. Collaboration on next-generation technologies requires government
and industries to balance interests in cooperation and competition. However, each country has
a major stake in the other’s success in reducing its carbon emissions.
A memorable aspect of the Hainan workshop was attendance of Kejun Jiang and discussion
with him. I had asked Junji Cao to invite Jiang, because of my impression that Jiang had the
greatest expertise on China’s energy and carbon emissions planning. Remarkably, Jiang made
the trip from Beijing for our brief side-discussion in which I described the potential merits of
fee-and-dividend for China and the U.S., as both countries have growing wealth disparities in
their public and both need to reduce CO2 emissions. Jiang agreed, saying “let’s write a paper
on it.” That paper never happened. We first had long fights to publish the nuclear workshop
paper (we learned that the liberal media bias against nuclear power extends to some scientific
journals, as will be described in Sophie’s Planet, which will eventually be published) and an
equally difficult battle to publish and defend our Ice Melt paper. Then Donald Trump was
elected President of the U.S., practically eliminating cooperation with China.
2.2 Scientific Reticence and Policy
Scientific reticence is discussed in section 7.2 of Pipeline.1 Here I give more revealing detail
on a specific case. In 2015 we submitted our Ice Melt paper44 to Atmospheric Chemistry and
Physics with the title Ice melt, sea level rise and superstorms: evidence from paleoclimate
data, climate modeling, and modern observations implies that 2°C warming is highly
dangerous. The paper was extensively peer-reviewed. Three of four reviewers agreed that it
should be published; the fourth reviewer, an IPCC lead author, seemed particularly incensed
by the paper’s title. The most offending phrase was “2°C warming is highly dangerous.”
In discussion with the editor, it became clear that the crucial issue was interpretation of the
word “dangerous.” That word appears only once in the 1992 United Nations Framework
Convention on Climate Change (UNFCCC),45 as follows “…to achieve, in accordance with
the relevant provisions of the Convention, stabilization of greenhouse gas concentrations in
the atmosphere at a level that would prevent dangerous anthropogenic interference with the
climate system. Such a level should be achieved within a time frame sufficient to allow
ecosystems to adapt naturally to climate change, to ensure that food production is not
threatened and to enable economic development to proceed in a sustainable manner.” The
12
word “dangerous” is not further defined, and that is probably for good reason: the public
understands what “dangerous” means. Thus, I wrote46 to the editor:
“Let us give an analogy. Say there is a poorly lit street frequented by gangs, thugs and
robbers, which police reports indicate has continually been the location of crimes, and
newspapers have reported such. A person looking down this street late at night and seeing a
number of loiterers, one seemingly with a blackjack in his hand, might well conclude that it
would be dangerous to go down that street. What is shown in our paper is analogous. It [a
planet 2°C warmer] would be dangerous in a sense that people would understand.”
Here is the kicker: the editor told me that if I included the above paragraph in my official
letter of response (which would be published on the ACP website, as all correspondence is to
be published in their open review process), they would not accept our paper for publication.
Today, I am still bothered by the censorship of that paragraph. What was the editorial board
thinking? Did they realize that they may be wrong and did not want to be exposed? In any
case, I was exhausted and had to accept their constraints or not publish the paper. We had
worked on Ice Melt for several years, taking pains to minimize unphysical ocean mixing that
reduced the sensitivity of most GCMs to growing freshwater injection from ice melt and from
an amplifying hydrologic cycle. We were confident in our conclusion that continued high
GHG climate forcing would cause shutdown of the AMOC (Atlantic Meridional Overturning
Circulation) and its Southern Hemisphere cousin (SMOC) this century, possibly by
midcentury, and then sea level rise of several meters on a time scale of 50-150 years. These
conclusions were based not only on our GCM modeling, but also on paleoclimate evidence,
especially from the Eemian period, and on ongoing observations.
The result was we had to change the title from “…is highly dangerous,” not to my proposed
compromise (“…is dangerous,”) but rather to “…could be dangerous,” a nothing-burger – we
could have concluded that without writing a scientific paper. To top it off, the IPCC/GCM
community was ready with a publication intended to kill our paper. The 15 authors, from
leading GCM groups, used 21 climate projections from eight “…state-of-the-science, IPCC
class…” GCMs to conclude that the “…likelihood of an AMOC collapse remains very small
(<1% probability) if global warming is below ~5K… ”.47 They treated the ensemble of their
model results as if it were the probability distribution for the real world! Their paper must
have been the basis on which IPCC blackballed our paper.1 We expect that IPCC, over the
next couple of decades, will reach conclusions similar to ours. The problem is that, given a
climate system with delayed response and amplifying feedbacks, it is possible to be too late.
2.3 Three Fundamental Policy Requirements
Section 7.5 of the Pipeline paper, “climate and energy policy,” discusses three main policy
needs: (1) a rising price on GHG emissions, enforced by border duties on products from
nations without a carbon fee; but that is not enough – long-term energy planning consistent
with phaseout of fossil fuel emissions over the next few decades is needed, which in many
nations, if not most, implies the need for rapid development of modern nuclear power, (2)
real global cooperation, in particular between the United States and China, countries with the
two largest economies and the greatest GHG emissions; the present politically-driven choice
to define China as an enemy, rather than negotiate fair relationships, threatens the future of
young people, (3) a multitude of actions are required within less than a decade to reduce and
even reverse Earth’s energy imbalance for the sake of minimizing the enormous ongoing
geoengineering of the planet; specifically, we will need to cool the planet to avoid
consequences for young people that all people would find unconscionable.
13
For the sake of getting this done today, I am also writing these latter sections in first person,
but my opinions and the technical information are influenced by my long-time colleagues,
especially Pushker Kharecha, Makiko Sato, Daniel Galpern, and Eunbi Jeong.
2.4 Is there a solution with a happy ending?
None of the three fundamental policy actions are presently occurring. Nor are they even on
the agenda of the COP (UN Conference of the Parties) meetings. Our ex-student and research
scientist, Surabi Menon, now a staff member of ClimateWorks on leave of absence to work
six months for COP28, tried to get me on the agenda there. Unsurprisingly, given the Pipeline
message that I would have been carrying, she did not succeed in getting me on the agenda.
That’s one reason I decided not to attend COP28. I believe that it’s more important to try to
help young people understand what is driving climate change, what are the consequences,
and what we can do to obtain the most beneficial outcome.
Why am I optimistic about the possibility of a happy ending to the climate crisis? Mainly
because of all the bright young people who can understand what is needed and are willing to
work to make it happen. More than 350 college student body presidents, from all 50 U.S.
states, have come out in favor of following the science, specifically in support of carbon fee
and dividend.48 Even high school students49 can understand the matter. Young people are not
carrying all the baggage, the indoctrinations about what constitutes clean energy, that older
people seem to be saddled with.50 Young people can see and understand that the old geezers
running the world are geoengineering the planet to destruction.51
Here we are out of time for discussion, but we note that we are planning a paper with some of
these young people, and scientists such as Eric Rignot who have the best present estimates for
how much time we have before it is too late to take actions to cool Earth. It may take at least
several years for nature and science to help the powers that be (mostly the public) understand
the need to cool the planet Such actions only make sense if we are simultaneously doing
everything possible to reduce atmospheric GHGs. These matters will be discussed in Sophie’s
Planet,52 which should be finished in 2024.
2.5 Response to Scientific Commentary
It should be sufficient to respond to the main comments of Zeke Hausfather, Michael Mann,
and Johan Rockstrom. They are all outstanding scientists and exceptional communicators.
Zeke Hausfather’s comment relayed to me was that we are “out of the mainstream.” That is a
correct summary. We use well-founded paleoclimate data to infer that equilibrium climate
sensitivity is much higher than IPCC’s best estimate. Also, we use multiple lines of evidence
to infer that the (negative) aerosol climate forcing is larger than IPCC estimates. High climate
sensitivity and declining aerosol forcing are the reason for the shocking increase of Earth’s
energy imbalance and, thus, the acceleration of global warming that is now underway.
Johan Rockstrom, in contrasting his view of the situation and mine, notes that global fossil
fuel emissions declined 7% in the first covid year and suggests that a similar reduction every
year would largely deal with the climate problem. I don’t disagree with that, but no energy
expert I have met believes that such a rapid reduction is plausible, and the West has no right
to demand that nations working to raise their living standards reduce their emissions 7% per
year. Also, I note that our data show only a 5% drop of emissions in the covid year, not 7%.
Michael Mann says that he doesn’t see any acceleration of global warming. Some people
would say that the acceleration is already apparent, but the level to which global temperature
rises by next May and then falls in the next La Nina, will firmly settle that matter.
14
Mike also says that he doesn’t see an increase in the rate of heat uptake by the ocean. Fig. 4
(by Li et al.) in our prior communication21 shows that there is evidence of increased heat
uptake even in the long-term in situ ocean data, which have large error bars because of the
difficulty in obtaining adequate coverage of the global ocean with consistent instrument
calibrations. However, our analysis refers to the changes underway in the 21st century, when
we have much more precise data on Earth’s energy imbalance from the combination of in situ
Argo ocean data and satellite CERES data (Fig. 7 above). The increase of absorption of solar
radiation by Earth and the increase of Earth’s energy imbalance are much larger than the
measurement uncertainty. The measurements need to be continued!53
Mike also says that our paper is “wrong” because nations and industries and businesses are
promising to go to zero emissions or net zero emissions in the future (sometimes in the far
future, when the promiser will be dead or at least out of office). Leaving aside whether the
promisers can all be trusted to deliver and whether their concept of “net zero” is really zero
(very big assumptions!), the present global warming and planetary energy imbalance assure
that we will hit 2°C global warming. Present knowledge of the consequences of ZEC (Zero
Emissions Commitment, the change in global mean temperature expected to occur following
cessation of net CO2 emissions), MacDougall et al.,54 indicates an approximate stabilization
of global temperature from the time at which ZEC is achieved. As for the realism of the
assumptions of near-term achievement of ZEC, one would be wise to read the opinions of
Dyke, Watson and Knorr,55 who have had the real-world experiences needed to grasp the
nature of the present situation.
2.6 Appeal for financial support
In 2023, we (CSAS, Climate Science, Awareness and Solutions) made good progress thanks
to an especially generous contribution from our long-term supporter Jeremy Grantham and a
new supporter (Eric Lemelson). I hope that we did not overlook other supporters from the
past few years in the acknowledgements of our Pipeline paper: [CSAS is a 501(C3) non-
profit supported 100% by public donations. Principal supporters in the past few years have
been the Grantham Foundation, Frank Batten, Eric Lemelson, James and Krisann Miller, Carl
Page, Peter Joseph, Ian Cumming, Gary and Claire Russell, Donald and Jeanne Keith Ferris,
Aleksandar Totic, Chris Arndt, Jeffrey Miller, Morris Bradley and about 150 contributors to
annual appeals.] The extra support in 2023 allowed us to hire two talented young people,
Isabelle Sangha and Joe Kelly, whose help was essential in completing the Pipeline paper.
In 2024, we have a special, one-time, need because Makiko Sato will be retiring at the end of
the year and we need to continue her remarkable work in obtaining, updating, and helping us
understand the huge number of data sets that are needed to analyze climate change. We need
temporal overlap of a new person with Makiko, preferably for at least six months.
Contributions are equally useful to CSAS at Columbia University or CSAS.inc, both of
which are 501(C3) non-profits. CSAS at Columbia supports those people with Columbia
University appointments while CSAS.inc supports all other costs without overhead.
Instructions for donations are at:
CSAS-CU: https://csas.earth.columbia.edu/giving
Inc donations: https://www.climatescienceawarenesssolutions.org/donate
1 Hansen J, Sato M, Simons L et al. Global warming in the pipeline. Oxford Open Clim Chan
2023;3(1):kgad008, doi.org/10.1093/oxfclm/kgad008
2 CFC use was curtailed after it was realized that disassociation of CFC molecules in the stratosphere led to
destruction of the ozone layer, which protects life on Earth from harmful ultraviolet sunlight. If CFCs had not
been curtailed, today’s climate would now be so warm as to constitute a different planet
15
3 The noise is due mainly to fluctuations in the annual growth of atmospheric CO2. Fossil fuel emissions of CO2
change only slowly, a few percent per year, but growth of CO2 in the air is erratic. Averaged over several years
the growth of CO2 in the air averages only about 55% of the fossil fuel emissions, because CO2 “sinks” – the
ocean, soil and biosphere – take up a large portion of the CO2 emissions. These sinks fluctuate from year-to-year
mainly because of climate variability, e.g., droughts can turn a CO2 sink into a temporary source.
4 Hansen J, Sato M. Greenhouse gas growth rates. Proc Natl Acad Sci 2004;101:16109-14
5 IPCC. Climate Change 2021: The Physical Science Basis [Masson-Delmotte V, Zhai P, Pirani A et al. (eds)].
Cambridge and New York: Cambridge University Press, 2021
6 A small part of this 4.1 W/m2 is “slow feedback,” i.e., it is engendered by climate change. The convention is to
use the precisely observed GHG amounts in calculating climate change. Separate studies including the carbon
and nitrogen cycles are needed to estimate the portions of CO2, CH4, and N2O change from feedbacks. Most of
the observed increases are traced to human-made sources.
7 Creutzig F, Erb KH, Haberl H et al. Considering sustainability thresholds for BECCS in IPCC and biodiversity
assessments. GCB Bioenergy 2021;13:510-5
8 Hansen J, Rossow W, Fung I. Long-term monitoring of global climate forcings and feedbacks. Washington:
NASA Conference Publication 3234, 1993
9 In a National Science Foundation project (CLIMAP) culminating in about 1980, it was estimated that the LGM
was 3.5°C colder than the Holocene. That result depended on LGM sea surface temperatures (SSTs) inferred
from the geographical distribution of microscopic biological species recorded in ocean sediment cores, based on
the assumption that these species migrated to stay within temperature zones where they exist today. But what if
species partly adapt on millennial time scales to change of temperature? In such case, CLIMAP underestimated
LGM cooling. Recent LGM studies (Tierney JE, Zhu J et al. Nature 2020;584:569-73; Osman MB, Tierney JE
et al. Nature 2021;599:239-44) exclude the biological species data, instead relying on chemical tracers and
using a GCM to get a realistic global pattern of temperature change, thus finding global cooling of 7°C at the
LGM maximum. An independent analysis (Seltzer AM, Ng J et al. Nature 2021;593:228-32) of temperature-
dependent noble gas abundances in groundwater deposited during the LGM finds cooling of 6°C on land areas
45S to 35N, which is consistent with the global full-LGM cooling found by Tierney et al., as shown in Pipeline.
10 Sustainable Development Solutions Network (SDSN). Nov 3, 2023. An Intimate Conversation with Leading
Climate Scientists to Discuss New Research on Global Warming [Webinar]. YouTube:
https://www.youtube.com/watch?v=NXDWpBlPCY8
11 Zelinka MD, Myers TA, McCoy DT et al. Causes of higher climate sensitivity in CMIP6 models. Geophys
Res Lett 2020;47:e2019GL085782
12 Jiang X, Su H, Jiang JH, et al. Muted extratropical low cloud seasonal cycle is closely linked to
underestimated climate sensitivity in models. Nat Comm 2023;14(1):5586, doi:10.1038/s41467-023-41360-0.
13 References for the International Maritime Organization documents are in our Pipeline paper (ref. 1 above)
14 Ship changes likely phased in the year leading up to January 2015 and January 2020.
15 Jin Q, Grandey BS, Rothenberg D et al. Impacts on cloud radiative effects induced by coexisting aerosols
converted from international shipping and maritime DMS emissions. Atmos Chem Phys 2018;18:16793-16808
16 Loeb NG, Johnson GC, Thorsen, TJ et al. Satellite and ocean data reveal marked increase in Earth’s heating
rate. Geophys Res Lett 2021;48:e2021GL09304
17 The average solar energy incident on Earth is about 340 W/m2.
18 Lenssen NJL, Schmidt GA, Hansen JE et al. Improvements in the GISTEMP uncertainty model, J Geophys
Res Atmos 2019;124(12):6307-26
19 Hansen J, Ruedy R, Sato M et al. Global surface temperature change. Rev Geophys 2010;48:RG4004
20 As an alternative derivation, let’s first assume that the aerosol forcing (added in two steps, 2015 and 2020) is
1 W/m2. That forcing would produce 0.3-0.4°C global warming by now (Fig. 3). With El Nino raising global
temperature 0.1-0.2°C above the trend line and aerosol forcing adding 0.3-0.4°C to global warming, we would
project peak 12-month-mean warming 0.4-0.6°C above the trend line in May 2024, thus including the possibility
of warming slightly above the 1.6-1.7°C range estimated in Pipeline (pink region, Fig. 4).
21 Hansen J, Kharecha P, Loeb N, Sato M, Simons L, Tselioudis G, von Schuckmann K, How we know that
global warming is accelerating and that the goal of the Paris Agreement is dead, 10 November 2023
22That dynamical process is normal during an El Nino, but the weekly forecasts of the NOAA model have
persistently failed to simulate this amplifying feedback during the current El Nino, instead suggesting that the El
Nino already peaked. The average of other dynamical models seems to be more realistic, yielding a peak
Nino3.4 of ~2.0, which would make this El Nino comparable in strength to the 2015-16 El Nino. Weekly
updates of forecasts and observed data are available at:
https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf
23 Myhre G, Shindell D, Breon FM et al. Anthropogenic and natural radiative forcing, in IPCC AR6, 2016.
24 Bellouin N, Quaas J, Gryspeerdt E et al. Bounding global aerosol radiative forcing of climate change. Rev
Geophys 2020;58:e2019RG000660
16
25 Diamond MS. Detection of large-scale cloud microphysical changes within a major shipping corridor after
implementation of the International Maritime Organization 2020 fuel sulfur regulations. Atmos Chem Phys
2023;23:8259-69
26 Hansen J, Sato M, Ruedy R, Lacis A and Oinas V. Global warming in the twenty-first century: an alternative
scenario. Proc Natl Acad Sci USA 2000;97:9875-80
27 Hansen, J.E. (Ed.), 2002: Air Pollution as a Climate Forcing: A Workshop. NASA Goddard Institute for
Space Studies.
28 Hansen, J. Dangerous Anthropogenic Interference: A Discussion of Humanity’s Faustian Climate Bargain and
the Payments Coming Due, Distinguished Public Lecture, University of Iowa, Iowa City, 26 October 2004.
29 Hansen, J. Is There Still Time to Avoid “Dangerous Anthropogenic Interference” with Global Climate?
American Geophysical Union, San Francisco, 6 December 2005.
30 Bowen, Mark, Censoring Science: Inside the Political Attack on Dr. James Hansen and the Truth of Global
Warming. New York: Dutton, 2008.
31 Hansen J. Storms of My Grandchildren. ISBN 978-1-60819-502-2. New York: Bloomsbury, 2009
32 Hansen, J. Dear Prime Minister Fukuda: A letter to leader of Japan before the G8 meeting, 3 July 2008
33 Economists’ statement on carbon dividends (28 November 2022, date last accessed)
34 The letter to Fukuda ended: Finally, Prime Minister Fukuda, I would like to thank you for helping make clear
to the other leaders of the eight nations the great urgency of the actions needed to address climate change. Might
I make one suggestion for an approach you could use in drawing their attention? If the leaders find that the
concept of phasing out all emissions from coal, and taking measures to ensure that unconventional fossil fuels
are left in the ground or used only with zero-carbon emissions, is too inconvenient, then, in that case, they could
instead spend a small amount of time composing a letter to be left for future generations. This letter should
explain that the leaders realized their failure to take these actions would cause our descendants to inherit a planet
with a warming ocean, disintegrating ice sheets, rising sea level, increasing climate extremes, and vanishing
species, but it would have been too much trouble to make changes to our energy systems and to oppose the
business interests who insisted on burning every last bit of fossil fuels. By composing this letter, the leaders will
at least achieve an accurate view of their place in history.
35 Hansen J. Dear Michelle and Barack and Tell Barack Obama the Truth — the Whole Truth, 9 December 2008
36 Hansen J, Sato M, Ruedy R et al. Dangerous human-made interference with climate: A GISS modelE
study. Atmos Chem Phys 2007;7:2287-312
37 Matthews HD, Gillett NP, Stott PA et al. The proportionality of global warming to cumulative carbon
emissions. Nature 2009;459:829-832
38 Hefner M, Marland G, Boden T et al. Global, Regional, and National Fossil-Fuel CO2 Emissions, Research
Institute for Environment, Energy, and Economics, Appalachian State University, Boone, NC, USA.
https://energy.appstate.edu/cdiac-appstate/data-products (20 August 2023, date last accessed)
39 Energy Institute. 2023 Statistical Review of World Energy (20 August 2023, date last accessed)
40 Tom Frieden did not attend, but he was represented by another scientist from the Center for Disease Control. I
had just retired from NASA and did not need permission to attend.
41 Hansen J. Symposium on a New Type of Major Power Relationship, Beijing, China, 24 February 2014.
42 Hansen J, Sato M Regional Climate Change and National Responsibilities. Environ Res Lett 2016;11:034009
43 Cao J, Cohen A, Hansen J et al. China-U.S. cooperation to advance nuclear power. Science 2016;353:547-8
44 Hansen J, Sato M, Hearty P et al. Ice melt, sea level rise and superstorms: evidence from paleoclimate data,
climate modeling, and modern observations that 2 C global warming could be dangerous. Atmos Chem Phys
2016;16:3761-812
45 United Nations Framework Convention on Climate Change. What is the United Nations Framework
Convention on Climate Change? https://unfccc.int/process-and-meetings/what-is-the-united-nations-framework-
convention-on-climate-change) (30 November 2022, date last accessed)
46 Fuller discussion at Hansen, J. Dangerous Scientific Reticence, 23 March 2016
47 Bakker P, Schmittner A, Lenaerts JTM et al. Fate of the Atlantic Meridional Overturning Circulation: strong
decline under continued warming and Greenland melting. Geophy Res Lett 2016;43:12252-60
48 Hansen J. Student leadership on climate solutions. 31 July 2020
49 Hansen J. Can young people save democracy and the planet? 8 October 2021
50 Hansen J. Why are you optimistic? 11 August 2020
51 Operaatio Arktis
52 Hansen J. Sophie’s Planet, draft.
53 Hansen J, Sato M, Ruedy R, Simons L. Global warming is accelerating. Why? Will we fly blind? 14
September 2023
54 MacDougall AH, Frolicher TL, Jones CD et al. Is there warming in the pipeline? A multi-model analysis of
the Zero Emissions Commitment from CO2. Biogeosciences 2020;17(11):2987-3016
55 Dyke J, Watson R, Knorr W. Climate scientists: concept of net zero is a dangerous trap. The Conversation. 22
April 2021