liVtaking Things Clearer: Exaggeration, Jumping the Gun, 

and The Venus Syndrome 

15 Apri12013 

James Hansen 

I"retired" so that I can focus my time better on (1) climate science, (2) communications 

thereof, and (3) policy implications. I will do this via research published in the scientific 

literature and translations for a wider audience: 

I have had the good fortune of my research being reported by top science writers: Walter 

Sullivan on the first major climate paper that my colleagues and I published l , Richard Kerr2on 

my congressional testimony in the late 1980s, and Justin Gillis 3on my retirement. Their articles 

raised some issues and queries, which are relevant to the task of getting the public to understand 

the urgency of effective policy actions. 

1. Exaggeration? 

I have been told of specific well-respected people who have asserted that "Jim Hansen 

exaggerates" the magnitude and imminence of the climate threat. If only that were true, I would 

be happy. 

"Magnitude and imminence" compose most of the climate story. 

Magnitude. CO2, the dominant climate forcing on the long run, will stay in the climate 

system for millennia. The magnitude of the eventual climate response to increasing CO2 

depends especially on climate sensitivity. Our best evaluation of climate sensitivity comes from 

Earth's paleoclimate history, via cornparisons of periods with differing climate forcings. a 

Unfortunately, paleoclimate data show that our early estimates of climate sensitivity were 

not an exaggeration. This is made clear in a paper 4in press at the Philosophical Transactions of 

the Royal Society (world's oldest scientific journal). The journal issue containing our paper will 

not appear until this summer. However, the publishers have allowed us to make available a 

nearly final version of the paper on the arXiv website for preprints. 

This paper concludes, among other things, that climate sensitivity is in the upper half of 

the range that has usually been estimated. Furthermore, siow feedbacks, such as change of ice 

sheet size and methane emissions, make the sensitivity still higher. 

Before the paper is published we will write a summary for a broader audience. 

Imminence- Recently a smart young person told me that she tends to discount global 

warming as a concern, because of prior assertions that we only had 5 years or 10 years before 

disastrous consequences -- and her observation that not much has changed in the past 5 years. 

That exposes another communications problem. Scientists did not expect sea level rise of 

meters or "a different planet" in 5 or 10 or 20 years. In 2005 (AGU meeting) I noted that we 

needed to get on a different global emissions path, with decreasing emissions, within 10 years -- 

not because dramatic climate change would occur in 10 years, but because otherwise we will 

build into the climate system future changes that will be out of our control. 

Climate effects are occurring already and are generally consistent with expectations. The 

perceptive person should notice that the clirnate dice are now loaded. However, changes so far 

aA frequently cited alternative, use of observed climate change of the past century, does not yield a useful constraint 

because the net climate forcing is unknown (assumed aerosol forcing can be described best as an educated guess) 

and inferred sensitivity also depend on uncertain transient ocean mixing. 

I 
are small compared with what will happen if we are so foolish as to continue down the path of 

extracting and burning every fossil fuel we can find. See below. 

2. Jumping the Gun 

It has been said that I reach conclusions before the evidence warrants them. Two 

examples suffice to illustrate the predicament that we face. 

Late 1980s. Dick Kerr colorfully titled a 5-day scientific meeting after my 1988/89 

congressional testimonies as "Hansen vs. the World on the Greenhouse Threat" 2 . Yet one of the 

participants told him "if there were a secret ballot at this meeting on the question, most people 

would say the greenhouse warming is probably there." 

Scientific conclusions are based on integration of multiple sources of information: 

climate changes observed today, Earth's history, basic theory, models, etc. Interpretation 

inherently involves assumptions and subjectivity, yet valid conclusions are possible. 

Communication of developing science might be affected by the phenomenon of scientific 

reticence. 5In the 1980s I could shrug off criticism with "It's just a logical, well-reasoned 

conclusion that the greenhouse is here now,i 2go back to research, and let nature clarify matters. 

Today it is different. The science is much clearer. And we are running out of time. 

Today. I was recently at a meeting that included many of the top researchers in climate 

change. There was universal agreement about the urgency of the climate crisis. 

Certainty of our predicament follows from basic considerations including: (1) huge 

inertia and thus slow response of key parts of the climate system, especially the ocean and ice 

sheets, and improving observations by Argo floats and gravity satellites that confirm trends and 

the existence of further change in the pipeline, (2) long lifetime of any ocean warming that is 

allowed to occur, (3) millennial time scale that fossil fuel COz will stay in the climate system, (4) 

paleoclimate confirmation of the magnitude of the eventual climate response to large CO2 

increase. 

These scientists, people who know what they are talking about, were not concerned 

about jumping the gun, but rather about whether the race might already be over. So they were 

considering the potential for air capture of COZ, in effect geo-engineering to counteract our 

unintended geo-engineering. 

What's wrong with this picture? We can pass from "jumping the gun" to unavoidable 

deleterious consequences without passing through demands for common sense policy actions? 

Let's come back to this matter after "The Venus Syndrome". 

3. The Venus Syndrome 

I get questions from the public about the Venus Syndrome: is there a danger of 

"runaway" greenhouse warming on Earth leading to Venus-like conditions? Related questions 

concern specific positive (amplifying) feedbacks such as methane hydrates: as warming thaws 

tundra and destabilizes methane hydrates on continental shelves, thus releasing methane, won't 

this cause more warming, thus more methane release, thus more warming -- a runaway warming? 

Amplifying feedbacks. Let's consider a positive climate forcing (say a solar irradiance 

increase or CO2 increase) that causes a unit of warming. Let's ask how this unit warming will be 

amplified by a very strong feedback, one that increases the initial warming by 50%. The added 

warming of 0.5 induces more feedback, by 0.5X0.5 = 0.25, and so on, the final response being 

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1+ 0.5 + 0.25 + 0.125 +... = 2. So this very strong feedback causes the final warming to be 

twice as large as it would have been without the feedback. But it is not a runaway effect. 

The strongest feedback that we observe on Earth today, from water vapor, is almost as 

strong as this example. Other feedbacks are occurring at the same time, some amplifying and 

some diminishing (negative). The net effect of all fast feedbacks can be assessed by comparing 

different well-characterized climate states in Earth's history, as described in our paper, 4treating 

slow changes such as ice sheet size as specified boundary conditions. It turns out that the net 

effect of fast feedbacks is to amplify the global temperature response by about a factor of 2-3. b 

Other feedbacks become important on longer time scales. As the planet becomes warmer 

the ice sheet area tends to decrease, exposing a darker surface that absorbs more sunlight. And 

as the planet warms the ocean and land release long-lived greenhouse gases, mainly CO2 and 

CH4 (methane). Thus Earth's climate is dominated by amplifying feedbacks on time scales of 

10-100,000 years and less. For this reason, Earth can be whipsawed between glacial and 

interglacial conditions by the small climate forcings caused by perturbations of Earth's orbit. 6 

The dominance of amplifying feedbacks and the resulting high climate sensitivity make 

Earth susceptible to what we can call a mini-runaway. By mini-runaway, I refer to a case with 

an amplifying feedback large enough that the total feedback reaches runaway (the infinite series 

above does not converge), but eventually that process runs out of fuel. Evidence of such 

behavior is provided by hyperthermal events °in Earth's history, sudden rapid warmings that 

occurred during periods of more gradual warming. 

The most studied hyperthermal is the PETM (Paleocene Eocene Thermal Maximum), 

which occurred in the middle of a 10 million year period of gradual warming. A rapid warming 

spike occurred in conjunction with injection of a large amount of CO2 into the climate system on 

a time scale of the order of a millennium. The source of the rapid CO 2increase is most 

commonly suggested to have been the melting of inethane hydrates due to a warming ocean, with 

an alternative suggestion being incineration of large peat deposits, especially on Antarctica. 

Whatever the CO2 source, global temperature increased about 6°C over several millennia. 

The continental weathering process provided a negative feedback, as a pumped-up hydrologic 

cycle drew down atmospheric CO2 and deposited it as carbonate on the ocean floor. However, 

this feedback requires tens of thousands of years, so the rapid warming stopped only when the 

fuel source was depleted. 

Are hyperthermals relevant now, as a possible amplification of fossil fuel warming? 

Unfortunately, they may be. Burning all fossil fuels would produce such large ocean warming, 

which would continue to exist for centuries, that ignition of a hyperthermal amplification of 

global warming is a possibility. Consequences are unclear. Carbon release in prior 

hyperthermals occurred over a millennium or more, at a rate up to — 5 GtC/year. This can be 

compared with the present global rate of fossil fuel burning, which is — 9 GtC/year. 

It is instructive to consider the task of dealing with such continuing carbon release, in the 

event that we did set it off. Humanity could defuse a continuous release of 5 GtC/year, thus 

avoiding hyperthermal warming, by capturing and sequestering the carbon. The American 

Physical Society estimates 7 the cost of capture and sequestration as —$2 trillion per GtC. Given 

that the United States is responsible for 26% of the fossil fuel CO2 in the air today g, the U.S. cost 

share for removing 5 GtC/year would be —$2.6 trillion each year. Technology development 

b Global warming in response to doubled CO Zor a 2% increase of solar irradiance would be 1.2°C in the absence of 

climate feedbacks. Thus the net fast feedback factor of 2-3 yields 2.4-3.6°C warming for doubled CO Z . 

` A brief discussion of hyperthermals on page 3 of our paper4 includes many references to the scientific literature. 

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might be able to lower that cost, but fundamental energy constraints imply that cost reduction at 

most will be a factor of a few. 9 

We had better be sure to avoid a mini-runaway. If we phase out fossil fuels rapidly and 

move to a clean energy future in accord with a scenario that my colleagues and I have described 8 

we could be reasonably confident of avoiding that situation. We know that prior interglacial 

periods were moderately warmer than the current (Holocene) interglacial. A fossil fuel 

emissions scenario similar to the one we have defined is needed for other reasons, especially for 

the purpose of maintaining reasonably stable shorelines, i.e., avoiding sea level rise of many 

meters, which would destroy thousands of coastal cities all around the world. 

In contrast, if we burn all the fossil fuels it is certain that sea level would eventually rise 

by tens of ineters. The only argument is how soon the rise of several meters needed to destroy 

habitability of all coastal cities would occur. It is also possible that burning all fossil fuels would 

eventually set off a hyperthermal event, a mini-runaway. Is it conceivable that we could get a 

runaway leading all the way to the Venus Syndrome, Venus-like conditions on Earth? 

Runaway Greenhouse. Venus today has a surface pressure of about 90 bars, compared 

with 1 bar on Earth. The Venus atmosphere is mostly CO 2 . The huge atmospheric depth and 

CO2amount are the reason Venus has a surface temperature of nearly 500°C. 

Venus and Earth probably had similar early atmospheric compositions, but on Earth the 

carbon is mostly in Earth's crust, not in the atmosphere. As long as Earth has an ocean most of 

the carbon will continue to be in the crust, because, although volcanoes inject crustal carbon into 

the atmosphere as CO2, the weathering process removes CO2 from the air and deposits it on the 

ocean floor as carbonates. Venus once had an ocean, but being closer to the Sun, its atmosphere 

became hot enough that hydrogen could escape from the upper atmosphere, as confirmed today 

by the extreme depletion on Venus of normal hydrogen relative to heavy hydrogen (deuterium), 

the lighter hydrogen being able to escape the gravitational field of Venus more readily. 

Earth can "achieve" Venus-like conditions, in the sense of —90 bar surface pressure, only 

after first getting rid of its ocean via escape of hydrogen to space. This is conceivable if the 

atmosphere warms enough that the troposphere expands into the present stratosphere, thus 

eliminating the tropopause (see Fig. 7 in our paper 4in press), causing water vapor to be 

transported more rapidly to the upper atmosphere, where it can be dissociated and the hydrogen 

can then escape to space. Thus extreme warming of the lower atmosphere with elimination of 

the cold-trap tropopause seems to be the essential physical process required for transition from 

Earth-like to Venus-like conditions. 

If Earth's lower atmosphere did warm enough to accelerate escape of hydrogen it would 

still take at least hundreds of millions of years for the ocean to be lost to space. Additional time 

would be needed for massive amounts of CO2 to accumulate in the atmosphere from volcanoes 

associated with plate tectonics and convection in Earth's mantle. So Venus-like conditions in the 

sense of 90 bar surface pressure and surface temperature of several hundred degrees are only 

plausible on billion-year time scales. 

Is it possible, with the present surface pressure of — 1 bar, for Earth's surface to become 

so hot that that the planet is practically uninhabitable by humans? That is the situation I depicted 

in "Storms of My Grandchildren" 10, which was presumed to be a consequence of burning all 

fossil fuels over a period of several centuries, with warming further amplified by ignition of 

PETM-like hyperthermal warming. Support for the possibility of large warming was provided 

by global climate model simulations indicating an upturn in climate sensitivity at climate 

forcings —10 W/mZ(Fig. 30 in "Stormsi 10). If other forcings are unchanged, a 10 W/m 2forcing 

requires a CO2 increase by a factor of 4-8 times its pre-industrial amount (-280 ppm) -- an 
increase that is possible if all extractable fossil fuels are burned 4°

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. Other complex global climate 

models also find an upturn in climate sensitivity or climate model "crash" when CO2 amount 

reaches such high levels 1 1 , raising the question of whether such a level of climate forcing is 

already trending toward a runaway greenhouse effect. 

The concept of a runaway greenhouse effect was introduced 12by considering a highly 

idealized situation with specified troposphere-stratosphere atmospheric structure, a simple 

approximation for atmospheric radiation, and no inclusion of how clouds might change as 

climate changes, as is appropriate for introduction of a concept. More recent studies 13relax 

some of the idealizations and are sufficient to show that Earth is not now near a runaway 

situation, but the idealizations are still sufficient that the studies do not provide a picture of 

where Earth is headed if all fossil fuels are burned. 

An alternative promising approach is to employ the fundamental equations for 

atmospheric structure and motions, i.e., the conservation equations for energy, momentum, mass, 

and water, and the ideal gas law. These equations form the core of atmospheric general 

circulation models and global climate models. However, today's global models generally contain 

representations of so many additional physical processes that the models are difficult to use for 

investigations of extreme climatic situations, because invariably some approximations in the 

scores of equations become invalid in extreme climates. In contrast, my long-term colleague 

Gary Russell has developed a global model that solves the fundamental equations with the 

minimum additional physics needed to investigate climate sensitivity over the full range from 

snowball Earth to a hothouse uninhabitable planet. The additional physics includes accurate 

spectral dependence of solar and thermal radiation, convection, and clouds. Although the 

precision of the results depends on the representation of clouds, we suggest that the simple 

prescription employed is likely to correctly capture essence of cloud change in response to 

climate change. 

We use the Russell model in our paper to show that the tropopause rises in response to 

the global warming that occurs with larger and larger CO Zamounts (Fig. 7 in our paper4), and 

cloud cover decreases with increasing CO2. In consequence climate sensitivity initially increases 

as CO2increases, consistent with the upturn of sensitivity found in more complex global climate 

modelsl l . With the more realistic physics in the Russell model the runaway water vapor 

feedback that exists with idealized concepts 12does not occur. However, the high climate 

sensitivity has implications for the habitability of the planet, should all fossil fuels actually be 

burned. Furthermore, we show that the calculated climate sensitivity is consistent with global 

temperature and CO2 amounts that are estimated to have existed at earlier times in Earth's history 

when the planet was ice-free. 

One implication is that if we should "succeed" in digging up and burning all fossil fuels, 

some parts of the planet would become literally uninhabitable, with some time in the year having 

wet bulb temperature exceeding 35°C. At such temperatures, for reasons of physiology and 

physics, humans cannot survive, because even under ideal conditions of rest and ventilation, it is 

physically impossible for the environment to carry away the 100 W of inetabolic heat that a 

human body generates when it is at rest 14 . Thus even a person lying quietly naked in hurricane 

force winds would be unable to survive. Temperatures even several degrees below this extreme 

limit would be sufficient to make a region practically uninhabitable for living and working. 

The picture that emerges for Earth sometime in the distant future, if we should dig up and 

burn every fossil fuel, is thus consistent with that depicted in "Storms" -- an ice-free Antarctica 

and a desolate planet without human inhabitants. Although temperatures in the Himalayas may 

have become seductive, it is doubtful that the many would allow the wealthy few to appropriate 

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this territory to themselves or that humans would survive with the extermination of most other 

species on the planet. At least one sentence in "Storms" will need to be corrected in the next 

edition: even with burning of all fossil fuels the tropical ocean does not "boil". But it is not an 

exaggeration to suggest, based on best available scientific evidence, that burning all fossil fuels 

could result in the planet being not only ice-free but human-free. 

4. Summary Discussion 

The inertia of the climate system is not our friend. Because climate responds slowly, we 

have felt so far only about half of the effect of gases already in the air. This limited response 

makes it easier for people to believe that we are exaggerating the climate threat. 

Climate system inertia means that it will take several centuries for the eventual extrerne 

global warming mentioned above to occur, if we are so foolish as to burn all of the fossil fuel 

resources. Unfortunately, despite the ocean's thermal inertia, the transient climate phase this 

century, if we continue business-as-usual fossil fuel burning, is likely to cause an extended phase 

of extreme climate chaos. As ice sheets begin to shed ice more and more rapidly, our climate 

simulations indicate that a point will be reached when the high latitude ocean surface cools while 

low latitudes surfaces are warming. An increased temperature gradient, i.e., larger temperature 

contrast between low and high latitudes, will drive more powerful storms, as discussed in 

"Storms of My Grandchildren".' o 

The science of climate change, especially because of the unprecedented human-made 

climate forcing, includes many complex aspects. This complexity conspires with the nature of 

reporting and the scientific method itself, with its inherent emphasis of caveats and continual 

reassessment of conclusions, to make communications with the public difficult, even when the 

overall picture is reasonably clear. 

My principal objective in "retiring", i.e., in leaving government service, is to create more 

time that will allow me to try to contribute more effectively to this communications effort. I also 

had concluded that the future of the Goddard Institute for Space Studies and its people would be 

better served by a younger person who could be focused on leading GISS. My own heart is 

turning more and more toward trying to make the science and its implications for policy clearer. 

I appreciate very much the well wishes I have received from many of you. I am not good 

at keeping up with e-mail correspondence, so I apologize to anyone who I may have failed to 

respond to. I also realize that the interview I gave regarding my retirement 3may have left the 

impression that I would now be working mainly on specific actions to stem fossil fuel extraction 

and use. I believe all the individual actions occurring at many places are very important and the 

sum of them may help turn the tide to clean energies. But I must keep up with and contribute to 

climate science or I cannot be effective, so I hope to be doing more science rather than less -- and 

science requires more than 40 hours a week -- so it is not practical for me to respond to all the 

requests that I am receiving. I also want to support two or three people working with me, so I 

need to spend time in fund raising - and I am finding that it is not easy to get foundation support. 

I hope that papers and testimony that I provide for cases of Our Children's Trust, or cases 

regarding coal exports, tar sands and other unconventional fossil fuels, can find wider use with 

little modification. I will continue to support the growing 350.org movement. I support 

CitizensClimateLobby.orgespecially, because of their focus on fee-and-dividend, which I 

believe is the sine qua non for phase out of fossil fuels. I hope you noticed the op-ed supporting 

fee-and-dividend in the Wall Street Journal by George Shultz and Gary Becker 15 , who point out 

that fee-and-dividend plus removal of energy subsidies would provide a level playing field and 

be good for the economy and jobs. There is also a Democratic (Boxer/Sanders) bill in Congress, 

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but as usual they cannot keep their hands off our wallets, proposing to take 40% of the money to 

make government bigger, including congressional specification of how 15% of the money is to 

be spent. Washington seems likely to remain dysfunctional on climate policy, so when we get a 

bit closer to 2016 I will argue why I think we need a third parry. In the meantime we must try to 

do what we can with what we have. Somebody with access to the President should wake him up 

to the implications of going down the unconventional fossil fuels route (I have tried, but failed to 

get access). He will have a heck of a lousy legacy if he takes the big step down that road with 

the Keystone pipeline. This is an area where he could reach across the aisle, suggesting that he is 

open to the idea of a revenue neutral carbon fee, which would save much more carbon than the 

Keystone pipeline would carry, but he would have to give up the Democratic penchant for telling 

us how to spend our money. 

Finally, I recognize that in this Summary Discussion I failed to answer the question "How 

can we pass from "jumping the gun" to unavoidable deleterious consequences without passing 

through demands for common sense policy actions?" Delving into that matter requires getting 

into how our government functions and fails to function. That is a big subject and, as you can 

see, I am running out of steam for this present communication. 

1 Hansen, J, D Johnson, A Lacis, S Lebedeff, P Lee, D Rind, G Russell, 1981: 

Climate impact of increasing atmospheric carbon dioxide 

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