NASA’s James Hansen, et al, on what’s needed to avoid a total ice-melt on Earth


An important scientific paper by James Hansen of NASA and colleagues, released last week, suggests that the world must organize around achieving a maximum atmospheric carbon dioxide concentration of 350 ppm to avoid crossing over into the climate conditions that existed millions of years ago when the Earth was ice-free (the current level has already risen to about 385 ppm).  An ice-free planet is nearly inconceivable:  sea level would rise nearly 200 feet and many ecological systems as we know them would no longer exist.  The article calls for nothing short of a Herculean effort, similar to the way the US rallied for World War II, with a grave warning that “the greatest danger is continued ignorance and denial, which could make tragic consequences unavoidable.”

Post by Anne Polansky

Lead author James Hansen of the NASA Goddard Institute and Columbia University was joined by nine other scientists from six other US and European research institutions including Yale and Boston University to publish “Target Atmospheric CO2: Where Should Humanity Aim?” in The Open Atmospheric Science Journal .  The authors identify coal as the most essential fossil fuel to focus on, calling for a rapid phase-out of coal fired power plants unless and until the CO2 can be captured and stored (still a big “if”), along with altered forestry/agricultural practices to sequester CO2. 

It used to be that only white-coated lab scientists used “ppm” as part of their daily conversation— in mainstream America, a ppm (part per million) was as obscure as a Btu (British thermal unit).  Now, an entire sociopolitical movement is organizing around one concentration level:  350 ppm.  Author and activist Bill McKibben was one of the first, just a year or so ago, to break with the thinking that approaching 450 ppm of carbon dioxide from our current level of 385 ppm still left us in a relatively safe zone, but that levels of 500 and higher were too dangerous, could lead to “tipping points” and points of no return in terms of perturbing the global climate system in a way that threatened life on the planet.  Dr. Joe Romm of Climate Progress and others reacted skeptically to Hansen’s and McKibben’s 350 ppm stated goal, questioning whether the draconian 350 ppm target was necessary and/or even remotely achievable.  Meanwhile, grassroots organizations all over the world (such as those who have joined are pushing hard for national and international reforms to draw down CO2 to reach the 350 mark, calling for nothing less than a fundamental change in the way the world does business, with our heavy reliance on coal, oil, and gas being the main root of the problem to be solved.  Such a revolutionary transformation would require a massive deployment of low- and zero-carbon energy sources on a timescale that makes even the most entrepreneurial, imaginative head spin. 

Thus, it is essential that we ask and answer:  Where is the scientific evidence backing the 350 target?  This paper represents some of the best thinking and analysis based on hard data, model runs, and the experience and wisdom of top researchers.  We will be watching with interest to see what sort of reception this paper will have among Hansen’s scientific colleagues and the NGO community. 

Unlike many peer-reviewed scientific papers, the abstract is written in plain English and is unmistakable in the gravity of the pending climate crisis and what we must do to avert it. 


Paleoclimate data show that climate sensitivity is ?3°C for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ?6°C for doubled CO2 for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO2 fell to 450 ± 100 ppm; barring prompt policy changes, that critical level will be passed, in the opposite direction, within decades. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.

Here are some key excerpts with a few interspersed comments: 

>> We use paleoclimate data to show that long-term climate has high sensitivity to climate forcings and that the present global mean CO2, 385 ppm, is already in the dangerous zone.  Despite rapid current CO2 growth, ~2 ppm/year, we show that it is conceivable to reduce CO2 this century to less than the current amount, but only via prompt policy changes.

>>Human-made global climate forcings now prevail over natural forcings… Earth may have entered the Anthropocene era 6-8 ky [ky = kiloyears or 1000s of years] ago, but the net humanmade forcing was small … prior to the industrial era. GHG forcing overwhelmed natural and negative human-made forcings only in the past quarter century… Human-made climate change is delayed by ocean….and ice sheet response times. Warming ‘in the pipeline,’ mostly attributable to slow feedbacks, is now about 2°C…  No additional forcing is required to raise global temperature to at least the level of the Pliocene, 2-3 million years ago, a degree of warming that would surely yield ‘dangerous’ climate impacts.

>> Realization that today’s climate is far out of equilibrium with current climate forcings raises the specter of ‘tipping points’, the concept that climate can reach a point where, without additional forcing, rapid changes proceed practically out of our control….Arctic sea ice and the West Antarctic Ice Sheet are examples of potential tipping points. Arctic sea ice loss is magnified by the positive feedback of increased absorption of sunlight as global warming initiates sea ice retreat… West Antarctic ice loss can be accelerated by several feedbacks, once ice loss is substantial… We define: (1) the tipping level, the global climate forcing that, if long maintained, gives rise to a specific consequence, and (2) the point of no return, a climate state beyond which the consequence is inevitable, even if climate forcings are reduced. A point of no return can be avoided, even if the tipping level is temporarily exceeded. Ocean and ice sheet inertia permit overshoot, provided the climate forcing is returned below the tipping level before initiating irreversible dynamic change.

>> Our estimated history of CO2 through the Cenozoic Era provides a sobering perspective for assessing an appropriate target for future CO2 levels. A CO2 amount of order 450 ppm or larger, if long maintained, would push Earth toward the ice-free state. Although ocean and ice sheet inertia limit then rate of climate change, such a CO2 level likely would cause the passing of climate tipping points and initiate dynamic responses that could be out of humanity’s control.  The climate system, because of its inertia, has not yet fully responded to the recent increase of human-made climate forcings… Yet climate impacts are already occurring that allow us to make an initial estimate for a target atmospheric CO2 level. No doubt the target will need to be adjusted as climate data and knowledge improve, but the urgency and difficulty of reducing the human-made forcing will be less, and more likely manageable, if excess forcing is limited soon.

>> Desire to reduce airborne CO2 raises the question of whether CO2 could be drawn from the air artificially. There are no large-scale technologies for CO2 air capture now, but with strong research and development support and industrial scale pilot projects sustained over decades it may be possible to achieve costs ~$200/tC or perhaps less…. At $200/tC, the cost of removing 50 ppm of CO2 is ~$20 trillion. 

CSW Comment:  This statement, while it is not substantiated well enough to be taken at face value, is an important one for policymakers to take in, especially those who are holding out for a wham-bam technological fix at some undefined future moment ~~ Hansen is saying that even if we were to develop an artificial means of sequestration, it would probably be prohibitively expensive, much more so than relying on more natural, inexpensive means sooner. 

>> A rising price on carbon emissions and payment for carbon sequestration is surely needed to make drawdown of airborne CO2 a reality. A 50 ppm drawdown via agricultural and forestry practices seems plausible. But if most of the CO2 in coal is put into the air, no such “natural” drawdown of CO2 to 350 ppm is feasible. Indeed, if the world continues on a business-as-usual path for even another decade without initiating phase-out of unconstrained coal use, prospects for avoiding a dangerously large, extended overshoot of the 350 ppm level will be dim.

  CSW comment, echoing a Joe Romm truism: “The time to act is yesterday.”

>> “… [I]t appears still feasible to avert catastrophic climate change…. Present policies, with continued construction of coalfired power plants without CO2 capture, suggest that decision-makers do not appreciate the gravity of the situation.…  We must begin to move now toward the era beyond fossil fuels. Continued growth of greenhouse gas emissions, for just another decade, practically eliminates the possibility of near-term return of atmospheric composition beneath the tipping level for catastrophic effects….  The most difficult task, phase-out over the next 20-25 years of coal use that does not capture CO2, is Herculean, yet feasible when compared with the efforts that went into World War II.  The stakes, for all life on the planet, surpass those of any previous crisis. The greatest danger is continued ignorance and denial, which could make tragic consequences unavoidable.

What Hansen et al quite clearly present is a moral argument for a much closer examination by world, national, state, and local leaders of the scientific underpinnings behind an unprecedented transformation of all of human society.  What Hansen does not do in this piece is to make the argument for a campaign to prepare ourselves for a set of inevitable impacts:  droughts, floods, sea level rise, more frequent and intense storms and hurricanes, species extinction, and so on.  We would argue that a strong emphasis on preparedness for climatic disruption is the writing between the lines on this paper that should be taken just as seriously as the call to swift and strong action to slash CO2 emissions.  This is the essential point of our Climate Change Preparedness Initiative and upcoming recommendations to the Obama/Biden administration. 

Hansen’s co-authors include: 

Makiko Sato—NASA/Goddard Institute for Space Studies and Columbia University Earth Institute, NY
Pushker Kharecha – NASA/Goddard Institute for Space Studies and Columbia University Earth Institute, NY
David Beerling—Department of Animal and Plant Sciences, University of Sheffield, UK
Robert Berner—Department of Geology and Geophysics, Yale University
Valerie Masson-Delmotte – Universite de Versailles
Mark Pagani—Department of Geology and Geophysics, Yale University
Maureen Raymo—Department of Earth Sciences, Boston University
Dana L. Royer—Department of Earth and Environmental Sciences, Wesleyan University
James C. Zachos —Earth & Planetary Sciences Dept., University of California, Santa Cruz

Yale’s press release, here.

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