Release of controversial climate paper by James Hansen on sea level rise suggesting major coastal impacts in near term

By Magdalena A K Muir, Climate Editor,
With his 1988 congressional testimony, then NASA scientist James Hansen is credited with putting the global warming issue on the map by saying that a warming trend had already begun. Now James Hansen — who retired in 2013 from his NASA post, and is currently an adjunct professor at Columbia University’s Earth Institute — is publishing what he says may be his most important paper. 
Along with 16 other researchers — including leading experts on the Greenland and Antarctic ice sheets — he has authored a lengthy study outlining an scenario of potentially rapid sea level rise combined with more intense storm systems.
If correct, his paper could have very strong implications for the Europe and the world’s coasts. The paper was released this week prior to peer review by other scientists, and has scenarios of sea level rise that occur more rapidly than those ratified by the United Nations’ Intergovernmental Panel on Climate Change in its latest assessment of the state of climate science, (2013). 
In this recent paper, the authors conclude that 2 degrees Celsius global warming—the widely accepted international target for how much the world should limit global warming—is highly dangerous for the world’s coasts. The paper takes, as one of its starting points, evidence regarding accelerating ice loss from some parts of the planet’s ice sheets, especially West Antarctica. One of Hansen’s co-authors on the new paper, Eric Rignot of NASA, was the lead author of a 2014 study suggesting that the decline of West Antarctica could now be irreversible. In the new paper, Hansen and his colleagues suggest that the “doubling time” for ice loss from West Antarctica — the time period over which the amount of loss could double — could be as short as 10 years. In other words, a non-linear process could be at work, triggering major sea level rise in a time frame of 50 to 200 years. By contrast, the IPCC assumed more of a linear process, suggesting only around 1 meter of sea level rise, at most, by 2100. 
“If the ocean continues to accumulate heat and increase melting of marine-terminating ice shelves of Antarctica and Greenland, a point will be reached at which it is impossible to avoid large scale ice sheet disintegration with sea level rise of at least several meters,” the new paper says.Using climate models and an analogy with the so-called Eemian period or “Marine isotope stage 5e” — an interglacial period some 120,000 years ago that featured considerable sea level rise — the paper goes on to suggest that major ice loss from both Antarctica and Greenland will change the circulation of the oceans, as large volumes of cold, fresh water pour into the seas. This freshening or decreasing saltiness of the ocean, at both poles, could ultimately block the oceans’ overturning circulation, in which (in the northern hemisphere) warm water travels northward, and then colder, denser water sinks and travels back south again.
This paper was released prior to peer review, and some comments are starting to be made  well known climate scientists in public media, such as the Washington post article. Some comments by reknown climate scientists follow:
Michael Mann, a climate researcher at Penn State University who reviewed the paper at the Post’s request, commented by email that “their case is most compelling when it comes to the matter of West Antarctic ice sheet collapse and the substantial sea level rise that would result, potentially on a timescale as short as a century or two.” But Mann was more skeptical of other aspects of the work.“Their climate model scenario wherein Greenland and Antarctic meltwater caused by warming poles, leads to a near total shutdown of ocean heat transport to higher latitudes, cooling most of the globe (particularly the extratropics), seems rather far-fetched to me,” Mann said. Nonetheless, Mann said, “Whether or not all of the specifics of the study prove to be correct, the authors have initiated an absolutely critical discussion.”
Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, an expert on sea level rise and the oceans’ overturning circulation, commented by email that “I agree that 2 C warming is dangerous and will very likely commit our home planet to meters of sea-level rise.” Rahmstorf had not yet had time to review the full Hansen paper Monday, so his comment was strictly about the danger of major sea level rise, not the other scenarios outlined in the study. Rahmstorf has previously suggested that among past periods featuring higher seas, the Eemian may not be the best analogy for where the planet is headed, given that changes at that time were driven by planetary orbital cycles, rather than carbon dioxide emissions.
Kevin Trenberth, an influential climate researcher at the National Center for Atmospheric Research in Boulder, Colorado, was critical of the paper, calling it “provocative and intriguing but rife with speculation and ‘what if’ scenarios.” Trenberth objected in particular to the climate modeling scenarios used to study freshwater injection as ice sheets melt. “These experiments introduce a lot of very cold fresh water in various places, and then they see what happens,” he wrote by email. “The question is how relevant these are to the real world and what is happening as global warming progresses?   They do not seem at all realistic to me.”“There are way too many assumptions and extrapolations for anything here to be taken seriously other than to promote further studies,” Trenberth wrote.
Richard Alley, a glaciologist at Penn State University and an expert on the planet’s ice sheets, said that the study by Hansen and his colleagues was likely to prompt a lot of thought. “Many parts of the new paper are likely to stimulate much technical discussion and further research in our community, as we try to weave together the deep-time and recent history to provide useful projections for the future,” he said by email.“This new paper is not ‘the answer,’” Alley continued. “Particularly, replacing the simple assumptions about doubling times of ice loss with physically based insights is a major focus of our field, but is not yet done and not likely to be ready really quickly.” Alley acknowledged that the IPCC’s sea level rise estimate “is well on the optimistic low-rise side of the possible outcomes,” and added that “the estimates in the new paper of freshening, and discussion of stabilization of the southern ocean and influences on precipitation, are interesting and important.”
Michael Oppenheimer, Albert G. Milbank Professor of Geosciences and International Affairs in the Woodrow Wilson School at Princeton University, commented by email that “If we cook the planet long enough at about two degrees warming, there is likely to be a staggering amount of sea level rise.  Key questions are when would greenhouse-gas emissions lock in this sea level rise and how fast would it happen? The latter point is critical to understanding whether and how we would be able to deal with such a threat.“The paper takes a stab at answering the ‘how soon?’ question but we remain largely in the dark.  Giving the state of uncertainty and the high risk, humanity better get its collective foot off the accelerator.”
 

There is also a very interesting discussion of this recent paper in an influential climate blog by Andrew Revkin, with some text excerpted below: A Rocky First Review for a Climate Paper Warning of a Stormy Coastal Crisis  http://dotearth.blogs.nytimes.com/?_r=0

On July 23, I wrote about the rocky rollout, prior to peer review, of “Ice Melt, Sea Level Rise and Superstorms: Evidence from Paleoclimate Data, Climate Modeling, and Modern Observations that 2°C Global Warming is Highly Dangerous.” The 66-page, 17-author paper was posted Thursday in Atmospheric Chemistry and Physics Discussions, the pre-publication forum for papers submitted to the European Geosciences Union journal Atmospheric Chemistry and Physics. (You’ll hear more on the benefits and problems of such open-review journals toward the end of this post.)The paper is a sweeping cross-disciplinary challenge to status-quo science on risks posed by the building greenhouse effect. The authors, led by James E. Hansen, the veteran climatologist-turned-campaigner, stitch a variety of findings and simulations into a worrisome vision of a looming and abrupt collapse of Antarctic ice sheets and a multi-meter rise in storm-raked seas. They directly call for urgent action by the world’s nations at the Paris treaty talks in December. It’s no wonder the paper made headlines. However less than two days of public review, the paper is being revealed as much more of a rough sketch, a provocation, than a thorough, deeply grounded new thesis. In science, that’s not a bad thing. It is how progress gets made. Dip the gutsy new idea in the acid bath of peer review. What’s left is new knowledge.But in the public sphere, with consequential science, the result can be whiplash, at best, and confusion and disengagement at worst. 
Further information:
A recording of a phone conference call with James E Hansen on July 20, 2015
In this call, James E. Hansen of Columbia University (and formerly NASA’s lead climate scientist) discussed a new discussion paper positing that abrupt sea level rise is a significant prospect with unabated greenhouse gas emissions. Two reporters expressed concerns about the lack of peer review.

The world’s most famous climate scientist just outlined an alarming scenario for our planet’s future
A new study predicting 10 feet of sea level rise by the century’s end isn’t supported by the mainstream scientific community.
Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 ◦C global warming is highly dangerous J. Hansen , M. Sato , P. Hearty , R. Ruedy, M. Kelley , V. Masson-Delmotte, G. Russell4 , G. Tselioudis , J. Cao , E. Rignot, I. Velicogna, E. Kandiano , K. von Schuckmann, P. Kharecha , A. N. Legrande , M. Bauer, and K.-W. Lo
Abstract of paper
There is evidence of ice melt, sea level rise to +5–9 m, and extreme storms in the prior interglacial period that was less than 1 ◦C warmer than today. Human-made climate forcing is stronger and more rapid than paleo forcings, but much can be learned by 5 combining insights from paleoclimate, climate modeling, and on-going observations. We argue that ice sheets in contact with the ocean are vulnerable to non-linear disintegration in response to ocean warming, and we posit that ice sheet mass loss can be approximated by a doubling time up to sea level rise of at least several meters. Doubling times of 10, 20 or 40 years yield sea level rise of several meters in 50, 100 or 10 200 years. Paleoclimate data reveal that subsurface ocean warming causes ice shelf melt and ice sheet discharge. Our climate model exposes amplifying feedbacks in the Southern Ocean that slow Antarctic bottom water formation and increase ocean temperature near ice shelf grounding lines, while cooling the surface ocean and increasing sea ice cover and water column stability. Ocean surface cooling, in the North Atlantic 15 as well as the Southern Ocean, increases tropospheric horizontal temperature gradients, eddy kinetic energy and baroclinicity, which drive more powerful storms. We focus attention on the Southern Ocean’s role in affecting atmospheric CO2 amount, which in turn is a tight control knob on global climate. The millennial (500–2000 year) time scale of deep ocean ventilation affects the time scale for natural CO2 change, thus the time 20 scale for paleo global climate, ice sheet and sea level changes. This millennial carbon cycle time scale should not be misinterpreted as the ice sheet time scale for response to a rapid human-made climate forcing. Recent ice sheet melt rates have a doubling time near the lower end of the 10–40 year range. We conclude that 2 ◦C global warming above the preindustrial level, which would spur more ice shelf melt, is highly danger- 25 ous. Earth’s energy imbalance, which must be eliminated to stabilize climate, provides a crucial metric.
Conclusions of paper
Humanity faces near certainty of eventual sea level rise of at least Eemian proportions, 15 5–9 m, if fossil fuel emissions continue on a business-as-usual course, e.g., IPCC scenario A1B that has CO2 ∼ 700 ppm in 2100 (Fig. S21). It is unlikely that coastal cities or low-lying areas such as Bangladesh, European lowlands, and large portions of the United States eastern coast and northeast China plains (Fig. S22) could be protected against such large sea level rise. 20 Rapid large sea level rise may begin sooner than generally assumed. Amplifying feedbacks, including slowdown of SMOC and cooling of the near-Antarctic ocean surface with increasing sea ice, may spur nonlinear growth of Antarctic ice sheet mass loss. Deep submarine valleys in West Antarctica and the Wilkes Basin of East Antarctica, each with access to ice amounting to several meters of sea level, provide gateways 25 to the ocean. If the Southern Ocean forcing (subsurface warming) of the Antarctic ice sheets continues to grow, it likely will become impossible to avoid sea level rise of several meters, with the largest uncertainty being how rapidly it will occur.
The Greenland ice sheet does not have as much ice subject to rapid nonlinear disintegration, so the speed at which it adds to 21st century sea level rise may be limited. However, even a slower Greenland ice sheet response is expected to be faster than carbon cycle or ocean thermal recovery times. Therefore, if climate forcing continues 5 to grow rapidly, amplifying feedbacks will assure large eventual mass loss. Also with present growth of freshwater injection from Greenland,
in combination with increasing North Atlantic precipitation, we already may be on the verge of substantial North Atlantic climate disruption. Storms conjoin with sea level rise to cause the most devastating coastal damage. 10 End-Eemian and projected 21st century conditions are similar in having warm tropics and increased freshwater injection. Our simulations imply increasing storm strengths for such situations, as a stronger temperature gradient caused by ice melt increases baroclinicity and provides energy for more severe weather events. A strengthened Bermuda High in the warm season increases prevailing northeasterlies that can help 15 account for stronger end-Eemian storms. Weakened cold season sea level pressure south of Greenland favors occurrence of atmospheric blocking that can increase wintertime Arctic cold air intrusions into northern midlatitudes. Effects of freshwater injection and resulting ocean stratification are occurring sooner in the real world than in our model. We suggest that this is an effect of excessive small 20 scale mixing in our model that limits stratification, a problem that may exist in other models (Hansen et al., 2011). We encourage similar simulations with other models, with special attention to the model’s ability to maintain realistic stratification and perturbations. This issue may be addressed in our model with increased vertical resolution, more accurate finite differencing method in ocean dynamics that reduces noise, and 25 use of a smaller background diffusivity. There are many other practical impacts of continued high fossil fuel emissions via climate change and ocean acidification, including irreplaceable loss of many species, as reviewed elsewhere (IPCC, 2013, 2014; Hansen et al., 2013a). However, sea level rise sets the lowest limit on allowable human-made climate forcing and CO2 , because of the extreme sensitivity of sea level to ocean warming and the devastating economic and humanitarian impacts of a multi-meter sea level rise. Ice sheet response time is shorter than the time for natural geologic processes to remove CO2 from the climate system, so there is no morally defensible excuse to delay phase-out of fossil fuel emissions as 5 rapidly as possible. We conclude that the 2 ◦C global warming “guardrail”, affirmed in the Copenhagen Accord (2009), does not provide safety, as such warming would likely yield sea level rise of several meters along with numerous other severely disruptive consequences for human society and ecosystems. The Eemian, less than 2 ◦C warmer than pre-industrial 10 Earth, itself provides a clear indication of the danger, even though the orbital drive for Eemian warming differed from today’s human-made climate forcing. Ongoing changes in the Southern Ocean, while global warming is less than 1 ◦C, provide a strong warning, as observed changes tend to confirm the mechanisms amplifying change. Predicted effects, such as cooling of the surface ocean around Antarctica, are occurring 15 even faster than modeled. Our finding of global cooling from ice melt calls into question whether global temperature is the most fundamental metric for global climate in the 21st century. The first order requirement to stabilize climate is to remove Earth’s energy imbalance, which is now about +0.6 W m−2 , more energy coming in than going out. If other forcings are unchanged, removing this imbalance requires reducing atmospheric CO2 20 from ∼ 400 to ∼ 350 ppm (Hansen et al., 2008, 2013a).
The message that the climate science delivers to policymakers, instead of defining a safe “guardrail”, is that fossil fuel CO2 emissions must be reduced as rapidly as practical. Hansen et al. (2013a) conclude that this implies a need for a rising carbon 25 fee or tax, an approach that has the potential to be near-global, as opposed to national caps or goals for emission reductions. Although a carbon fee is the sine qua non for phasing out emissions, the urgency of slowing emissions also implies other needs including widespread technical cooperation in clean energy technologies (Hansen et al., 2013a).
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