This story has been updated. The new paper by Hansen and colleagues can be read online here.

James Hansen has often been out ahead of his scientific colleagues.

With his 1988 congressional testimony, the then-NASA scientist is credited with putting the global warming issue on the map by saying that a warming trend had already begun. “It is time to stop waffling so much and say that the evidence is pretty strong that the greenhouse effect is here,” Hansen famously testified.

Now 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.

It’s an alarming picture of where the planet could be headed — and hard to ignore, given its author. But it may also meet with considerable skepticism in the broader scientific community, given that its scenarios of sea level rise 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, published in 2013.

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.”

The research is slated to appear online this week in Atmospheric Chemistry and Physics Discussion, an open-access journal published by the European Geosciences Union in which much of the peer review process, in effect, happens in public — a paper is uploaded, and then other scientists submit comments on it, and then the authors respond. (Because the work is undergoing a public peer review, we solicited comments on the paper from five prominent climate and ice sheet scientists — their varying reactions are quoted more extensively below.)

The new 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, as one NASA press release put it, the decline of West Antarctica could now be “irreversible.”

In the new study, 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, Hansen and colleagues note, 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.

As the paper notes, there is already evidence of such cooling in the north Atlantic — presumably due to ice loss from Greenland. Note the large blue (cold) anomaly from this recent analysis of last winter’s global temperatures by the National Climatic Data Center:

Around Antarctica, meanwhile, sea ice has been growing — potentially another indicator of cooling and freshening at the ocean surface due to ice loss from the frozen continent.

In the model employed by Hansen and his coauthors, this cooling and freshening of the oceans eventually leads to a shutdown of the oceans’ circulation, and warm waters trapped at depth below a cold fresh surface layer in the Antarctic region, continually eating away at ice sheets from below. It also triggers a globe with ever-warming tropics but cold poles — leading to a large contrast in temperatures between the mid-latitudes and the polar regions.

A larger temperature contrast between the tropics and the poles, the researchers posit, would then strengthen winter storms or so-called extratropical cyclones, which draw their energy from such contrasts. The study therefore contemplates more powerful storms. It notes research suggesting that in the Eemian period, the last time the world saw major sea level rise of as much as 5 to 9 meters (between 16 and 30 feet), gigantic waves apparently moved huge boulders from the seafloor to the top of hills in the Bahamas.

“During this last warm interglacial, not much warmer than the present, [the world saw] not only a higher than present average sea level, but ultimately a significantly higher sea level that required the melting and or collapse of probably both Greenland and West Antarctica, along with basically this great oceanic disturbance,” says Paul Hearty, a geologist at the University of North Carolina, Wilmington who is one of Hansen’s co-authors on the new paper, and conducted the research on the Bahama boulders. “There were storms, and a lot of more catastrophic type events associated with this big climate shift.”

The Eemian, the research suggests, may have only reached global average temperatures about 1 degree Celsius warmer than today — but nonetheless, featured these major changes.

This scenario all depends, of course, on major ice loss from Greenland or Antarctica happening relatively quickly — an assumption that lies at the center of the new paper. “Ice mass losses from Greenland, West Antarctica and Totten/Aurora basin in East Antarctica are growing nonlinearly with doubling times of order 10 years,” notes the study. Elsewhere, it notes that “Doubling times of 10, 20 or 40 years yield sea level rise of several meters in 50, 100 or 200 years.”

And while the paper doubts whether a continued, non-linear growth will be possible for Greenland, it concludes that “if [greenhouse gases] continue to grow, the amplifying feedbacks in the Southern Ocean, including expanded sea ice and [Southern ocean overturning circulation] slowdown likely will continue to grow and facilitate increasing Antarctic mass loss.”

So is this abrupt climate change scenario really something we should take seriously? We received reactions to the paper from a number of top climate researchers — and their responses varied:

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.”

Thus, in light of these comments, it seems safe to say that the intellectual process of digesting the new paper has already begun. And while some researchers are already calling into question some of the more novel scenarios involving meltwater freshening, ocean circulations and storms, the work’s concerns about major sea level rise from melting ice sheets appear hard for scientists to rule out.

Granted, the paper is long and very detailed, synthesizing past research on ice sheets, the oceans, storms, and past climates, while also introducing new climate model experiments — so it will surely take a while for scientists as a whole to determine what they think. But skepticism is probably likely, given not only the nature of scientists in general, but also the significant departures from the view of a consensus body — the IPCC.

“You can see a lot of different points in this paper, and it’s going to take a while for the community to sort them out, but actually, the story is clear,” says Hansen. The bottom line conclusion, he says, is that sea level rise is “the big impact of human made climate change.”

Joby Warrick contributed to this report.

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