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 climate change issue on the map by saying that a warming trend had already begun.
Now Hansen — who retired in 2013 from his NASA post and is now 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 a 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 Hansen's reputation.
But it may also meet considerable skepticism in the broader scientific community, given that its scenarios for 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 limiting 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, other scientists submit comments on it, and the authors respond.
The new paper takes, as one of its starting points, evidence regarding accelerating ice loss from parts of the planet's ice sheets, especially West Antarctica. One of the co-authors on the new paper, Eric Rignot of NASA, was the lead author of a 2014 study that suggested, as one NASA news release put it, that 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 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 about 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 Eemian period — an interglacial period 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 in. 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 north and then colder, denser water sinks and travels back south.
As the paper notes, there is already evidence of such cooling in the North Atlantic — presumably because of ice loss from Greenland.
Around Antarctica, meanwhile, sea ice has been growing — potentially another indicator of cooling and freshening at the ocean surface because of ice loss from the frozen continent.
In the model employed by Hansen and his co-authors, this cooling and freshening of the oceans eventually leads to a shutdown of the oceans' circulation, with warm waters trapped 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.
This scenario depends on major ice loss from Greenland or Antarctica happening relatively quickly — an assumption that lies at the center of the new paper.
And while the paper expresses doubts whether a continued non-linear growth will occur in 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?
Michael Mann, a climate researcher at Penn State University who reviewed the paper at The Washington 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 skeptical of other aspects of the work, including whether ice loss from Greenland and Antarctica would lead to a near-total shutdown in the circulation of the oceans.
Mann said, "Whether or not all of the specifics of the study prove to be correct, the authors have initiated an absolutely critical discussion."