Leading climate-change scientist outlines an alarming scenario

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 U.N. Intergovernmental Panel on Climate Change (IPCC) 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 was to appear online 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. 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 nonlinear 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.

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 suggests 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. 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.

So is this abrupt climate change scenario something we should take seriously?

Michael Mann, a climate researcher at Penn State University who reviewed the paper, commented by e-mail 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." While Mann was skeptical of other aspects of the work, he said, "The authors have initiated an absolutely critical discussion."