An iceberg floats through the water on July 21, 2013, in Ilulissat, Greenland. If James Hansen is correct, get ready for a lot more of these. Joe Raedle/Getty Images.

James Hansen’s bombshell climate warning is now part of the scientific canon

Last summer, James Hansen—the pioneer of modern climate science—pieced together a research-based revelation: a little-known feedback cycle between the oceans and massive ice sheets in Antarctica and Greenland might have already jump-started an exponential surge of sea levels. That would mean huge levels of sea level rise will happen sooner—much sooner than expected. Hansen’s best estimate was 2 to 5 meters (6–15 feet) by the end of the century: five to 10 times faster than mainstream science has heretofore predicted.

The result was so important that Hansen didn’t want to wait. So he called a press conference and distributed a draft of his findings before they could be peer-reviewed—a very nontraditional approach for a study with such far-reaching consequence. Now, after months of intense and uncharacteristically public scrutiny by the scientific community, the findings by Hansen and his 18 co-authors have passed formal peer review and were published Tuesday in the journal Atmospheric Chemistry and Physics.

That’s bad news for those of us rooting for a stable planet. With Hansen’s paper now through peer review, its dire conclusions are difficult to ignore. And the scientific community, many of whom were initially wary of Hansen’s paper when it came out this summer, is starting to take serious note.

In an email to Slate, Ruth Mottram, a climate scientist who was skeptical of the initial draft, calls the final study “considerably improved.” Mottram, who specializes in studying the Greenland ice sheet, said “the scenario they sketch out is implausible, though perhaps not impossible … it’s frankly terrifying.”

Richard Alley, a key figure in the polar research community, also gave the Hansen study cautious praise. “It usefully reminds us that large and rapid changes are possible,” Alley said in an email, and that “uncertainties are clearly loaded on the “bad” side.” Alley stressed, though, that the Hansen result was only a single study, and wasn’t detailed enough to be used as a firm prediction.

Hansen and his co-authors describe a world that may quickly start to spin out of control if humans keep burning fossil fuels at close to our current rate. “It is not difficult to imagine that conflicts arising from forced migrations and economic collapse might make the planet ungovernable, threatening the fabric of civilization,” the study reads. And given the assumed accelerated pace of melting, all this could happen just decades from now, not centuries.

The world Hansen and his colleagues describe reads like a sci-fi plot synopsis—and it’s now officially part of the scientific canon (though peer review doesn’t necessarily guarantee that a paper is infallible). If Hansen and his colleagues are correct, this paper is likely one of the most important scientific contributions in history—and a stark warning to world governments to speed up the transition to carbon-free energy.

The biggest scientific contribution that Hansen and his colleagues make is an attempt to nail down a Moore’s law (which models nonlinear rates of growth in computer chips) to ice sheets: Assuming non-linear processes have already begun, how fast will Greenland and Antarctica melt? Through scrutiny of climate models, evidence of ancient sharp shifts in climate, and observations of rapid change over the past few decades, Hansen lays out possible futures that put us past the point of no return.

The paper’s conclusions on sea level rise go far beyond current scientific consensus. Hansen suggests “several meters” of sea level rise over the next 50 to 150 years given a continued high rate of fossil fuel emissions, well above the accepted 1 meter or so by the end of the century. He admits the ice sheet models he uses “are still very primitive.”

Hansen’s co-author Eric Rignot said in an email that the exploration of such extreme scenarios was justified “because they are not unlikely, and they are more likely than the more conservative scenarios branded by [United Nations] reports.”



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