The environmental scientist James Hansen participates in a mock funeral procession during an action day against climate change in 2009. PHOTOGRAPH BY CHRISTOPHER FURLONG/GETTY

How much does the climate have to change for it to be “dangerous”? This question has vexed scientists ever since the first climate models were developed, back in the nineteen-seventies. It was provisionally answered in 2009, though by politicians rather than scientists. According to an agreement known as the Copenhagen Accord, which was brokered by President Barack Obama, to avoid danger, the world needs “to hold the increase in global temperature below 2 degrees Celsius” (3.6 degrees Fahrenheit).

Now a group of climate modellers is arguing that the danger point is, in fact, a lot lower than that. In a paper set to appear online this week in Atmospheric Chemistry and Physics, the modellers, led by James Hansen, the former director of NASA’s Goddard Institute for Space Studies, warn that an increase of two degrees Celsius could still be enough to melt large portions of Antarctica, which, in turn, could result in several metres’ worth of sea-level rise in a matter of decades. What’s important about the paper from a layperson’s perspective—besides the fate of the world’s major coastal cities, many of which would be swamped if the oceans rose that high—is that it shows just how far from resolved, scientifically speaking, the question of danger levels remains. And this has important political implications, though it seems doubtful that politicians will heed them.

To understand the significance of the new paper, it helps to go back to a pair of earlier papers on Antarctic melt, which appeared last year. In those papers, two teams of scientists independently reached the same conclusion: the disintegration of a major portion of the West Antarctic Ice Sheet is probably already under way. “Early stage collapse has begun,” one of the teams wrote in the journal Science. The leader of the other team seconded that view, saying, “The collapse of this sector of West Antarctica appears unstoppable.”

The two papers were, to put it mildly, bad news. “This is what a holy shit moment for global warming looks like” is how Mother Jones put it. All told, the West Antarctic Ice Sheet contains enough water to raise global sea levels by more than ten feet. Still, both of last year’s papers concluded that the melt of a major portion of the ice sheet, while perhaps already irreversible, would likely take centuries to play out.

What the new paper does is look back at a previous relatively warm period, known as the Eemian, or, even less melodically, as Marine Isotope Stage 5e, which took place before the last ice age, about a hundred and twenty thousand years ago. During the Eemian, average global temperatures seem to have been only about one degree Celsius above today’s, but sea levels were several metres higher. The explanation for this, the new paper suggests, is that melt from Antarctica is a non-linear process. Its rate accelerates as fresh water spills off the ice sheet, producing a sort of “lid” that keeps heat locked in the ocean and helps to melt more ice from below. From this, the authors conclude that “rapid sea level rise may begin sooner than is generally assumed,” and also that a temperature increase of two degrees Celsius would put the world well beyond “danger.”

“We conclude that the 2°C global warming ‘guardrail,’ affirmed in the Copenhagen Accord, does not provide safety, as such warming would likely yield sea level rise of several metres along with numerous other severely disruptive consequences for human society and ecosystems,” Hansen and his colleagues wrote.

The new paper has received a lot of attention because, as Eric Holthaus put it for Slate_,_ Hansen is “known for being alarmist and also right.” (I wrote a Profile of Hansen for the magazine, in 2009.) The paper has not been peer-reviewed—it is being published in a “discussion” journal—and several other scientists have called its methods iffy. Kevin Trenberth, a prominent researcher at the National Center for Atmospheric Research, in Boulder, Colorado, for example, told the Washington Post that the paper was “rife with speculation and ‘what if’ scenarios,’ ” and that many of the scenarios “did not seem at all realistic.”

But whether or not Hansen is in this case right, his new paper highlights a crucial point, one that even those who question his methods would probably agree on. The two-degree goal offered in the Copenhagen Accord is more a reflection of what seemed politically feasible than what is scientifically advisable. A group of prominent climatologists put it this way a few months before the accord was drafted: “We feel compelled to note that even a ‘moderate’ warming of 2°C stands a strong chance of provoking drought and storm responses that could challenge civilized society, leading potentially to the conflict and suffering that go with failed states and mass migrations.”

Meanwhile, holding warming to two degrees would, at this point, require a herculean effort—one that the same world leaders who agreed to the Copenhagen Accord now seem unwilling or unable to make. A number of commentators have recently questioned whether, practically speaking, it is even still possible. “The goal is effectively unachievable,” David Victor, of the University of California, San Diego, and Charles Kennel, of the Scripps Institution, wrote recently in Nature. (The commentary was accompanied by a drawing of a feverish and exhausted-looking globe hooked up to a variety of life-support systems.) Thus, whether the “danger” zone lies below two degrees Celsius or above, the world seems bent on reaching it—with all the suffering and challenges to “civilized society” that go with it.