If you hang around climate scientists, you often hear the saying “Uncertainty is not our friend.” It came to mind yesterday, when two teams of scientists released papers that reached the same terrifying conclusion. A significant chunk of the West Antarctic Ice Sheet has begun to disintegrate and, owing to the ice sheet’s peculiar topography (much of it lies below sea level), this process, having begun, has now also become unstoppable. “Today we present observational evidence that a large section of the West Antarctic Ice Sheet has gone into irreversible retreat,” the lead author of one of the papers, Eric Rignot, a glaciologist at NASA’s Jet Propulsion Laboratory, said at a news conference. “It has passed the point of no return.” Rignot said that melting in the section of West Antarctica that his team had studied could cause global sea levels to rise by four feet over the course of a couple of centuries. Since the disappearance of some of its major glaciers could quite possibly destabilize the entire ice sheet, the ultimate sea level rise from West Antarctica, he said, could be triple that.

“Scary,” Stefan Rahmstorf, a professor of physics of the oceans at Potsdam University, who was not involved in either paper, tweeted. “One of the feared tipping points of the climate system appears to have been crossed.”

“This Is What a Holy Shit Moment for Global Warming Looks Like,” read a headline on the Web site of Mother Jones.

The vulnerability of the West Antarctic Ice Sheet, or WAIS, has been appreciated for a long time; all the way back in 1968, an eccentric Ohio State glaciologist named John Mercer observed that the WAIS was peculiarly unstable, and that it may have melted away in the geologically recent past. But Mercer (who, interestingly enough for a glaciologist, liked to do field work in the nude) published his observations in an obscure journal, and, according to the historian of science Spencer Weart, “did not push his views on colleagues.”

In more recent years, even as forecasts of global sea-level rise have been notched up, most projections have not taken into account the possibility of a significant, near-term ice loss from the West Antarctic. The most recent analysis by the U.N.’s Intergovernmental Panel on Climate Change forecasts a global sea-level rise for this century of somewhere between one and three feet; the new findings, according to Rignot, will require these figures to be revised upward.

Of the many inane arguments that are made against taking action on climate change, perhaps the most fatuous is that the projections climate models offer about the future are too uncertain to justify taking steps that might inconvenience us in the present. The implicit assumption here is that the problem will turn out to be less serious than the models predict; thus, any carbon we have chosen to leave in the ground out of fear for the consequences of global warming will have gone uncombusted for nothing.

But the unfortunate fact about uncertainty is that the error bars always go in both directions. While it is possible that the problem could turn out to be less serious than the consensus forecast, it is equally likely to turn out to be more serious. In fact, it increasingly appears that, if there is any systemic bias in the climate models, it’s that they understate the gravity of the situation. In an interesting paper that appeared in the journal Global Environmental Change, a group of scholars, including Naomi Oreskes, a historian of science at Harvard, and Michael Oppenheimer, a geoscientist at Princeton, note that so-called climate skeptics frequently accuse climate scientists of “alarmism” and “overreacting to evidence of human impacts on the climate system.” But, when you actually measure the predictions that climate scientists have made against observations of how the climate has already changed, you find the exact opposite: a pattern “of under- rather than over-prediction” emerges. The scholars attribute this bias to the norms of scientific discourse: “The scientific values of rationality, dispassion, and self-restraint tend to lead scientists to demand greater levels of evidence in support of surprising, dramatic, or alarming conclusions.” They call this tendency “erring on the side of least drama,” or E.S.L.D. for short.

Unfortunately, we live in dramatic times. Yesterday’s news about the West Antarctic Ice Sheet is just the latest reminder of this; there will, almost certainly, be much more “surprising” and “alarming” news to follow. Which is why counting on uncertainty is such a dangerous idea.

Photograph: Jefferson Beck/GSFC/NASA