Reuters A lake of meltwater on the surface of the Petermann Glacier in Greenland.
As I report in Sunday’s Times, many climate scientists are frustrated by the lack of good information about the status of the world’s land ice. Yet they have nonetheless come to an informal consensus about the extent to which the sea may rise in the course of this century: roughly three feet.
Where did that figure come from?
The short answer is that it is based on calculations derived from fairly simple observations about the behavior of land ice and sea level. For that reason, no climate scientist professes huge confidence in the number. It is simply the best the researchers can do with the limited information they have today about the world’s ice.
Yet the calculations themselves are intriguing. The three-foot estimate is based on two distinct methodologies that give essentially the same answer. My interviews with climate scientists over the past several months suggested that it was the overlap between these methods that gave them enough confidence to begin offering the three-foot estimate as their best advice to governments and coastal communities.
One method was pioneered by Stefan Rahmstorf, a professor at the Institute for Climate Impact Research and at nearby Potsdam University in Germany. He told me in an interview that he went to bed one September night a few years back, thinking about the problem of sea level. In the middle of the night, his baby daughter awakened him, and he could not go back to sleep.
As he lay in bed, he was seized by a flash of inspiration. If a cube of ice is placed in a warm room, the rate of melting will depend on the temperature of the room. What if the entire earth worked essentially the same way, Dr. Rahmstorf thought, with the melting of land ice dependent upon the exact temperature of the planet?
Before going back to sleep, he thought up an equation that might describe the relationship between temperature and sea level. The next morning, he analyzed data on his computer and found that there was, indeed, a close correlation between the two. “I thought, hey, there’s something to this,” Dr. Rahmstorf recalled.
He published his method in 2007, encountered some criticism and published a refined version with a colleague in 2009. It calls for using widely accepted projections of the earth’s future temperature to make forecasts about how much the sea will rise in response. His technique has gradually won acceptance as one way to approach the sea-level problem.
A second method was pioneered by a University of Colorado glaciologist named Tad Pfeffer, who is a fellow at the university’s Institute of Arctic and Alpine Research. A few years ago, when rapid changes going on in Greenland and Antarctica led to scary talk among scientists about the possibility of a 15-foot rise in sea level this century, Dr. Pfeffer began to wonder if that was physically possible.
He knew the Greenland ice sheet sits in what amounts to a giant bowl ringed by mountains. If warmed only by the air, it would be slow to melt. To reach the sea unmelted, Greenland’s ice has to flow into glaciers and squeeze through narrow mountain passes. With colleagues, Dr. Pfeffer worked out the physics of how quickly he thought that could happen, and made assumptions about other contributors to sea-level rise.
Dr. Pfeffer found that any rise of sea level higher than about 6 ½ feet in the 21st century would be implausible, and a more conservative set of assumptions produced an estimate of 2 ½ feet.
“My purpose was to try to bring down the loose talk,” Dr. Pfeffer said. “I think that’s happened.”
Dr. Rahmstorf’s method yields a projection of 2½ to 6¼ feet, overlapping Dr. Pfeffer’s range almost exactly. It might be chance, of course, but it might not. A lot of climate scientists are impressed that such different methods yield basically the same answer.
Dr. Pfeffer found that the low end of his range was more likely to come to pass than the high end, which may help to explain why scientists have settled on three feet, not six, as the most likely scenario.