lunes 16/5/22

Greenland without a return: the thaw reaches the point of no return

Its glaciers have shrunk so much that even if global warming were to stop today, the ice sheet would continue to shrink

Nearly 40 years of satellite data from Greenland show that its glaciers have shrunk so much that even if global warming were to stop today, the ice sheet would continue to shrink.

The finding, published in the journal Nature Communications Earth and Environment, means that Greenland's glaciers have passed a kind of tipping point, where the snowfall that replenishes the ice sheet each year cannot keep up with the ice flowing into the ocean.

"We have been analyzing these remote-sensing observations to study how the discharge and accumulation of ice has changed," Michalea King, lead author of the study and a researcher at Ohio State University's Byrd Polar and Climate Research Center, said in a statement. "And what we have found is that the ice that is discharged into the ocean far exceeds the snow that accumulates on the surface of the ice sheet.

King and other researchers analyzed monthly satellite data from more than 200 large glaciers that flow into the ocean around Greenland. Their observations show how much ice breaks down into icebergs or melts from glaciers into the ocean. They also show how much snow is falling each year, how these glaciers are replenishing themselves.

Research Deductions

The researchers found that, throughout the 1980s and 1990s, snow gained through accumulation and ice melted or detached from glaciers was mostly in balance, keeping the ice sheet intact. Throughout those decades, the researchers discovered, the ice sheets generally lost about 450 gigatons (about 450 billion tons) of ice each year to the outflow of glaciers, which were replaced by snowfall.

"We are measuring the pulse of the ice sheet, the amount of ice drained from the glaciers at the edges of the ice sheet, which increases in the summer. And what we see is that it remained relatively stable until there was a large increase in the discharge of ice to the ocean for a short period of five to six years," King said.

The researchers' analysis found that the baseline of that pulse, the amount of ice lost each year, began to steadily increase around 2000, so the glaciers were losing about 500 gigatons each year. Snowfalls did not increase at the same time, and over the last decade, the rate of ice loss from glaciers has remained more or less the same, meaning that the ice sheet has been losing ice faster than it is replenishing itself.

"Glaciers have been sensitive to seasonal melting ever since we could observe it, with peaks in ice discharge in the summer," he said. "But from 2000, you start to overlap that seasonal melt at a higher baseline, so you're going to have even more losses.

Before 2000, the ice sheet would have about the same chance of gaining or losing mass each year. In today's climate, the ice sheet will gain mass in only one year out of 100.

"Touch and go

King said Greenland's large glaciers have retreated an average of 3 kilometers since 1985 -- "that's a long distance," he said. The glaciers have shrunk enough that many of them are in deeper waters, which means there is more ice in contact with the water. The warm ocean water melts the ice on the glaciers and also makes it difficult for the glaciers to grow back to their previous positions.

That means that even if humans were somehow miraculously able to stop climate change in its tracks, the ice lost to the glaciers that drain the ice into the ocean would probably still exceed the ice gained from snow accumulation, and the ice sheet would continue to shrink for some time.

"The retreat of glaciers has hit the dynamics of the entire ice sheet in a constant state of loss," said paper co-author Ian Howat, a professor of earth sciences and distinguished university scholar at Ohio State. "Even if the climate stayed the same or even cooled down a little bit, the ice sheet would still be losing mass.

The shrinking of Greenland's glaciers is a problem for the whole planet. The ice that melts or breaks off from Greenland's ice sheets ends up in the Atlantic Ocean and eventually in all the world's oceans.

Greenland's ice is one of the main contributors to the rise in sea level: last year, enough ice melted or broke off from the Greenland ice sheet.

Greenland without a return: the thaw reaches the point of no return