Greenland’s enormous ice sheet is melting at such an accelerated rate that it may have reached a “tipping point” and could become a major factor in sea-level rise around the world within two decades, scientists said in a study published on Monday.
The Arctic is warming at twice the average rate of the rest of the planet, and the new research adds to the evidence that the ice loss in Greenland, which lies mainly above the Arctic Circle, is speeding up as the warming increases. The authors found that ice loss in 2012, more than 400 billion tons per year, was nearly four times the rate in 2003. After a lull in 2013-14, losses have resumed.
The study is the latest in a series of papers published this month suggesting that scientific estimates of the effects of a warming planet have been, if anything, too conservative. Just a week ago, a separate study of ice loss in Antarcticafound that the continent is contributing more to rising sea levels than previously thought.
Another new analysis suggested that the oceans are warming far faster than earlier estimates. Warming oceans are currently the leading cause of sea-level rise, since water expands as it warms.
Researchers said these findings underscored the need for action to curb emissions of planet-warming gases and avoid the worst effects of climate change.
Rising sea levels are one of the clearest consequences of global warming; they are caused both by thermal expansion of the oceans and by the melting of ice sheets on land. Current projections say that if the planet warms by two degrees Celsius (3.6 degrees Fahrenheit) over preindustrial times, average sea levels will rise by more than two feet, and 32 million to 80 million people will be exposed to coastal flooding.
Much of the previous research on Greenland’s ice has dealt with the southeast and northeast parts of the island, where large chunks of glacial ice calve into the sea. The new paper focuses on the ice-covered stretches of southwest Greenland, which has few large glaciers and was not generally considered as important a source of ice loss.
But as the earth warms, the paper concludes, the vast plains of southwestern ice will increasingly melt, with the meltwater flowing to the ocean. Within two decades, it says, the region “will become a major contributor to sea level rise.”
Greenland Is Melting Away
This river is one of a network of thousands at the front line of climate change.
The study, which appears in the Proceedings of the National Academy of Sciences, used satellite data and ground-based instruments to measure Greenland’s ice loss in the 21st century. It looked closely at what seemed to be a pause in the ice loss for about a year, beginning in 2013, that followed a stretch of greatly accelerated melting.
The researchers tied the pause in melting to a reversal of the cyclical weather phenomenon known as the North Atlantic Oscillation. Before the pause, the oscillation was in what is known as its negative phase, which is associated with warmer air hitting west Greenland, along with less snowfall and more sunlight, all of which contribute to ice loss. When the cycle shifted into a positive phase in 2013, an “abrupt slowdown” of melting occurred.
Yet, the slowdown was anything but good news, said Michael Bevis, the lead author of the paper and a professor in the School of Earth Sciences at Ohio State University.
The North Atlantic Oscillation has occurred throughout the historical record, he noted. But before 2000, overall average temperatures were cool enough that the N.A.O.’s positive and negative cycles did not have much of an effect on rates of melting in Greenland.