Multiple Recent Climate Findings Highlight The Path To Worst-Case Outcomes
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It's not the fact that ice loss was accelerating that was surprising, but, rather, where it was coming from. There are two sources of ice loss in Greenland, according to Michael Bevis, a professor at Ohio State University and lead author on the new study: chunks of glaciers falling directly into the ocean and runoff from melting, land-based ice. For years, climate scientists have known that the Arctic nation's glaciers were calving into the sea at an accelerating pace as the ocean around the island warmed, but its ice sheet was relatively insulated from increases in air temperatures, Bevis says.
So in this new study, Bevis and his colleagues were surprised to see that most of the ice loss occurred in the southwest corner of Greenland, a region with very few glaciers. In other words, most of the ice loss was in the form of meltwater runoff, draining from land-based ice sheets into the sea. "We could see a huge acceleration in ice loss due to melting: Greenland was losing 100 billion tons a year in 2003, and it was losing nearly 400 billion tons a year in the beginning of 2013," Bevis says. "Then, in the summer of 2013, it just stopped. It was astonishing."
What Bevis and his colleagues found was that the melting was being controlled by something called the North Atlantic Oscillation—an irregular fluctuation in atmospheric pressure over the Atlantic Ocean that influences the weather on several continents. The NAO has two phases: a negative phase, which brings warm air to Greenland, and a positive phase, which brings cold temperatures. The NAO has been around for thousands of years, with little impact on ice melt in Greenland. What changed, Bevis says, is our atmosphere.
"Global warming brought summertime temperatures just shy of the critical temperature at which massive melting would occur, and the NAO pushed it over this critical threshold," he says. When the NAO flipped back to the positive phase and colder air returned, the major melting stopped. This may sound like good news—that a positive NAO can buffer Greenland's ice sheet from global warming—but it's assuredly not, Bevis says. It means that the ice sheet is now sensitive to small fluctuations in summer temperatures, and if global temperatures continue to rise as predicted, soon Greenland's summers will be warm enough to cause massive melting regardless of the NAO's phase.
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https://psmag.com/environment/are-we-headed-toward-the-worst-case-climate-change-scenario