A series of recent studies and reports suggest that, without immediate and drastic action, the worst-case climate scenario will become the rule rather than the exception.

Sea ice is seen from NASA's Operation IceBridge research aircraft off the northwest coast on March 30th, 2017, above Greenland. (Photo: Mario Tama/Getty Images)

A record number of Americans say they accept that global warming is happening, according to a new survey from the Yale Program on Climate Change Communication and the George Mason University Center for Climate Change Communication, and nearly three-quarters of them now say it's an issue that's personally important to them. Both numbers have risen sharply over the last few years—a shift that could not have come soon enough, as the once-distant tipping points and worst-case climate scenarios researchers have been forecasting for decades become reality.

This week, for example, researchers warned that Greenland's ice sheet has reached a tipping point. Looking at more than a decade's worth of ice loss, an international team of scientists found that the rate of Greenland's ice loss in early 2013 was four times higher than in 2003.

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.

Researchers were already concerned about Greenland's contributions to sea-level rise when it was just shedding icebergs that would slowly melt in the ocean. But meltwater, though also a source of sea-level rise, is also worrying for other reasons. The longer-term concern, Bevis says, is that, maybe 50 or 100 years from now, "if you put enough freshwater into the oceans near Greenland, you can start to disturb the global ocean circulation system."

"It's the 900-pound gorilla of the climate system," Bevis says. "Basically, the climate system exists to spread heat out, so a lot of that heat transfer is done by the global ocean circulation systems." Hot water currents, like the Gulf Stream, carry warm water from the Equator toward the poles; when the warm water arrives around Greenland, it sinks. "If there's a whole bunch of freshwater, it won't want to sink, and that conveyor belt of heat is going to slow down," Bevis says. "It's hard to say what the consequences would be, but they would be major."

But here's the thing about those long-term climate change concerns: So far, they have a tendency to arrive much earlier than expected.

Earlier this month, researchers reported in the journal Science that the oceans are warming about 40 percent faster than the United Nations' climate panel predicted just a few years ago. Last year, a climate scientist told the Guardian that the Antarctic ice sheet is now melting at a "surprising" and accelerating rate. Bevis agreed that recent years have produced a "whole series of papers saying that the impacts of global warming have been underestimated and they are happening faster than expected."

While melting ice sheets and sea-level rise get lots of attention, Michael Mann, a climate scientist at Penn State University, points out that climate change is also already having dramatic effects on extreme summertime weather events. In a Science Advances article from October, Mann and his colleagues identified a key mechanism by which climate change influences extreme weather events that current climate models had failed to capture: a misbehaving jet stream. The team found that climate change was causing the once-meandering jet stream to stay in place, trapping high or low pressure systems in place in the atmosphere, which, in the summer of 2018, led to extreme heat waves, drought, wildfires, and flooding across the United States and abroad.

"In other words," Mann writes in an email, "the climate models have likely underestimated the impact that climate change is already having on extreme weather events like the devastating events that unfolded in [the summer of] 2018 and they are likely underestimating the future increases in these events."

Despite this sobering evidence, and growing recognition of the problem by the public, the current federal administration continues to punt on climate change, rolling out policies that weaken environmental protections, bolster fossil fuels, and, ultimately, slow down international progress toward the goals of the Paris Agreement. Indeed, Trump administration officials have tried to discredit some of the most rigorous climate science—the National Climate Assessment, which is based on the work of more than a dozen federal agencies—as "alarmist" or "radical." White House spokesperson Sarah Huckabee Sanders claimed—falsely—that the assessment was based only on "the most extreme" emissions scenario.

"Trump and the fossil fuel lobbyists and advocates who run his administration have done everything possible to deny the science, bury government reports warning of the impacts, and reverse previous policies aimed at curbing carbon emissions and stabilizing warming below dangerous levels," Mann writes. "In that sense—that is to say, from a political perspective, yes—we are witnessing a worst-case scenario."

Indeed, it's becoming ever more clear that, without immediate and drastic action, the worst-case climate scenario will become the rule rather than the exception.

"We're already in a situation where there are going to be quite serious consequences to what we've done to the atmosphere," Bevis says, "but the longer we delay, the worse the consequences will be."