Turns out, when you heat up ice, it melts. And with 2017 likely going down as one of the warmest years on record worldwide, this year's climate change signal was amplified at the Earth's poles.
There, decades-old predictions of intense warming have been coming true. The ice-covered poles, both north and south, continue to change at a breathtaking pace, with profound long-term consequences, according to the scientists who study them closely.
And the consequences are destined to spill over into other parts of the globe, through changing atmospheric patterns, sea currents and feedback loops of ever intensifying melting.
The past year may not have broken annual records, but it provided ample evidence of where long-term trends are heading. "Even though we're not setting records every year—and we don't expect to because of natural variability—we're not any where close to the averages we saw in the 1980s, 1990s and before," said Walt Meier, a senior researcher at the National Snow and Ice Data Center.
The area covered by sea ice in the Arctic hit record lows through the winter of 2017. In March, when the sea ice hit its largest extent of the year, it was lower than it ever had been in the nearly 40-year satellite record. The spring melt began a month earlier than normal, and though the pace of decline slowed some over the summer, the Bering and Chukchi Seas along Alaska's coast remained ice-free longer into the fall than ever before.
In November, NASA reported that two to four times as many coastal glaciers around Greenland are at risk of accelerated melting than previously thought. Greenland is losing an average of 260 billion tons of ice each year. In mid-September, a surge of warm air caused a spike in surface melting in southern Greenland—one of the largest spikes to occur in September since 1978.
In Antarctica, the ice also recorded new lows. The annual low-point in ice coverage, which happened in early March, was the lowest on record. A few months later, in July, a trillion-ton section of Antarctica's Larsen C ice shelf broke off.
"There's no evidence that anything is recovering here," said Mark Serreze, the director of the NSIDC. "What we've seen historically is a downward trend in ice extent in all months. Superimposed on that are the ups and downs of natural variability. We're going to continue to head downward."
When the Jet Stream Gets Loopy
The consequences are profound.
The modern weather system has been defined by cold poles and warmer mid-latitudes. Along the boundary of the regions, where the cold air meets the warm, you'll find bands of strong wind, known as the jet stream.
The jet stream typically behaves in expected or at least understood ways, but every now and then something wacky happens. It dips, it wobbles, and suddenly the northeast finds itself in the chilly embrace of a polar vortex; or California finds itself in yet another drought; or Seattle is doused in days of rain.
"It's basically extreme weather when you get that loopy jetstream," said Meier.
A growing body of science—some of which was recently published—is finding that as there are more years with historically low sea ice levels, there is an associated uptick in wobbly jet streams.
"Trying to ferret out whether this is due to some real change in the system or if it's just random is difficult," Meier said. That has led to some debate among scientists, but increasingly, the picture is becoming clearer that the loss of ice and the wobblier jet streams seem to be correlated.
"When you're taking out 30, 40, almost 50 percent of the ice cover, that's a big change in the environment," Meier said. "Whether we're seeing it yet, there's still some debate, but whether there will be an effect as we continue to lose ice, I think that's pretty obvious."
Anxious About the Outlook
With the icy regions known as the cryosphere in a downward trend, the long-term outlook is disconcerting.
"We are looking at an ice-free Arctic Ocean sometime in the 2040s," said Serreze. "There's no evidence that we've seen anything like this before."
Over the past couple of years, a new body of research has emerged looking on the implications for sea rise. And many scientists are warning that the outlook is more dire than previously feared.
Ted Scambos, lead scientist with the National Snow and Ice Data Center, said that while the current pace of melting is not alarming, a series of papers "has led to a realization that the West Antarctic Ice Sheet may already be in an irreversible retreat."
If parts of the Antarctic ice sheets were to drain more quickly into the ocean, that could raise sea level by several feet within a few decades rather than centuries. Warning signs about a possible sudden disintegration in Antarctica suggest more than 3 feet of sea level rise is possible by the end of the century if societies continuing releasing greenhouse gases at a high rate.
The Biggest Threat
Greenland is melting, too—for now, it's the biggest threat. "Greenland has become Loserville," said Jason Box, who tracks ice for the Geological Survey of Denmark and Greenland.
"New observations from many different sources confirm that ice-sheet loss is accelerating," the United States Global Change Research Program said in its comprehensive special report on climate science. "Up to 8.5 feet of global sea level rise is possible by 2100" in a worst-case emissions scenario. That's almost 2 feet more than scientists expected just a few years ago.
"So we're guaranteed significant sea level rise no matter what we do, even under the optimistic Paris scenario," Box said. "We had better prepare."
Coming next: local implications of sea level rise.