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 January 23, 2019
Greenland's ice is melting faster than we thought. Here's why that's scary.

 The island of Greenland, population 56,000, is almost entirely covered in ice that is more than a mile thick in some areas. That's roughly 8 percent of all ice on Earth.

And since 1998, it's been melting due to climate change, adding about 0.027 inches a year to global sea levels. If the entire Greenland ice sheet were to melt, it would raise global sea levels by 20 feet. That's enough to inundate much of lower Manhattan in New York City and flood the National Mall in Washington, DC.

So figuring out how much and how fast this ice is thawing is crucial to the fate of the planet as we know it. And new research looking at these questions is pretty ominous.

In a paper published Monday in the Proceedings of the National Academy of Sciences researchers using satellite and ground measurements found ice loss increased fourfold between 2003 and 2012. The faster melting mainly occurred in the southwestern part of the ice sheet, a region that previously wasn't thought to be thawing so rapidly.

By 2013, however, the rate of melt appeared to pause, which the researchers linked to a cyclical weather pattern known as the North Atlantic Oscillation. In its negative phase, warm air, more sunlight, and less snow reach Western Greenland. The cycle swung into its positive phase in 2013.

Lead author Michael Bevis, a professor in the School of Earth Sciences at Ohio State University, told the New York Times that the pause is cold comfort as global average temperatures continue to rise:

"If a relatively minor cycle can cause massive melting," he said, "it means you've reached a point of amazing sensitivity" to warmer temperatures, which could represent "the tipping point."

And so, he said, "One degree of warming in the future will have way more impact than one degree of warming in the last century."

And as he noted in a press release, "We're going to see faster and faster sea level rise for the foreseeable future. Once you hit that tipping point, the only question is: How severe does it get?"

Changes in the climate are already having outsized consequences for the Arctic

Several studies lately have illustrated a disturbing fact about climate change: While the overall warming trend is slow and gradual, its effects can be sharp and sudden.

A paper published last year in Geophysical Research Letters reached a similar conclusion to the PNAS paper: the Western part of Greenland's ice sheet is currently melting at its fastest rate in at least 400 years, and the melt rate suddenly sped up in the early 1990s.

Erich Osterberg, an assistant professor of earth sciences at Dartmouth College and a co-author of the GRL paper, explained that the team behind the research wanted to fill in a crucial measurement gap in the record of Greenland's ice melt.

"The problem is the last time people were in this area was in the 1990s, and that's when it started to accelerate," Osterberg said.

So they examined composite snow and ice cores from a part of Greenland's Western ice sheet where the ice melted in the summer but refroze in the winter. These cores, which go back 400 years, allowed scientists to create an annual record of thaw and reconstruct ice patterns, building one of the most complete and longest records to date.

With these ice cores, the team was able to validate satellite measurements that showed Greenland's ice sheet was shrinking. They found year-to-year variation in the melt rate but observed that the overall transformation of ice into liquid water was speeding up. Greenland loses on average of 270 billion tons of ice each year.

The rapid increase in melting came as a decade-long pattern of atmospheric blocking --- where warm air stalls over Greenland --- converged with a warming phase in the ocean circulation cycle and the long-term warming trend in the climate.

The team also reported that Greenland is now 1.2 degrees Celsius warmer than when it experienced similar blocking conditions in the 1890s.

These findings clearly point to humanity as the culprit. "This 'extra' warming is most likely caused by human greenhouse gas emissions, leading to the unusual melt rates of recent years," the scientists reported in the paper.

The study adds to a body of work showing that Greenland's ice isn't steadily trickling into the ocean; it's melting in big spikes. Though it's not as spectacular as city block-sized icebergs calving from the ice sheet, the increasing runoff could be more consequential for the global climate system.

Late last year, another research team found using on-the-ground measurements that one catchment in Greenland abruptly started melting 80 percent faster starting in 2003.

Beyond rising seas, the increase in freshwater running into the ocean can slow down the Atlantic Conveyor, a massive pattern of currents in the Atlantic Ocean. "That can have global implications for climate," Osterberg said.

The findings align with other, aforementioned observations about the Arctic: that the sea ice is melting at its fastest rate in 1,500 years and the alarmingly warm winter in the Arctic, replete with a heat wave in February.

And almost everywhere scientists look in the frozen regions of the Earth, known as the cryosphere, they find it rapidly dribbling away. In another paper, Osterberg reported that the glaciers in Alaska's Denali National Park are melting at their fastest rate in 400 years. Summer temperatures are now melting 60 times more snow on Mt. Hunter than they did 150 years ago.

All this goes to show that the world is changing, often faster than we realized, and we still don't fully know what's still to come.