The ice that sits atop Greenland spans an area more than three times the size of Texas and almost two miles deep at its thickest. If this frigid white expanse ridged with crystal blue rivers of meltwater were to thaw completely, it would raise global sea levels by more than 20 feet.

Which is why it’s alarming that scientists are repoting that this sheet of ice is thawing, and doing so at an accelerating rate not seen for more than 350 years. Already, scientists have found that Greenland’s ice melt is the main reason the rate of average sea level rise has sped up since 1993.

There’s been a steady drip of research hinting that a major shift is taking place throughout Greenland’s ice. “We’ve seen quite profound changes, I would say, across Greenland,” Luke Trusel, an assistant geology professor at Rowan University, said.

A paper earlier this year reported that Western Greenland is melting at its fastest rate in 400 years. Another study last year found that one section of Greenland suddenly started melting 80 percent faster, suggesting that the rate of thaw in parts of the Greenland ice sheet isn’t steadily increasing, but surging.

However, these studies only looked at sections of the ice sheet. Trusel wanted to find out what we can conclude about melting in Greenland’s ice sheet as a whole. What he and his team realized was that global warming is already leading to drastic alterations within Greenland’s ice and that these changes are swift and sudden. It’s a finding with consequences for coastal regions around the world, where the twin threats of erosion and inundation grow greater and greater.

It’s not just the heat that’s making Greenland’s ice sheet melt faster

In a study published December 5 in the journal Nature, Trusel and his collaborators reconstructed the changes in Greenland’s ice sheet over the past 350 years using ice cores and satellite measurements. These measurements cataloged a history of variations in Greenland’s ice.

“What we were able to do and take it a step further was to show that our ice cores were not just recording what was happening at that particular location but rather more broadly across Greenland,” Trusel said.

The team found that melting in Greenland started to pick up shortly after the dawn of the Industrial Revolution in the mid-1800s, when humans started burning coal, oil, and natural gas in huge quantities, sending tons of heat-trapping carbon dioxide into the atmosphere.

However, it’s only over the last 20 years that the melt rate has definitively increased beyond natural variability. One section of the ice sheet saw its melt intensity surge 575 percent over the last 20 years compared to pre-industrial times.

There are several mechanisms at work here. Greenland gains some ice in the winter and loses some in the summer, but as the planet has warmed, the latter has outpaced the former. The ice sheet itself is also changing. The firn layer in the ice sheet, the boundary between snow and ice, is heating up and becoming denser. So water that would ordinarily trickle down through the snow and refreeze as it contacts ice runs off the ice sheet instead.

The ice in Greenland is also getting darker, as soot carried through the air and microorganisms like algae coat the ice. This allows the ice to absorb more solar energy, thereby heating up and melting faster, which in turn allows the microorganisms to spread further.

All this creates a feedback loop where changes in the climate and in the ice cause the thaw to speed up. “As air temperatures warm, melting outpaces that warming,” Trusel said.

That’s why “only limited additional warming will greatly enhance the area of the ice sheet subject to meltwater runoff,” according to the paper. More melting ice means higher sea levels, which would lead to higher storm surges and more coastal flooding, putting millions more people at risk around the world.

In addition to Greenland’s ice sheet, parts of the Antarctic ice sheet are melting as well. The key uncertainty for sea level rise is what the world will do to limit global warming, but on our current trajectory, we can expect upward of three feet of sea level rise by the end of the century.

There is a lot of grim news for the Arctic climate

The latest findings aren’t all that surprising for scientists, but they do give us greater insight into the unprecedented changes underway at one of the most important polar ice sheets.

The Arctic region as a whole is warming faster than the rest of the world, and we’re seeing the consequences of climate change throughout the Arctic Circle, not just Greenland. Sea ice in the Arctic is declining at its fastest rate in 1,500 years. Earlier this year, we saw a heat wave in the Arctic. That heat in turn helped fuel wildfires in the Arctic.

And as temperatures rise further, scientists expect these shifts to become more drastic. In another paper last month published in Nature Climate Change, Trusel and his co-authors pointed out that the world is now committed to a huge amount of melting in the Greenland ice sheet as well as the Antarctic ice sheet, even if we manage to limit warming to 2 degrees Celsius above preindustrial levels.

However, letting temperatures get any higher could lead to “irreversible mass loss due to the surface mass balance–elevation feedback, whereas for Antarctica, this could result in a collapse of major drainage basins due to ice-shelf weakening.”

Trusel explained that there is likely a tipping point in the ice melt mechanism that we’ll cross in the 1.5 degrees Celsius to 2 degree Celsius range of warming, so to protect coastal areas, it makes sense to cut greenhouse gas emissions aggressively as possible to limit climate change.

However, as the Intergovernmental Panel on Climate Change pointed out in October, capping warming this century to 1.5 degrees Celsius would require an unprecedented global coordinated effort. We’re nowhere close to getting on that pathway. And some countries, including the United States, are actually taking steps to move in the opposite direction.