The irreversible melting of a massive cluster of glaciers in Antarctica has begun, and could cause sea levels to rise 1.2 metres within 200 years, scientists conclude in two grim studies.
Researchers had previously estimated the cluster in the Amundsen Sea region of the West Antarctic Ice Sheet would last for thousands of years despite global climate change. But the new studies found that their loss is underway already as warming ocean water melts away the base of the ice shelf, and is occurring far more rapidly than expected.
The studies found warming water was tied to several phenomena, including global warming from human activity and depleted ozone that has changed wind patterns in the area.
"There is no red button to stop this," said Eric Rignot, a University of California, Irvine professor of earth system science and the lead author of one of the studies, conducted with Nasa's Jet Propulsion Laboratory.
The six glaciers had passed "the point of no return", Rignot said, meaning total collapse - the melted retreat of the glaciers - could not be prevented. "The only question is how fast it's going to go."
Antarctica is the largest mass of ice on the planet, containing an estimated 80 per cent of the world's fresh water.
The loss of even a portion of that ice would have consequences across the globe. Scientists have surmised its possibility for decades, and have braced for confirmation, which arrived with the studies on Monday.
For the UCI-JPL study, scientists used 40 years' worth of measurements, much of it data from satellite radar systems that can measure changes on the earth's surface to within a centimetre.
The data was used to measure the precise location of the glaciers' so-called grounding lines - where glaciers connect to a land mass. It is at this nexus where warmer water encounters the ancient ice and causes it to retreat.
The problem compounded itself in several ways. The more grounding lines recede, for instance, the less glaciers weigh, which lifts them farther off the earth's bedrock, which allows even more warm water to erode their foundations. Similarly, as the glaciers retreat into deeper portions of the ocean, their ice faces become steeper, rendering them increasingly unstable and exposed to warmer water.
The second study, conducted by researchers at the University of Washington, focused largely on one of the six glaciers, the Thwaites Glacier. Scientists tried to pinpoint how quickly the giant Thwaites might disappear altogether, a development that by itself could cause global sea levels to rise by 60cm.
That amount of sea level rise would have a chaotic impact, swamping dozens of low-lying cities around the world.
The study used satellite measurements and computer models to determine that the Thwaites could melt in as little as 200 years, or the melting could take as long as 1,000 years.
Ian Joughin, a university glaciologist and the lead author of the University of Washington study, said the most likely scenario was at the lower end of that range.
"There is quite a bit of ongoing destabilisation," he said, and the disappearance would begin slowly and accelerate over time, with no available "stabilising mechanism".
The Thwaites is an important test case because it is viewed as particularly unstable, and a linchpin for the stability of the rest of the West Antarctic Ice Sheet. As goes the Thwaites, many scientists fear, so goes the rest of the ice sheet - its disappearance would undermine the entire glacial system, exposing many more kilometres of grounding lines to the warming ocean water.
The scientists were careful to point out that an inevitability is not cause for surrender. The 800-year range for the time frame is enormous, they pointed out - and was driven by computer models that "turned up the knob and turned down the knob" on global temperatures driven by climate change, Joughin said. More emissions meant more melting and faster collapse, the researchers said, but the inverse was true, too.