From the frozen crags of the Andes and Rockies to country-sized ice sheets covering Greenland and Antarctica, global warming is melting the world's ice at a dizzying rate. In the last five years, mountain glaciers have unexpectedly disintegrated and collapsed, including a pair of deadly ice avalanches in Tibet. In Alaska, a quarter-mile section of the Flat Creek glacier broke away and oozed down the valley, mowing down 400-year-old trees.
This ice loss is worrying for many reasons. Modern humans evolved on a planet where ice has been a crucial regulator, reflecting some of the sun's heat back to space, and storing huge amounts of moisture—about 69 percent of the world's freshwater is stored in glaciers and ice sheets. Slow melting and replenishment were in balance for 10,000 years or so, until human-caused global warming disrupted the cycle.
The meltdown is having impacts across the planet. As ice melts off Greenland and Antarctica their gravitational mass decreases, sending the water surging toward the equator, where sea level rising two or three times as fast as the global average is already swamping islands. A study published April 30 in the journal Science helps show from where the water is coming, and exactly which processes are causing it.
The loss of mountain glaciers is disconcerting for cities and farming areas in the Western United States and other areas that rely on slow-melting mountain ice. Alpine towns that have faced giant avalanches of ice, mud, rocks and snow are also anxious and in South America, mountain towns are threatened by sudden floods from collapsing glaciers.
How Fast Will it Melt?
Knowing how fast global warming will speed up the melting is crucial for people trying to plan future water supplies for cities and farms, or for when they might have to abandon low-lying coastal areas and islands. The new study uses data from NASA's ICESat ( Ice, Cloud, and Land Elevation Satellite) missions to make increasingly accurate calculations of how much ice melted off Greenland and Antarctica from 2003 to 2019, as human-caused warming really kicked into high gear. The study calculated that Greenland lost about 200 gigatons per year during the study period and Antarctica about 118 gigatons annually, together raising sea level by 0.55 inches.
The laser sensors can peer into the steep ice canyons along the edges of Greenland and Antarctica, said NASA scientist Alex Gardner, a co-author of the new paper.
"The numbers are quite consistent between different sensors," Gardner said. "People are really starting to converge on what the central estimates are and we have increasing confidence in those numbers."
Flying 310 miles high at 3.4 miles per second, ICESat measures a grid of the entire Earth every three months, sending trillions of photons down to the surface.
"They bounce photons off the surface like they're ping pong balls and maybe 12 bounce back to the telescope," Gardner said. "From that, they can measure the height of the ice down to a distance of a few centimeters. It's unprecedented. It's helping us understand, what are the processes that are leading to those changes, adding spatial details and insights into what's happening and why."
That includes the vast frozen expanse of East Antarctica, long thought to be a bastion of stable ice. "Our thinking on that is quickly evolving," Gardner said. "For the most part, East Antarctica still seems fairly stable," but there are signs that warmer water incursions could make some of the ice vulnerable to irreversible retreat."
The new study helps differentiate between the processes that cause the melting—in some cases shifts in winds that push warmer water toward the ice, or, in other cases, changes within the ice sheets and glaciers themselves. For example, warming at the surface can send more meltwater to the base of the glaciers. That, Gardner said, "jacks up" the glacier, making it flow faster.
"What we thought would have been a really anomalous year back then is getting to be normal. Now, it's rapidly losing ice each year," Gardner said.
With the data, the researchers were able to show how some polar melting is caused by shifting winds and ocean currents, or by rising air temperature alone. And that helps determine if the ice is near a tipping point that could lead to faster melting.
"Beyond that tipping point, even if you turn the climate back to normal, the ice sheet will melt away forever," he said.
With Seas Rising, Some Don't Have the Option to Stay Put
If a tipping point happened, it would intensify the disastrous sea level rise already swamping some islands in the tropical Pacific. The 0.55 inches of sea level rise from 2003 to 2019 caused by melting in Greenland and Antarctica calculated by the study may not sound like much. But in some places in the tropical Pacific, sea level is going up by that much each year, said Fiji-based marine scientist Stacy Jupiter, director of the Wildlife Conservation Society's Melanesia Program, who was not involved in the new study.
"I think studies like this makes the communications about sea level rise all the more urgent and helps people who are planning for the future take into account what their community is going to look like in 10, 20, or 30 years," Jupiter said. With good data, communities can map out exactly how high sea level will reach.
Some islands are already running out of room.
"They don't really have options to stay," she said. Along with the physical impact of dislocation, rising sea levels are affecting the social fabric of island communities. "Some people are starting to marry their children to people in the highlands to ensure their future safety."
As the rising sea floods coastal villages, some communities are moving their homes into agricultural lands that are a little higher up and farther from the sea. That can affect food production and also changes social relationships, because people aren't living in a cluster anymore.
"It won't be as easy for the community to come together. They would miss out on extended family interactions that drive that extended sense of community," Jupiter said.
Sea level rise also intrudes into very limited underground freshwater reservoirs, affecting water for drinking and crop irrigation.
Mike MacFerrin, a climate scientist at the University of Colorado, Boulder, said the new study is the most detailed summary of ice sheet changes over the past two decades, and that matters because those changes will be the biggest contributors to sea level rise.
One thing the study doesn't identify exactly is how water-logged snow layers atop the ice affect the measurements, said MacFerrin, who wasn't involved in the new study.
Bert Wouters, a climate scientist with Utrecht University and Delft University of Technology, said the new research provides more detailed information than has been available for ice melt in crucial areas, where small, hard-to-measure changes can have outsized impacts on the melt rate.
Improving the understanding of the processes that drive melting in those areas is "the backbone of all the projections," said Ingo Sasgen, a climate scientist with the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research.
Small Compared to Ice Sheets, Glaciers are Just as Important
Sea level rise from melting of the giant polar ice sheets will be the biggest factor in coming centuries. But for the next few decades, the water pouring off mountain glaciers is just as important, said University of Zürich glaciologist Michael Zemp, director of the World Glacier Monitoring Service (WGMS).
If the polar ice sheets are giant pancakes of ice, the world's 200,000 glaciers would be a cornflakes, an individual glacier a sugar crystal. But their small size, relative to the ice sheets, makes them very vulnerable to melting at today's level of warming. When all the world's glaciers melt, it will raise sea level by about 20 inches.
The WGMS tracks the melting of about 30,000 glaciers worldwide, mostly with on-the-ground measurements. A 2019 study published in the journal Nature, based on that data, shows that Alaska is the super-contributor in terms of melt and sea level rise. The melting glaciers of the Southern Andes and Greenland (distinct from the Greenland Ice Sheet) are the next most important sources, followed by the melting from glaciers in the Russian Arctic and the Svalbard and Jan Mayen Arctic Ocean island groups in Scandinavia.
"Right now sea level is rising about 3.5 millimeters per year from all sources (glacier and ice sheet melting, plus thermal expansion)," Zemp said. " We're not really worried about that. But it's not going to stop. That's what we're worried about."
He added, "Generations of glaciologists before me have been interested in the ice age theories of how glaciers respond to climate change, but never had a chance to see if their theories were right, until now."