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The hidden melting of the most important ice on Earth, explained

The future of sea level rise is being written underneath Antarctica and Greenland.

Color satellite image of breakup of ice pack at the mouth of the Pine Island Glacier in Antarctica.
The ice shelf around Antarctica’s Pine Island Glacier lost about 20 percent of its area between 2017 and 2020.
Planet Observer/Universal Images Group via Getty Images
Umair Irfan is a correspondent at Vox writing about climate change, Covid-19, and energy policy. Irfan is also a regular contributor to the radio program Science Friday. Prior to Vox, he was a reporter for ClimateWire at E&E News.

The largest ice masses on the planet contain so much water that they’re increasing sea levels around the globe as temperatures rise. Satellites can see these drastic changes from space.

But new research finds that some of the most profound changes to Earth’s ice are largely invisible because they’re happening far beneath the surface. Land ice and ice shelves are wearing thin from below, and it’s happening much faster than previously expected.

Ice is at once extremely simple and extraordinarily complicated. It’s just frozen water. But as it gathers in miles-thick sheets near the planet’s poles, it becomes a geological force that can move mountains and reshape the contours of the planet.

The sheer weight of ice presses down on the land and carves it over millennia as the ice slides. Ice holds more than three-quarters of the world’s fresh water.

And when it melts, it can threaten the lives and livelihoods of billions of people. More than one-third of humanity lives within 60 miles (100 kilometers) of a coastline. As average temperatures continue to rise, so will the oceans.

Warmer temperatures are melting solid ice into liquid water that flows into the seas. The oceans themselves are heating up, too, causing the water to expand. Together, these factors are pushing water levels ever higher. The National Oceanic and Atmospheric Administration reported recently that the next 30 years could cause seas to rise in the coastal United States as much as they did in the past century — about 10 to 12 inches (25 to 30 cm).

“By 2050, moderate flooding ⁠— which is typically disruptive and damaging by today’s weather, sea level and infrastructure standards ⁠— is expected to occur more than 10 times as often as it does today,” said Nicole LeBoeuf, NOAA national ocean service director, in a press release. “These numbers mean a change from a single event every 2-5 years to multiple events each year, in some places.”

Despite the enormous consequences of melting polar ice, there’s a lot scientists still don’t know — including some of the mechanisms behind it, where tipping points may lie, and its ripple effects over the whole planet. But recent studies are bringing one of the most difficult-to-study regions into sharper focus: what scientists can’t see with their own eyes. Their findings could change how much the oceans are expected to rise in the coming decades.

What’s hiding beneath the ice

There are two main kinds of ice that shape sea levels. The first is sea ice, which comes from ocean water that freezes solid. It makes up most of the ice at the North Pole. As it forms, it changes the saltiness of seawater and helps shape powerful ocean currents.

Melting sea ice doesn’t change the overall amount of water in the ocean, just as melting ice cubes don’t change the water level in a glass of water. But sea ice tends to reflect sunlight, while the darker ocean tends to soak up its heat. That speeds up warming and drives more ice melt in a worrying feedback loop. The warmer temperatures also contribute to the thermal expansion of water, which in turn can raise sea levels.

The second kind of ice is land ice, which builds up in sheets over thousands of years from compacted snow. In Antarctica, the ice sheet is 1.5 miles thick (2.4 km) on average, reaching up to 3 miles (5 km) in some areas. Greenland’s ice sheet averages a mile in thickness. When land ice starts to jut out over the ocean, it creates a floating ice shelf.

Most of the world’s ice shelves are in Antarctica, where they span more than a million square kilometers, or 386,000 square miles. They act as a buttress, slowing down glaciers that would otherwise flow more quickly into the ocean. But as they get thinner or break apart, the glaciers flow into the ocean at a faster rate, raising sea levels.

Ice loss has accelerated in Antarctica in recent years. Every 40 hours, Antarctica loses a billion metric tons of ice, according to a 2018 study, and at least half that loss comes from ice shelves.

Many of these staggering losses are occurring in places that are very hard to monitor. “It’s a hidden world,” said Robert Larter, a geophysicist at the British Antarctic Survey. “We can see from satellites that the ice is thinning quite dramatically in certain areas, but it’s happening from the bottom up rather than the surface down.”

Scientists are finding innovative new ways to deepen their understanding of these crucial ice shelves, Larter wrote in a recent commentary in the journal Geophysical Research Letters. The key is measuring the melting that happens below rather than above, a phenomenon called basal melting.

The chilly air above the South Pole tends to keep ice frozen from above and around its edges. But deep Antarctic waters aren’t quite as frigid. “At depth in the Southern Ocean, there is a tremendous amount of heat energy below a few hundred meters down,” said Larter. This warmer water can then come into contact with the underside of ice shelves, causing them to melt.

“Warm” by Antarctic standards means “barely above freezing,” but it’s enough to thin ice shelves. “That is in fact what is the driver of most of the serious ice loss that’s happening in Antarctica at the moment,” Larter said.

Schematic view and section through Thwaites Glacier Eastern Ice Shelf.
There are several mechanisms that shape the formation and melt of ice shelves in Antarctica, as seen in this cross-section of the Thwaites Glacier Eastern Ice Shelf.
Marlo Garnsworthy/Geophysical Research Letters

Some of the most intense basal melting is happening at the ice shelves around the Thwaites Glacier and the Pine Island Glacier in West Antarctica. For 60 years, the ice front around the Pine Island Glacier stood in place, but between 2015 and 2020, its northern region suddenly retreated more than 30 kilometers. It’s an example of how changes in ice aren’t always slow and steady but can be sudden.

Scientists are probing the melting depths of ice shelves in several ways. They are drilling holes through ice shelves and lowering instruments and robots down below, for example.

But researchers have also found that melting beneath ice shelves can leave telltale signs above. Ice shelves tend to have a smooth surface, but they get rougher as they melt from below, according to a study in Geophysical Research Letters last year. Measuring surface roughness of ice shelves could become an easy way to gauge how much basal melting is occurring far below. The roughness could be an early warning sign of destabilizing fractures in the ice that could lead to a collapse.

Climate change is squeezing Greenland’s ice sheet from above and below

Greenland is home to the second-largest ice sheet on Earth, accounting for 8 percent of the world’s ice, and it too is melting ever faster. But Greenland’s ice loss is different from Antarctica’s in crucial ways.

One is that almost all of Greenland’s ice is on land, with few sections floating on water. The air over Greenland is also warmer, so melting at the surface of the ice sheet is a much more significant driver of ice loss than at the South Pole. In fact, during the summer, thousands of meltwater lakes and streams form on the surface of the ice sheet.

“The ice sheet is melting fast on the surface, and that’s something we don’t see in Antarctica,” said Poul Christoffersen, a glaciologist at the Scott Polar Research Institute at the University of Cambridge.

Sarah Das from the Woods Hole Oceanographic Institution walks though a surface meltwater lake on July 16, 2013 on the Glacial Ice Sheet, Greenland.
Meltwater forms lakes on the surface of Greenland’s ice sheet, but the ice is also melting below, far out of sight.
Joe Raedle/Getty Images

The water doesn’t stay on top. It pours through cracks and fissures in the ice, falling more than a mile in some places to the rocky ground below. In a study this week published in the Proceedings of the National Academy of Sciences, Christoffersen and his colleagues revealed that this is melting Greenland’s ice sheet from below.

Like the water that rushes through a hydroelectric dam, falling meltwater carries an immense amount of kinetic energy. That energy causes water to warm as it pools beneath the ice sheet. This in turn triggers basal melting. “The melt rates are actually astounding,” said Christoffersen. He estimated peak basal melt rates on the Greenland ice sheet to be 100 times greater than previous estimates that didn’t include this heat source.

The water sandwiched between the ground and the ice sheet also acts as a lubricant, allowing the ice sheet to slide more easily toward the ocean. But because this water is hidden from view, researchers only have a spotty picture of what’s going on. “We don’t really know a lot about these systems,” Christoffersen said. “Are they large rivers or a myriad of small streams, or even tiny films?”

Researchers estimated that these factors would increase the overall melt rate of Greenland by 8 percent. “It doesn’t sound like much, but anybody [who] has ever had a mortgage at 8 percent, they know it’s pretty painful,” Christoffersen said. That means over the coming years, Greenland’s contributions to sea level rise around the world may be greater than previously thought.

There are still more mysteries locked in the ice

These latest findings further confirm that Earth’s cryosphere — its frozen regions — is in trouble. There are forces at work that scientists are only now starting to appreciate.

A better understanding of melting ice helps us imagine the future and prepare for what’s coming. It could help people decide whether to adapt to rising seas, for example with sea walls and elevating buildings, or retreat from coastal areas altogether. But researchers caution that there’s a lot left to study, and the ice could cross a threshold of no return.

For instance, the thinning ice shelves in West Antarctica could enter a cycle of collapse. They could lose enough mass that they fall apart, and the glaciers they keep on land would flow into the ocean much faster.

“There are theoretical scenarios where it could run away,” Larter said. “Once it starts, it would be very difficult to stop.” These potential tipping points are some of the biggest uncertainties for predicting sea level rise, particularly after 2050.

The other major uncertainty — and potential source of hope — is what humans will do about climate change. Confronted with the ice already lost and the growing threats of rising seas, people could start cutting greenhouse gas emissions drastically enough to stave off some of the worst possibilities for sea level rise.

Or countries could continue on the path toward disaster, allowing the planet to heat up further. For billions of people around the world, the future is on thin ice.

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