The vast ice sheets that cover Greenland and the Antarctic have begun melting and sliding into the ocean as heat-trapping greenhouse gases continue to build in the atmosphere. How much and how fast the ice is disappearing, however, has been poorly understood, because the satellites that measure it haven’t always agreed. But a report published Thursday in Science has cleared up much of the uncertainty.
A team of no fewer than 47 scientists from 36 laboratories, looking at data from 10 different satellites, has come up with numbers everyone is on board with: between 1992 and 2011, Greenland has lost 152 billion metric tonnes of ice, while Antarctica has shed 71 billion, contributing 11 millimeters to the rise in sea level over that period – about a fifth of the total (the rest has come from seawater expanding as it warms).
“The new estimates,” lead author Andrew Shepherd, of the University of Leeds, said in a press conference, “are the most reliable to date. They end 20 years of uncertainty.”
The estimates don't change projections about what’s likely to happen for the rest of this century. Scientists’ best estimate is that that the ocean, which has risen an average of 8 inches since 1900, should rise another 3 feet or so by 2100. But without a truly accurate record of how much melting has already occurred, it’s hard to know how good that projection is. It will also be hard to be certain if the melting starts to speed, or slow.
“This [study] provides a nice baseline. It’s really the start of the reliable record,” said co-author Ben Smith, of the University of Washington, at the press conference.
One problem with using information from individual satellites is that they’ve been in orbit at different times over the decades since satellite observations began. Another is that they use entirely different techniques for measuring ice loss – looking at changes in gravitational pull as the mass of ice changes, or bouncing radar beams off the surface to see if the distance between satellite and ice gets bigger as the ice shrinks away. It’s like comparing apples to oranges, with the added confusion that the apples and oranges aren’t on the table at the same time.
“What we found,” Shepherd said in an interview, “was that there was a sweet spot in the mid-2000s when 10 satellites were flying at once.”
Scientists working on data from all 10 came together, and compared results from exactly the same time periods for exactly the same regions within Antarctica and Greenland. It was somewhat analogous to photographing a scene not with a single camera, but with 10 different cameras looking from 10 different perspectives: you get a much richer picture of what’s going on.
In particular, Shepherd said, scientists can now start to pinpoint changes in ice region by region, and figure out what’s happening in each place.
For example, the ice sheet in East Antarctica, which holds by far the most ice, is actually growing slightly, because of an increase in snowfall. “That’s consistent with climate warming because you expect more evaporation and therefore more rain and snow,” Shepherd said.
By contrast, the smaller ice sheets in West Antarctic and the Antarctic Peninsula are losing ice more quickly. “In general,” said co-author Ian Joughin of the University of Washington in an interview, “this is due to heat from the ocean. We’re doing lots of work trying to understand the melting going on beneath ice shelves.”
In fact, Joughin also served as lead author on a second paper in the same issue of Science that tackles exactly this question.
“It’s not as simple as just saying that the ocean warms up,” he said. “The main effect in Antarctica seems to be warmer water from offshore moving up under the ice shelves [that ring the continent] as changing winds blow colder surface water away.”
Joughlin said there’s another big concern. “The warm water is affecting only a few ice shelves so far. If winds bring in warm water more extensively, it could cause a big ice loss.”
In Greenland, meanwhile, where the ice losses have been most dramatic, the situation is much more complicated. Unlike in more frigid Antarctica, temperatures rise high enough in summer that ice melts on the top surface of the ice sheet. Some of the meltwater percolates down to lubricate the base of seaward-flowing glaciers, which may make them move faster. Down where those glaciers meet the sea, warmer ocean water has led to faster calving of ice chunks into the sea – but nobody knows exactly how it all works.
“There are lots of processes in play and it remains unclear whether the ice loss will level out or slow down or speed up,” Joughin said. “We thought we understood ice sheets, but it’s clear we don’t. We need a lot more observations and a lot better modeling.”