Beneath the ocean's surface, glaciers may be melting 10 to 100 times faster than previously believed, new research shows.
Until now, scientists had a limited understanding of what happens under the water at the point where land-based glaciers meet the sea. Using a combination of radar, sonar and time-lapse photography, a team of researchers has now provided the first detailed measurements of the underwater changes over time. Their findings suggest that the theories currently used to gauge glacier change are underestimating glaciers' ice loss.
"The overall trend of glacier retreat around the world is due to both warming air and warming oceans," said David Sutherland, an oceanographer at the University of Oregon and lead author of the new study, published July 25 in the journal Science.
"They're getting eaten away on both ends," he said.
The warming atmosphere melts glaciers from above, while ocean water can erode the ice along the glacier's face. Researchers have been studying similar effects of ocean water beneath the ice shelves in Antarctica, which slow the flow of the glaciers on land behind them. Last year, a study there found that warming ocean waters are contributing to glacier changes that can increase the rate of ice flow and, subsequently, the rate of sea level rise.
The new study, conducted in Alaska, could help researchers better understand tidewater glaciers elsewhere, including in Greenland and Patagonia.
Over two years, a group of glaciologists, oceanographers and engineers measured melting of the LeConte Glacier in Southeast Alaska. Their study is the first to directly measure underwater melt rates of tidewater glaciers: Previously, scientists relied on estimates of how water and air temperature and ocean currents influence melting.
The researchers found that those theories oversimplify tidewater glacier melting patterns. For example, the theories assume that ice will melt more quickly in places where water is flowing faster. But that's not always the case, the new findings show.
"The theory we've been relying on for these melt rates is wrong," said Rebecca Jackson, an oceanographer at Rutgers University and co-author of the study. "We should be able to predict melt rates based on ocean conditions ... [but] they're not at all related in the way we expected."
In addition to air and water temperature, ocean salinity, currents and the glacier's shape can all play a role in influencing tidewater glacier melt.
Signs of Climate Change
Signs that global warming is influencing glacier melt are evident already. The overall retreat of glaciers is one example.
"If the climate is steady, you expect 90 percent [of glaciers] to be advancing at any time," said Jason Amundson, a glaciologist at the University of Alaska and co-author of the study. "What we're seeing now is the opposite." Only a few tidewater glaciers are advancing; most are retreating, he said.
The tidewater glaciers, which form vertical cliffs in the ocean, lose ice through both melting and a process called calving, as massive chunks break off into the water.
Underwater melting can also intensify calving in the way that chipping away at a building's base would encourage it to topple. "If you start taking away its foundation, it's going to tip over," Sutherland said.
Impact Goes Beyond Sea Level Rise
As fresh water from melting glaciers enters the ocean, it does more than increase sea level. "Plumes" of fast-moving runoff stir up nutrients locked deep in the water, which then feed phytoplankton and zooplankton near the surface, spurring population booms.
"Usually, it's all about sea level rise," Sutherland said. "That would be important if it was the only thing, but all of a sudden we have these ecosystem impacts as well, and that's something we want to quantify."
Changes in tidewater glaciers can have an impact on people living along the Alaskan coast, altering patterns in the ocean water that provides food and livelihood for many. Longer melt seasons mean more fresh water entering the ocean earlier in the year, Amundson explained. "Will that change things like salmon swimming up those streams?" he asked.
More Sensitive to Changes than Expected
As tidewater glaciers melt and contribute to sea level rise, the ability to predict their rate of melting is still developing. "One big open question is: What's the role of the ocean in driving those changes?" Jackson said.
The new direct measurements are giving researchers new insights into that role. "Maybe glaciers are more sensitive to changes in temperature than we might have expected before," Jackson said.
The study's findings suggest that as the planet warms, sea level could rise faster than currently estimated.
"Even if we know how much the ocean was going to warm, the paper shows that we don't have the ability to predict how that would translate into melt rate of the ice," said Jonathan Kingslake, a researcher at Columbia University who models the flow of ice and water in glaciers and was not involved in the study.
Improving those sea level estimates is important for coastal communities to prepare for the effects of climate change, and the new findings are adding to the knowledge base for increasingly detailed analyses. Just a small increase in sea level can exacerbate the effects of storms, for example, Kingslake said. "It makes planning really difficult if you don't have an accurate estimate," he said.
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