Washam also thinks this dynamic might be leading to the breakup of ice shelves, because it creates crevasses that propagate upwards through the ice until pieces crack into the sea. “Their main form of mass loss—how they lose their ice into the ocean—is actually from big old icebergs breaking off, because you have these crevasses that eventually break through,” he says.
A second paper published today in Science Advances offers more troubling news from the grounding line. In this one, a team from four institutions modeled the environment underneath the Denman and Scott Glaciers in East Antarctica. These two glaciers could together add 1.5 meters (5 feet) to sea level rise if they disappeared. The modeling found long rivers of freshwater flowing from the interior of the ice sheets toward the coast, caused by geothermal heat warming the underside of the glaciers, plus the friction of all that ice grinding against the ground.
When that freshwater dumps into the ocean at the grounding line, it provides turbulence that draws relatively warm ocean water closer to the grounding line, enhancing melting. “As we thin the ice shelf, we’re essentially weakening this dam,” says Scripps Institution of Oceanography glaciologist Tyler Pelle, lead author of the new paper. “This is especially important at the grounding line, just because it is that glacier’s very last contact point with the bedrock. We’re essentially, at this point, thinning the most sensitive part.”
Scientists know how freshwater drives melt, but “we’ve never modeled how these very localized melt enhancements could drive glacial retreat on century timescales, which is what’s important in terms of sea level rise,” Pelle says. The new modeling finds that such subglacial discharge could increase the sea level rise contribution of the Denman and Scott glaciers by about 16 percent by the year 2300 in scenarios of high greenhouse gas emissions. These rivers of subglacial water run beneath most Antarctic glaciers, including Thwaites. “We think that we could really be underestimating Antarctica’s global contribution to sea level rise, because we’re not accounting for this process,” Pelle adds.
Taken together, these papers add to our rapidly evolving understanding of the hidden processes driving the decline of Antarctica’s glaciers, and they underscore the urgent need to reduce carbon emissions. “These systems aren’t yet doomed to collapse and add meters to global sea level. It all depends on how much CO2 we continue to add to the atmosphere and the impact of that on ocean warming,” says University of Waterloo glaciologist Christine Dow, coauthor of the groundwater paper. “It’s not too late to prevent their collapse. But, as these models show, we’re running out of time.”