Significantly more ice in Greenland's glaciers may be exposed to warming ocean waters than previously thought, new research suggests. Indeed, more than half the ice sheet may be subject to the melting influence of the sea.
These are the latest conclusions of a detailed mapping project exploring the topography of the seafloor and bedrock around and beneath Greenland's glaciers.
Published in their final form last week in the journal Geophysical Research Letters, the maps draw on a variety of data sources, including satellite radar and aerial imagery, as well as special sonar data collected on ship expeditions to the front of the ice sheet. (An earlier, although nearly identical, version of the paper was published online in September.)
Included in the new paper are some of the most detailed data yet on the depths of the canyons and fjords scarring the Greenland coast, which carry water in from the sea to lap against the ice. The results suggest that the western and northern regions of Greenland are most exposed to the influence of ocean water. Out of 139 ocean-touching glaciers the team identified, they also found that 67 rest in waters 200 meters (about 650 feet) or more below sea level, where warm water is typically found—at least twice as many as previously thought.
And, worryingly, the research suggests that as these glaciers melt and retreat backward, the shape of the seabed will continue to expose many of them to warm ocean water for hundreds of miles as the ice moves inland.
"The primary uncertainty in sea level rise is what are the ice sheets going to do over the coming century," said Mathieu Morlighem, an expert in ice sheet modeling at the University of California, Irvine, who led the paper along with dozens of other contributors from institutions around the world. "We know that there could potentially be catastrophic collapse of the ice sheets—we know it has happened in the past—but we don't know how likely it is to happen over our time scale. So we've been doing a lot of progress in terms of better understanding how the ice is flowing."
The study also finds that the Greenland ice sheet may contain more ice, with a greater potential to raise global sea levels, than previous research has suggested—about 2.75 inches more, to be exact. Altogether, the new study suggests that the ice sheet has the potential to raise global sea levels by about 24.3 feet, should it melt entirely.
But the new information about the interaction between ocean and ice along the Greenland coast may be the maps' most important contribution.
"A lot of research has shown that intrusions of warm water are responsible for melting ice along the polar coastlines and that these intrusions are steered by the shape of the seafloor," said Jamin Greenbaum, an oceanography and geology expert at the University of Texas, Austin, who was not involved with the new study, in an email.
It's mainly in the last few years that the ocean's melting influence on glaciers has come to the forefront of scientific attention, but scientists now recognize it as a major—if not the primary—contributor to ice loss in both Greenland and parts of Antarctica.
There are many factors driving what scientists believe to be an increase in the amount of warm water (as opposed to colder water) that many glaciers are exposed to. Some of it is as simple as the fact that climate change is causing the oceans to warm, in addition to the outside air, and some of it may also involve the complex ways climate change is believed to be affecting atmospheric patterns, winds and the ocean currents that carry warm water around the world.
And scientists now know that the underwater topography—the hills, slopes and crevices at the bottom of the ocean, where the ice meets the sea—is a critical influence on just how much ice actually touches the water. So mapping projects like this one are critical for helping scientists figure out how much of the ice sheet is actually threatened.
And they can also significantly inform scientists' understanding of how glaciers will behave, and how quickly they'll lose ice, as they melt. This has to do with the physical processes that affect the way ice in a glacier moves and flows out to sea.
Glaciers that back up to the ocean tend to have similar physical structures. The ice is firmly grounded to the bedrock, up to a point known as the "grounding line." At this point, the ice becomes disconnected from the ground and turns into a kind of floating ledge, known as an ice shelf, that juts out into the ocean from the front of the glacier. These ice shelves are responsible for stabilizing the glacier and hold back the flood of ice behind them.
If they begin to melt, however—particularly as they're exposed to warmer ocean water—the shelves become thinner and the grounding line begins to retreat backward, causing the glacier to become less stable and making the ice shelf more likely to break. But the manner in which this retreat happens depends largely on the shape and contour of the bedrock beneath it, Morlighem noted. A downward slope, for instance, might cause the glacier to retreat more quickly, while ridges or other topographical features might help to slow or halt the backward motion.
"The bed topography controls how far the retreat is going to be and how fast also depending on the slope," Morlighem said. "The bed topography is a crucial data set for ice sheet modelers. Even today, the modeling community would tell you that it's the No. 1 data set that we need."
Growing interest in the connections between ice, ocean and bedrock has helped launch multiple research programs aimed at creating more detailed maps of the world's ice sheets. One of these is NASA's Oceans Melting Greenland, or "OMG," mission, which is collecting both aerial and ship-based observations of ocean temperatures and the shape of the seafloor. Data from the OMG mission helped inform the maps Morlighem and his colleagues have just published.
But scientists aren't only interested in Greenland. They believe similar processes are at play in Antarctica—in fact, "a lot of the big changes in Antarctica all seem to be ocean-driven," said Martin Truffer, an expert in glacier physics at the University of Alaska, Fairbanks. And many researchers are trying to improve maps of the Antarctic ice sheet, as well. Greenbaum, for instance, has conducted some of his own mapping research in Antarctica, and his work, published in Nature Geoscience in March 2015, has helped reveal that East Antarctica's biggest glacier—the Totten Glacier, sometimes referred to as the region's "sleeping giant"—is vulnerable to the intrusion of warm ocean water.
Meanwhile, the increasingly unstable glaciers in West Antarctica remain some of scientists' biggest concerns about future sea-evel rise. Over the last few years, research has increasingly suggested that ice loss in this region is heavily driven by the influence of the warming ocean. And just last year, the United States and the United Kingdom announced an in-depth coordinated research project focused on one of West Antarctica's largest and most vulnerable glaciers, the Thwaites Glacier, which will focus in large part on the interaction of the ocean and the ice front.
Truffer noted that mapping in Antarctica tends to be more challenging than in Greenland because the Antarctic ice sheet is so much larger and so much more remote. But these projects are no less important.
Morlighem and his colleagues are currently working on some similar efforts to map the Antarctic coast while continuing to update and improve their Greenland maps, as well. Ultimately, Morlighem says, all of the data will inform model projections of how the ice sheets will behave under future climate change and how much they may raise global sea levels over the next few centuries—the real question all this research is aimed at answering.
But he noted, "We can't understand how it works if we don't have a good description of the bed topography first."