The first, a comprehensive seabed mapping project, relying in part on new data from NASA’s OMG (“Oceans Melting Greenland”) mission, concludes that the Greenland ice sheet is far more exposed to the planet’s warming oceans than previously known — and has more ice to give up than, until now, has been recognized.
The massive study, published last month in Geophysical Research Letters, pulls together a large number of data records to provide a comprehensive map of the shape of the seabed around and lying beneath Greenland’s glaciers, based on state of the art soundings taken by ships and other data sources.
The research — which pulls together a body of evidence that has now been accumulating for a while — measures the depth and contours of the ocean floor both beneath liquid water in Greenland’s fjords and beneath ice in places where the ocean may someday flow. The work was led by Mathieu Morlighem of the University of California at Irvine, with no less than 31 other authors from institutions in the United States, Canada, Britain and across Europe.
The researchers have found that Greenland contains more total ice above sea level than previously thought — the entire ice mass is capable of raising sea levels by 24.3 feet, about three inches more than previously realized.
Still more significant is how much of that ice is vulnerable to warm water that reaches the bases of the ice sheet’s deeper glaciers. The new research finds that “between 30 and 100% more glaciers are potentially exposed to [warm Atlantic water] than suggested by previous mapping, which represents 55% of the ice sheet’s total drainage area.” In other words, more than half of Greenland’s ice lies in or flows through areas that could be influenced by warming seas.
Here are two figures produced from the research, using different color schemes to underscore the point:
“The typical result is that we find these fjords to be much deeper than represented in previous maps,” said Eric Rignot, a NASA and UCI scientist who has been working on mapping Greenland for a decade and is a co-author on the work. “They’re deeper because they’ve been carved by glacial cycles, multiple times.”
The study, which catalogues 243 separate Greenland glaciers, also underscores that the island has several vulnerable points where a submerged passageway penetrates into the center of the ice sheet, where the bedrock also lies below sea level. One is at Jakobshavn Isbrae (or Jakobshavn Glacier), the fastest flowing of all Greenland glaciers, racing outward at over eight miles per year, and lying over a deep channel that cuts into the center of Greenland from its western flank.
Another is at Petermann Glacier in northwest Greenland. Petermann is not flowing as rapidly as Jakobshavn, and isn’t currently as deep. But both glaciers have vast drainage areas, accounting for the potential for large sea level rise.
And those are just the goliaths of Greenland. There are hundreds of other glaciers and, the new research finds, 134 of them currently sit in the ocean waters, and thus are resting on a bed below sea level. Of those, 89 rest 200 or more meters below sea level, and 59 rest 300 meters or more below sea level, including some of Greenland’s fastest flowing glaciers (and Jakobshavn and Petermann). For dozens of these glaciers, the new study calculates a deeper depth than previously realized, which means they are all more potentially exposed than previously thought to warm seas.
All of this new data, Rignot said, will now be fed into sophisticated models that, he thinks, will probably lead to higher estimates of Greenland’s sea level rise potential in this century and beyond. “I think when they’re going to use these new bed models, the projections are going to change,” he said.
Meanwhile, on Wednesday, a separate team of scientists used another quite different large-scale mapping exercise to document a surprising — but closely related — change in Greenland’s above-water topography. Publishing in the journal Nature, they showed that the contours of the huge island are changing because with all the ice melt rushing from glaciers to the sea, river deltas are expanding outward — a rare occurrence these days when deltas around the world are generally retreating, threatened by rising seas (think of the Mississippi River delta, for instance, and its vanishing wetlands).
The study used aerial photos of Greenland taken in the 1940s by U.S. pilots and digitized their coordinates — so researchers were able to compare the extent of 121 Greenland deltas during two periods: from the 1940s to 1980, and the 1980s to the present.
In the latter period, the work determined, the deltas showed a pattern of marked expanse.
“Over the period of the 1980s to 2010s, rapid increase of meltwater and sediment fluxes caused dramatic advance of these deltas into the ocean,” said Irina Overeem, one of the study’s authors and a researcher at the Institute of Arctic and Alpine Research at the University of Colorado at Boulder.
The work was led by Mette Bendixen of the University of Copenhagen and had 11 other authors from Danish, U.S. and Greenlandic institutions.
This delta expansion is not just a curiosity, Overeem explained, it’s a confirmation that Greenland has been losing more ice in the past few decades, awakening large flows of water from land to sea that carry sediment with them along the way.
“It’s like a fire hose with slurry, and it’s been turned on way faster now, because there’s so much more meltwater,” she said.
Indeed, while direct satellite measurements of Greenland’s ice mass loss don’t go back very far in time, the delta research now suggests that it has been going on since the 1980s.
The researchers had expected that delta growth would be muted or counteracted by another factor — the decline of floating sea ice, which allows for more ocean wave action that can erode coasts. But in the relatively narrow and calm fjords of Greenland, that didn’t seem to matter as much, and in these protected spaces, deltas expanded rapidly.
Citing the new maps of Greenland’s seabed, Overeem sees an overlap: Regions with the deepest glaciers, and hence the most potential to feed ice into the ocean, often also support some of the largest deltas.
“Just from looking at where many of the biggest deltas are, if you look at the map of the bed, those are the valleys that go the furthest back under the ice sheet,” she said. “Those are the deepest gorges that go way under the ice. Often the biggest delta systems are associated with the places where the bedrock map shows that there’s large drainage basins.” Simply put, there’s just a lot of ice to melt in these areas, whether it enters the ocean directly in the form of icebergs or flows into it in the form of a river.
Together, the new studies underscore that Greenland’s very topography — and coastline — is changing before our eyes. And they suggest that it’s only the beginning.
Read more at Energy & Environment: