NASA has briefed the press on its “intensive research effort” into the rate and causes of sea level rise, releasing a suite of new graphics and visualizations showing how precisely the agency is measuring the upward creep of the oceans, currently at a rate of 3.21 millimeters per year.
It would be easy to lose yourself in all of the new material, but if there’s one slide above all that really matters, it’s this one:
NASA’s Gravity Recovery and Climate Experiment (GRACE) twin satellites have measured the loss of ice mass from Earth’s polar ice sheets since 2002. (Credit: Steve Nerem/CU-Boulder)
That’s an almost unfathomable amount — a gigaton is a billion metric tons — but spread around the world, it’s only equivalent to .74 millimeters of average sea-level rise per year (that’s the figure in the center of the graph). Thus, adding together Greenland and Antarctica’s contributions right now gives you a millimeter of annual sea level rise, roughly — and the remaining 2 millimeters comes from the expansion of ocean water as it warms, and from the melting of mountain and tidewater glaciers around the world (Alaska’s, for instance, are losing 75 gigatons a year).
The expansion of sea water will continue as the world warms further — but glaciers around the world will contribute less and less to sea level rise in the future, as they have less ice to lose. But the planet will still be able to look forward to the long melting of Greenland and Antarctica, which have 20 feet and 200 feet of potential sea level rise, respectively, contained in their ice sheets.
The critical question thus becomes: Is Greenland likely to lose even more ice than it’s currently losing per year — and could Antarctica do the same?
What’s pretty clear from NASA’s recent briefings and communications on this subject is that its scientists very much worry that they might.
NASA is flexing its muscles to study Greenland in particular, and that entails two major types of research: studying the melting that is occurring on top of the ice sheet, and studying the melting of its outlying, oceanfront glaciers, which often calve off gigaton-sized icebergs into the sea, with enough force to generate powerful earthquakes.
1. Water flow on the ice sheet’s surface. On top of the ice sheet, summer meltwater forms lakes and fast-running rivers, which sometimes plunge deep below the ice sheet when they hit sudden “moulins,” or crevices. Lakes also sometimes vanish suddenly, draining water down into the ice sheet below. Both of these mechanisms not only give the surface water access to the ocean, they also move the ice sheet itself, by lubricating its base. It’s a potential feedback and accelerator of Greenland’s melting, which is why the process is so important to further investigate.
This surface melt process is one way that the melting of the Greenland ice sheet could speed up. But it’s not the only one:
2. Ocean water melting glaciers. Along the outside of the ice sheet, multiple glaciers stretch finger-like towards the sea, often flowing out into deep fjords — submerged canyons scraped by glaciers of long-ago eras — with their bases anchored well below the level of the water. The rapidly retreating Jakobshavn glacier is one of these — it’s the fastest-moving glacier in Greenland, and single-handedly contributed about a millimeter to sea level rise from 2000 to 2011.
Currently, the glacier’s submerged bed is some 1,300 meters below sea level, and this great depth seems to be enabling its rapid retreat, because there is so much contact with the warmer ocean. “The potential for large losses from Greenland is likely to be determined by the depth and inland extent of the troughs through which its outlet glaciers drain,” noted a recent study of the Jakobshavn glacier.
“Observations suggest we should be very cautious to conclude too soon that conservative scenarios are reasonable. They may not be,” said Eric Rignot, a University of California Irvine and NASA glaciologist, at a NASA press call Wednesday in which he discussed changes to both Antarctica and Greenland. “And this is at the heart of what we at NASA, and other national and international agencies are working on right now.”
That’s what NASA’s aptly–named OMG (Oceans Melting Greenland) mission aims to study. Over the next five years, ships, aircraft overflights, and deployed sensors will attempt to map the depths and shapes of the ocean floor and undersea canyons all around Greenland’s glaciers, as well as the temperature and salinity characteristics of the water. The goal is to see just how much warm water is reaching them, which in turn will influence their capacity to melt.
The key thing to understand about this region is that in the oceans around Greenland, water has some strange characteristics. “What’s really interesting is that the water around Greenland is sort of upside down. You have warm water underneath a layer of cold water,” explained Josh Willis, NASA’s lead researcher on OMG, on the NASA briefing Friday.
“The warm water is at depth because it’s extra salty,” Willis continued. “The cold water comes from the Arctic and it’s very fresh.”
The question then becomes how much of the warm water manages to sneak up to to the glaciers. And that depends on a complicated system deep below the water surface where bedrock, glacier, and ocean meet. The nature of that system can be different at every glacier.
And there’s yet another complexity — water that originated on the ice sheet’s surface, but then escaped down to its base, can flow out at the location of the glaciers. Sometimes, this water “comes out right at the bottom of the ice, and it’s light and fresh, it surfaces,” Willis said on Friday. “That can pull warm water in towards the glacier.”
That’s where OMG comes in — trying to map and record all of this, and get a handle on the complexities of glacier position, seafloor features, and ocean water characteristics.
So in sum, as NASA deploys new resources to Greenland, we may soon know whether on the figure above, Greenland’s line will continue its current downward slope, or plunge more steeply.
No comments:
Post a Comment