The Washington Post
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:
Antarctica contains
vastly more ice than Greenland. However, Greenland is subjected to the
rapidly warming temperatures of the Arctic. The result is that for now
at least — and as you can see above — it is losing ice mass considerably
faster than Antarctica is, to the tune of several hundred gigatons a
year.
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.
Smith and his team camped atop the ice sheet and sought
to measure the flow rate. The ice sheet surface contains thousands of
moulins, notes Vena Chu, a
University of California-Berkeley researcher who collaborated with Smith
and also appeared on NASA TV Friday. When water falls down the moulins,
“it takes water into the bottom of the ice sheet, and that’s where it
can really affect how fast the ice is flowing.”
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.
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