getting under the glacier. Many large glaciers, such as Jakobshavn Isbrae
and the other large glaciers
of Greenland, run along deep channels in the
earth that are actually below
sea level
near their outlets. (In eastern
Antarctica, submarine flow beds extend all the way to the center of the
ice sheet.) Eventually, the glacial ice flowing down the channel floats
free of the ground along a front called the grounding line. Scientists
have proposed that warmer sea waters are melting ice along the
grounding lines of Greenland's glaciers, pushing the grounding lines
upstream. This makes the glaciers'
channels more slippery and
accelerates the ice flow.
Surface water temperatures in the vicinity of Greenland, however, are
not high enough to account for the shifting of glacial grounding lines.
Scientists explain how warmer water can nevertheless get to Greenland's
glaciers in the following way. Efforts to account for Earth's heat
budget—how much comes in from the sun, and where it goes—have
shown that atmospheric warming cannot account for all the heat Earth is
absorbing because of its increased greenhouse gases. The extra heat has
been absorbed by the oceans, mostly in the top 3,280 ft (1,000 m).
However, in the North
Atlantic Ocean
, the heat is carried to greater
depths. Warm, salty water is
denser than cold, freshwater from glacial
melting, so at the outflow of the glacier the warmer, mid-depth ocean
water slips down to the grounding line.
Glacial acceleration has been observed not only in Greenland but in the
western peninsula of Antarctica, which is the only part of the world to
have been warmed as much by global climate change as
the Arctic
(at
about twice the rate of the rest of the planet).
So heavy are the ice flows of glaciers that when they slip suddenly,
moving forward several feet or more in a quick jerk, a small earthquake
is produced. The existence of these glacial earthquakes was only
detected in 2003. By 2006, careful analysis of recordings
of vibrations in
Earth's crusthad revealedthatfrom1993to the late 1990s, there was a
modest increase in the number of glacial earthquakes. This was followed
by a rapid increase from 2002 onward, with nearly as many glacial
earthquakes in 2005 as in all of 1993–1996. Seventy-two percent of the
earthquakes came from Greenland's three largest glaciers: Jakobshavns
Isbrae, Kangerdlugssuaq, and Helheim. Although it is not clear whether
the accelerated motion of these glaciers
is producing more glacial
earthquakes or simply more glacial earthquakes that are easy to detect,
scientists take this activity as another sign of changing conditions under
the Greenland icesheet.
The earthquake and ice-flow data do indicate that modest changes in
temperature in the area of the ice sheet, such as have already been seen,
may lead to large increases in the amount of glacial ice discharged to the
sea. Computer models of ice sheets used until 2006, including those
used by contributors to the Intergovern-mental Panel on Climate Change
(IPCC), the world's largest and most authoritative body of climate and
weather scientists, have not taken this newly discovered sensitivity into
account. As such, they may have underestimated how much future
warming may accelerate Greenland's melting and thus its contribution to
sea-level rise.
Finally, another form of melting has recently been observed in
Greenland on an unprecedented scale— superficial melting, the direct
melting of ice on the surface of the sheet as
opposed to the shifting of
ice directly into the sea by glaciers. A re-analysis of satellite and other
data in 2007 showed that over the last 25 years, superficial melting in
Greenland has accelerated twice as fast as was previously thought. From
1979 to 2005, the average year-round temperature rose 3.6°F (2°C), and
the area of Greenland that saw superficial melting at least one day per
year grew by 42%. The earlier studies had underestimated the melting
because they were based on microwave radar waves beamed at Earth
from satellites. However, clouds
weaken such reflections, and the
increased melting was releasing more water into the air, thus creating
more clouds. In effect, the melting was partly hiding behind a screen of
clouds. As with accelerated glacial movement, the increased superficial-
melt area is around the edges of the ice cap, not at the center. Surface
melt area set an all-time record in 2002, then broke that record in 2005.
The annual difference between snowfall (which adds mass) and
superficial melting (which removes it) accounted for about a third of
Greenland's current mass loss; the rest is from glaciers, which dump
icebergs into the sea. Like an ice cube dropped in a drink, an iceberg
raises sea level as
soon as it is put in, not when it finally melts.
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