Most
widely recognized for its wildlife and its rough, untamed beauty, the Arctic
region is a sensitive indicator of global change. The northern latitudes amplify
shifts in temperature, ocean circulation, precipitation and evaporation that
occur elsewhere on the planet, making the region a kind of early warning system
for global climate change.
Alaska’s Muir Glacier has retreated
more than 120 kilometers (75 miles) in the past 200 years. Courtesy of Bruce
Molnia.
Over the past century, Arctic and sub-Arctic regions have warmed by 2 or 3 degrees
Celsius, two or three times the amount of warming that has occurred elsewhere
on the planet. That seemingly incremental shift upward in temperature has created
noticeable changes in the region.
Last year, Jonathan Overpeck, director of the Institute for the Study of Planet
Earth at the University of Arizona in Tucson, and his co-authors said that the
Arctic may have reached a point where it will tip into a “super interglacial”
state, a long warm period unlike anything experienced in the past 1 million
years. The drivers behind that shift are complex but not completely unclear,
Overpeck and co-authors concluded, in a paper published last August in Eos.
Surprisingly, human activities, such as the release of greenhouse gases, seem
to have less of a direct impact — except in how they affect physical processes
from far away, the scientists said.
Direct effects from ocean and air temperature changes, Overpeck and his co-workers
determined, mean that sea ice in the Arctic, which generally freezes the surface
of a significant portion of the North Pole’s open ocean no matter what
time of year, could disappear during the summer months in the next century,
resulting in completely open water in the Arctic for the first time in the past
800,000 years. Other researchers found similar timing in models for the degradation
of near-surface permafrost, the layer of long-frozen soil that underlies millions
of acres of Alaskan and Siberian tundra.
Even the massive ice sheet that covers Greenland seems to be feeling the effects
of warming, as the glaciers that ring the edge of the massive block of continental
ice flow faster. Greenland’s ice sheet is more than 2 million square kilometers,
and 85 percent of it is extremely thick: more than 3 kilometers deep in some
places. If the entire sheet were to melt, it would raise sea level by as much
as 6 meters (nearly 20 feet).
From the region’s glaciers and Greenland’s giant ice sheet to the
Arctic Ocean’s sea ice and the region’s permafrost, the icy landscape
is shifting dramatically, fulfilling many climate scientists’ predictions
that the Arctic would be the first to experience climate change, and that the
effects there would be most pronounced. The ultimate outcome of these changes
remains uncertain for plants, animals and ice, however, due to the complexities
of the physical and biological systems there. Whether or not climate changes
cause the Arctic to lose its ice completely, the warming temperatures have already
set off a complex web of events.
Moving glaciers
Perhaps
the most obvious change in the Arctic has been to the region’s glaciers.
Only recently have glaciologists determined how glaciers could start to move
and shrink very rapidly.
The fastest-moving glacier in the world
flows out of western Greenland. The glacier, called Jakobshavn Isbrae, moved
at its fastest velocity of 12.6 kilometers a year in 2003, and continues to
send its ice into the Arctic Ocean while receding inland. Image courtesy of
NASA/USGS.
In the early 1980s, scientists noticed that glaciers at the edge of Greenland,
as well as those in Alaska, had started to melt drastically, even though the
local climate had not dramatically warmed. By the 1990s, though, long-term changes
had thinned glacier ice to a threshold where a subtle change could set off massive
melting, according to Tad Pfeffer of the Institute of Arctic and Alpine Research
at the University of Colorado in Boulder.
“When it gets a little warmer, things don’t necessarily get only a
little worse,” Pfeffer said at a press conference at last December’s
annual meeting of the American Geophysical Union in San Francisco. He described
the retreat of the Columbia Glacier in Alaska, for example, that has uncovered
plants and soils for the first time since the mid-19th century. Disappearing
as quickly as 30 meters a day, Pfeffer said, Columbia’s retreat is not
“unique” or “unusual” in Alaska, where the termini of many
so-called tidewater glaciers have retreated kilometers up their valleys.
Melting Alaskan glaciers already contributed between one-tenth and one-fifth
of a millimeter to sea-level rise over the past half-century, according to Anthony
Arendt of University of Alaska in Fairbanks and his co-workers, and at increasing
rates in the 1990s. Using airborne laser altimetry to measure glacier ice thickness,
they extrapolated a loss of 35 cubic kilometers a year by 2002 — amounting
to almost 0.3 millimeters of sea-level rise, and nearly twice the annual loss
of ice from Greenland’s ice sheet.
Rapid melting and retreat is now also occurring on the “outlet” or
“exit” glaciers that rim Greenland. Fed by the huge continental ice
sheet, exit glaciers carry ice off the continent and into the ocean, where it
could melt and potentially raise sea level.
Cumulative research by glaciologists around the globe recently showed how such
land-to-sea glaciers speed up. Added weight on the glacier can drive the velocity
of an exit or tidewater glacier. Once it hits the ocean, the ice makes a transition
at what glaciologists call the grounding line, the boundary on the glacier between
its floating and grounded halves.
That line shifts with changes in ocean and air temperatures. A cold ocean allows
a glacier to push into the water without melting, bringing the grounding line
down toward the water with it as the glacier’s weight pushes into the land
and then into the water. But warmer ocean waters might melt the ice tongue,
thinning the glacier and buoying up the ice, moving the grounding line inland
as the glacier’s weight recedes. Warmer air temperatures could cause a
glacier to calve, decreasing its weight at the water end — which also would
make the glacier more buoyant and move the grounding line further inland.
These ice dynamics have dramatically thinned Jakobshavn Isbrae, a glacier in
West Greenland, making it the fastest-moving glacier in the world (its fastest
clip clocked well over 12 kilometers a year in 2003). In the past five years,
calving at its terminus effectively “took the brakes off” the glacier,
scientists from NASA and elsewhere have said, so that it both sped up and got
shorter while it thinned, despite all the ice behind it from the continental
sheet.
The weight and height of Greenland’s ice sheet itself affects the behavior
of the glaciers at its edges, even from its center, and the continental ice
sheet “is behaving curiously,” says Waleed Abdalati, head of NASA’s
Cryospheric Sciences Branch at the Goddard Space Flight Center in Greenbelt,
Md. Scientists have detected more snowfall inland on the ice sheet. “If
sea ice is retreating, that means there’s more water vapor,” Abdalati
says, and more vapor produces more precipitation, essentially depositing more
snow in the inland snow bank — which eventually could feed the rapidly
shrinking glaciers at the edge of the sheet.
The warming that produces more precipitation could also eventually change ocean
circulation patterns, producing a different cascade of perhaps surprising effects:
“It could eventually cool the Arctic,” Abdalati says. Disruptions,
for example, in the Atlantic’s thermohaline circulation — the so-called
ocean conveyor belt that brings warm water north from the tropics — could
occur due to a sudden influx of cold freshwater from melting glaciers or increased
discharge from large Arctic rivers. One such recent event (known as a Heinrich
event) took place 16,000 years ago, when thousands of melting icebergs from
North America changed ocean salinity and local air temperatures by as much as
10 degrees Celsius (see Geotimes,
February 2006).
Sea-ice reflections
Overpeck and colleagues’ study, which projected that the Arctic could be
completely ice-free during summer months by the end of the century, raises a
slew of questions about complex feedback systems in the region. For example,
Arctic researchers point out that losing ice means a change in the reflectivity
across the region “for several reasons,” says Terry Chapin, an ecologist
at the University of Alaska in Fairbanks, including the simplest: “When
it gets warmer anywhere, it changes snow cover, especially in the spring and
the fall, or it changes the amount of sea ice that is present.”
Snow and ice reflect the sun’s energy, but dark-colored areas absorb more
heat, and then heat the air above. “Any small amount of change” in
snow cover or ice from melting will dramatically change the amount of energy
reflected, or the albedo, from the Arctic, Chapin says. Shifting “from
a white surface to dark has tremendous amplification effects on the warming
that occurs.” Chapin estimates that the loss of all the Arctic Ocean’s
sea ice would increase the amount of energy absorbed by the water from 5 percent
to 70 to 90 percent.
Satellite measurements of ice covering the north polar ocean show smaller and
smaller coverage of the Arctic over the past 25 years. Analyses made every September,
the month of the year in the Arctic with the minimum annual ice cover, show
that the Arctic Ocean has lost more than 329,000 square kilometers of sea ice
per decade over the past 30 years, according to one NASA study. Last fall, researchers
at the National Snow and Ice Data Center at the University of Colorado in Boulder,
reported that September’s sea ice covering the Arctic measured 5 million
square kilometers — the smallest area since satellite measurements started
in 1978.
Changes to albedo from that loss promise to ripple through the rest of the planet’s
climate system, according to climate models. One preliminary study by David
Rind of NASA’s Goddard Institute for Space Studies in New York City and
co-workers shows that the complete disappearance of sea ice accounted for as
much as 37 percent of the global average temperature change in model runs. Another
by Jacob Sewall and Lisa Sloan of the University of California in Santa Cruz
shows that precipitation over the western United States could decrease dramatically,
possibly through teleconnections between shifting temperatures and effects on
weather in the northern latitudes and subsequent storm tracks over western North
America.
But scientists working in the Arctic say they do not know exactly what will
happen when sea ice disappears. So far, too few studies have been conducted,
says Marika Holland, a co-author of Overpeck’s at the National Center for
Atmospheric Research (NCAR) in Boulder, Colo. “It’s really difficult
to isolate the effects of the sea ice versus everything else that’s changing,”
she says.
Impermanent permafrost
As a former student and now as a hydrology professor at the University of Alaska
in Fairbanks, Larry Hinzman says that he has seen a variety of changes to the
regional landscape, some of which have been taking place over the past century.
But some changes seem to have happened overnight, Hinzman says — such as
the appearance of “thermokarst.”
The development of such sinkholes, which occurs as permafrost degrades, is one
of the most dramatic changes that Hinzman says he has seen since he first moved
to Alaska in the 1980s. Recent short winters, warmer summers and other aspects
of warming climate have thawed permafrost that then does not refreeze solidly
during the next cold season. The melt has left the Alaskan interior pockmarked
by thermokarst — sometimes creating small lakes and sinkholes underneath
houses, some large enough to swallow an 18-wheeler truck.
Defined as ground that has been frozen for at least two years, permafrost can
extend as deep as several kilometers in some regions, and its extent may be
patchy or continuous over large regions. The layer that is most at risk is the
top 3 to 5 meters, according to modeling by Dave Lawrence of NCAR and Andrew
Slater of the University of Colorado’s National Snow and Ice Data Center
in Boulder.
Using results from a global climate model to evaluate a variety of possible
future climate scenarios, Lawrence and Slater found that northern latitudes’
surface layer of permafrost could nearly disappear in the next 100 years, they
reported online Dec. 17 in Geophysical Research Letters. Other permafrost
researchers disagree, saying it will take much longer (from hundreds of years
to thousands), and the modelers themselves underscore the uncertainty of their
conclusions. Nevertheless, fallout from current permafrost loss or degradation
can be seen even now, says Glenn Juday, an ecologist at the University of Alaska
in Fairbanks.
“As thawing of permafrost continues, there are all kinds of implications,”
Juday says. In a landscape that is extremely cold, vegetation and other organic
matter doesn’t decompose, making permafrost “a giant storehouse of
carbon,” he says. “It’s the perfect carbon sequestration mechanism,
and we’re basically unwinding it, the product of thousands of years of
excess growth and decomposition,” and releasing greenhouse gases (carbon
dioxide and methane) to the atmosphere. The amount of carbon stored in the Arctic
remains under debate, with some researchers estimating that one-third of the
world’s soil-stored carbon is tucked away in western Siberia alone (see
Geotimes, July 2005).
Juday also cites previous work showing that permafrost thaw has led to a loss
of about 40 percent of some regions’ surface water. Water perched in lakes
and ponds that sat atop long-frozen soil percolated away once the permafrost
barrier melted.
Such thawing has led to dramatic changes in river systems and watersheds, Hinzman
says. He and his colleagues have found that softened permafrost allowed rivers
to change their courses: In some cases, streambeds shifted from straight parallel
lines, like the tines of a comb, to a more mature network, like the branches
of a tree. The resulting changes increase erosion, producing more silt and sediment,
carried downstream where it impacts fish, insects and plants, as well as river
and stream flows.
Shifts in permafrost may also allow darker plants to move northward into previously
lighter-colored regions, with the potential to increase the amount of heat absorbed
locally by Earth’s surface. Although these effects on albedo are relatively
small, warmer air temperatures could lead to more warming and melting.
Invisible shifts
Beyond their potential to affect the entire planet’s climate system, many
Arctic changes will remain particularly intense locally, as concluded in results
of the Arctic Climate Impact Assessment. The coalition of scientists from Arctic-ringing
nations released several reports last year detailing the effects of climate
change in the region. Biodiversity there might increase, as warmer weather allows
species that could not previously tolerate the cold to move in, while some native
species might disappear. Anecdotal evidence shows that indigenous people have
already lost some traditional hunting routes, as have polar bears, which rely
on sea ice as a platform for winter feeding.
Oil exploration and drilling seasons in Alaska have shortened with shrinking
winters, as thawed tundra cannot support heavy equipment in warmer temperatures.
Although an ice-free Arctic Ocean may mean expanded shipping passages, new fishing
grounds and access to previously ice-covered oil deposits, drawbacks include
political boundary disputes as international interest increases in getting access
there and the increased resources necessary to patrol Arctic waters, the Arctic
Climate Impact Assessment reports say.
“On the one hand, some in the Arctic region may be overjoyed by [an] Arctic
ice-free sea route,” says Mickey Glantz of NCAR. “On the other hand,
it’s devastating, because the loss of Arctic sea ice will alter the northern
hemisphere.” Meanwhile, all interested parties — scientists, policy-makers
and residents — will continue to watch the northern latitudes for changes,
and how those changes might translate to the rest of the planet.
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