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Arctic Amplification
Naomi Lubick

Sidebar: Heat brings fire

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.

Heat brings fire

Several unusual events have occurred with the warming in the Arctic, says Terry Chapin of the University of Alaska in Fairbanks. For example, warmer rivers allow a species of parasite to thrive on salmon, making the fishes’ flesh unacceptable for humans to eat or sell. But “the most obvious” change, Chapin says, is the subsequent increase in fire hazard for the region “right outside my doorstep here in Fairbanks.”

“Fire is no stranger to boreal forest,” where some trees need it to reproduce, says Chapin’s colleague, Glenn Juday. Black spruce, which have been encroaching farther north with the past century’s climate changes in the region, now make up more than half of forests there, and they need fires to open up their cones for reproduction.

In a strange evolutionary adaptation, as the spruce “start cranking away” to grow, Juday says, their photosynthetic processes grab carbon dioxide and release water. The forest soaks up radiant energy, thawing soils beneath the trees and the snow around them. The combination of water and radiant energy warms the air above the forest, creating convective cells in the atmosphere that set off lightning strikes — which then start life-giving fires, she says.

Juday says that normally sustainable cycles of insect booms, forest desiccation and fires have been thrown out of whack. The fire seasons of 2004 and 2005 were the first and third record years for acreage burned in the past half century. An “astounding” percentage of forest land went up in flames, Juday says: About a quarter of Alaska’s forest burned in the past two years, and across eastern Alaska and the Yukon Territory, forest fires covered an area comparable to Ireland.

“What’s really unusual” about the recent fire seasons, Juday says, “is that these were back-to-back years.” In the past, anomalously warm years “were infrequent and well-spaced. Now those are happening at extreme levels, closely spaced,” he says, shortening the recovery time for forests. “It’s changing the whole system.”

NL

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Lubick is a staff writer for Geotimes.

Links:
"Is ocean circulation slowing down?" Geotimes, February 2006
"Carbon leaching out of Siberian peat," Geotimes, July 2005

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