Frozen continent may not be immune to global warming
By Erin Wayman
Web edition: July 11, 2013
Print edition: July 27, 2013; Vol.184 #2 (p. 18)
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Various types of ice form off the coast of East Antarctica, as shown in this satellite image. In contrast to the rest of the continent, this region has seen no significant warming.
Credit: Jesse Allen, Robert Simmon/NASA Earth Observatory
Antarctica is a land of extremes: the driest, windiest, coldest place on Earth. The ice sheet that blankets the continent is, on average, 2 kilometers thick and covers nearly 14 million square kilometers. Antarctica is so remote and so isolated that as recently as 2007 scientists thought that it might be unaffected by global warming.
That year the Intergovernmental Panel on Climate Change concluded in its Fourth Assessment Report that Antarctica was the only continent where anthropogenic temperature change had not been detected. In contrast to the Arctic, the report said, ice in the far south wasn?t experiencing alarming, widespread melting. Some data even suggested that the continent was moderately cooling. ?As best we knew,? says David Bromwich, a climate scientist at Ohio State University, ?there was not much changing.?
Climate change skeptics latched onto the report to bolster their own conclusions. But in truth scientists didn?t really know much about Antarctica?s climate, past or future, at the time. Few long-term, on-the-ground temperature records exist, and those that do date back only to the 1940s and ?50s. And most of the long-running research stations that collected these data line the coastal perimeter; just a handful dot the expansive interior. It?s like trying to measure what?s happening on the coasts of the United States to determine what?s going on in North Dakota, Bromwich says.
Yet, over the last six years researchers have found clever ways to take Antarctica?s temperature and piece together its complicated climate history. Those efforts reveal that the continent is home to some of the most rapidly warming places on Earth. Whether natural or human-caused, Antarctica?s changing climate makes it clear that the continent isn?t as isolated as was once thought.
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Comparing the poles
The Antarctic ice sheet (bright white, left, surrounded by sea ice) is the largest single mass of ice on Earth. Should recent warming trends there continue, the glacial ice could begin sliding into the oceans and raise global sea level. Seasonal melting of sea ice (bright white, right) in the Arctic is more dramatic, but has little impact on sea level. Last summer, the Arctic?s sea ice reached the smallest extent ever recorded by satellites.
Credit: From left: NASA/Wikimedia Commons; GSFC Scientific Visualization Studio/NASA
If these warming trends continue, what happens in Antarctica will have important consequences for the rest of the world. The Antarctic ice sheet stores roughly 70 percent of the planet?s freshwater. If it melted entirely and drained into the ocean, global sea level would rise more than 60 meters ? enough to submerge New York City, London, Copenhagen, Bangkok, all of Florida, much of the Netherlands, Bangladesh and many other low-lying coastal and island locales.
Fortunately, researchers don?t expect the massive ice sheet to disappear anytime soon. But to plan for the future,officials need to know how much ice will melt and how quickly sea level will rise. When it comes to these sorts of estimates, Antarctica is the biggest unknown.
Geographic differences between Antarctica and the Arctic help explain why it has been easier to spot signs of climate change in the North. The Arctic is naturally warmer than the Antarctic ? Greenland, for example, sits at a lower latitude than Antarctica ? so it doesn?t require as much warming to thaw out. Thus, rising Arctic temperatures have already caused startling changes. Last summer, 97 percent of Greenland?s ice sheet experienced some degree of surface melting.
Another difference is that Antarctica is land surrounded by ocean while the Arctic is ocean surrounded by land. Much of the Arctic?s melting ice is sea ice, or frozen seawater. When sea ice melts, nothing happens to sea level because the ice is already in the ocean and its melting doesn?t change the ocean?s volume. Disappearing sea ice does lead to more warming, however. As the white ice thins and reveals the darker ocean below, the Arctic absorbs more solar radiation, leading to more warming and ultimately more melting.
Enlarge View larger image | Satellite images show the rapid collapse of the Larsen B Ice Shelf on the east coast of the Antarctic Peninsula in 2002. As air temperature warmed, ponds of meltwater formed on the ice shelf surface, flowing into cracks and splitting the shelf apart. Warmer oceans may also have contributed.
Credit: Courtesy of Ted Scambos/NSIDC and Univ. of Colorado, based on data from MODIS
The Arctic is also just simpler to study. A trip to the far north is effortless compared with the journey to Antarctica. And people live in the Arctic, so scientists have a much longer history of climate observations and measurements, including knowledge from native cultures that have inhabited the region for thousands of years.
?Antarctic science tends to lag at least a decade behind Arctic science,? says climate scientist David Schneider of the National Center for Atmospheric Research in Boulder, Colo. But Antarctic researchers are beginning to catch up.
Uneven warming
Almost twice the size of Australia, Antarctica is divided into three regions. East Antarctica accounts for two-thirds of the continent and touches the Indian and Atlantic oceans. With a higher elevation than the rest of Antarctica, the region is like a giant, flattened mountain, says Eric Steig, a geochemist at the University of Washington in Seattle. So far, scientists haven?t found much warming here. The east?s tall, steep coastline helps keep out warmer air coming from the north, he says. The westerly winds that flow around the continent also seem to prevent warmth from penetrating.
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The Transantarctic Mountains stretch 3,500 kilometers between the Ross Sea and the Weddell Sea and form the boundary between East Antarctica and West Antarctica. The summits (the tallest is 4,500 meters above sea level) are some of the few places on the continent not covered by ice. East Antarctica?s higher elevation helps keep warm air out and stabilizes climate.
Credit: From top: GSFC Scientific Visualization Studio/NASA; Rose Dominguez/UC Santa Cruz, NASA
Separated from the east by the Transantarctic Mountains, West Antarctica abuts the Pacific Ocean, and much of the region lies below sea level. The warming that?s been observed in Antarctica has occurred here and on the mountainous Antarctic Peninsula, the most northern part of the continent that sticks up from West Antarctica like a tail pointing toward South America.
The peninsula was the site of one of the earliest clues that warming might have reached Antarctica. Over 35 days in 2002, a chunk of floating ice covering an area larger than Rhode Island crumbled into countless icebergs that broke free from the coast. Tracked by satellite imagery, the breakup of the Larsen B Ice Shelf was Antarctica?s largest ice collapse in three decades. Scientists blamed surface melting for the demise of the ice shelf, an extension of an ice sheet or glacier that floats on the ocean. Pools of water on the surface probably filtered through cracks in the ice, triggering more melting. The water?s weight further fractured the shelf.
Scientists had corroborating data of warming elsewhere on the peninsula, where average temperatures in some areas have risen almost 3 degrees Celsius since the 1950s. Still, many researchers considered the region an outlier. Just looking at a map, Steig says: ?It?s pretty easy to connect East and West Antarctica as one thing and the peninsula as something else.?
But maps can deceive.
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Windy continent
View larger image | The westerly winds circle Antarctica, bringing warm air to the Antarctic Peninsula while shielding the higher eastern reaches. Starting in the 1940s and '50s, a handful of research stations (shown) began collecting weather data, providing the continent's few long-term climate records. Today, scientists keep better tabs on the far south with satellites and more than 160 automated weather stations.
Credit: Sources: COMNAP, P. Fretwell et al/The Cryosphere 2013; Map: Geoatlas/Graphi-Ogre, adapted by S. Egts
In 2009, Steig and colleagues reported in Nature that Antarctica as a whole has warmed about 0.12 degrees per decade since 1957, although warming across the continent hasn?t been uniform. The team found a statistical way to make up for the lack of long-term records. Since the 1980s, satellites have peered down on the South Pole to remotely take temperature measurements across all of Antarctica.
The team compared those data with ground-based measurements compiled at a few dozen weather stations from the same time period and figured out a mathematical relationship that links the satellite and ground data. Looking at ground data back to the 1950s, the researchers used the mathematical relationship to infer what satellite data would have looked like all across Antarctica had there been satellites focused on the continent back to that time.
The results were striking: West Antarctica has been warming along with the peninsula, about 0.17 degrees per decade since the late ?50s. Other studies relying on an assortment of methods have confirmed this, although the magnitude of the heat-up varies by study.
Most recently, Bromwich, Julien Nicolas, also of Ohio State, and colleagues calculated a temperature rise for West Antarctica that?s almost triple what Steig?s team found. They used a more direct approach to assess trends, analyzing temperatures collected by various methods at Byrd Station over the last half-century. Because Byrd Station sits on an exposed, flat area near the top of the ice sheet, its weather is representative of a large chunk of West Antarctica, Bromwich says. The team estimated that the region warmed an average of 0.47 degrees per decade from 1958 to 2010, for a total rise of 2.4 degrees. That puts both West Antarctica and the Antarctic Peninsula in the race for fastest-warming place on Earth, the researchers reported in February in Nature Geoscience. The global average is only 0.13 degrees of warming per decade over the same time period.?
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An ice core from James Ross Island records thousands of years of climate history off the Antarctic Peninsula's northeast coast. In recent years, average air temperature has risen quickly (top shows long-term temperature difference from recent average). At the same time, ice on the island is melting more than in any other period over the last 1,000 years (bottom).
Credit: N. Abram et al/Nature Geoscience 2013
The warming trend is most notable in West Antarctica during the spring, but temperatures are also on the rise in winter and summer. Summer warming is crucial, Bromwich says, because if the trend continues air temperatures could climb above freezing ? and West Antarctica could begin to melt. ?If we get a lot more melting in West Antarctica than we?ve got today, that may become a significant issue for the health of the ice sheet itself,? he says.
Frozen in time
With only 50 or 60 years of data, it?s difficult for scientists to gauge whether current temperature trends are the result of human activity or just natural climate patterns. ?If anything varies [naturally] on a timescale of decades, we have a problem because we have very short records,? Bromwich says.
To clarify the situation, polar researchers have found ways to travel back in time by looking for clues trapped in Antarctica?s ice. Ice sheets grow as layers of snow are compacted over time and turn to ice. The chemistry of the ice layers and air bubbles trapped in them archive information about climate at the time they formed. So scientists can drill long tubes of ice out of an ice sheet to assess changes in temperature over thousands of years.?
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Warming may threaten both Antarctica's coastal ice shelves and the glaciers behind them. When shelves disintegrate, glacial flow can accelerate, delivering more ice to the ocean. Colors represent speed of ice movement, where brown to green are slowest (1 meter per year) and blue and red are fastest (3,000 meters per year).
Credit: GSFC/NASA
In 2008, researchers drilled an ice core from James Ross Island, off the northeastern tip of the Antarctic Peninsula. The 364-meter-long tube records climate history back 50,000 years. Last September, researchers led by Robert Mulvaney of the British Antarctic Survey reported in Nature that temperatures were as warm as they are today during a stable period 9,200 to 2,500 years ago. The rate of temperature rise over the last century, however, is unusual. In the last 100 years, the average temperature on the island shot up nearly 1.6 degrees; over the last 50 years, the rate increased to 2.6 degrees per century. These are among the fastest known temperature increases for the peninsula, ranking in the top 0.3 percent of all century-scale warming events in the region over the last 2,000 years. If the warming continues at this pace, researchers warn, temperatures will surpass those of the previous warm period that ended 2,500 years ago.
In a related study, published in May in Nature Geoscience, Mulvaney?s team considered the impact of warming. It assessed summer melting trends by looking at the thickness of melt layers ? ice that melts and refreezes. ?The current high levels of melting on the Antarctic Peninsula are unprecedented over at least the last 1,000 years,? says study leader Nerilie Abram of the Australian National University in Canberra. In line with rising temperatures, melting has particularly sped up over the last half-century.
Scientists have also collected ice cores in West Antarctica. As on the peninsula, recent temperatures have been anomalously warm compared with the norm over the last 2,000 years. But, as observed with the James Ross Island core, the temperatures haven?t yet surpassed the upper limit of natural variability, Steig and colleagues reported in May in Nature Geoscience. For example, warm spikes in the 1830s and 1940s come close to recent temperature hikes.?
For some researchers, the fact that Antarctica?s temperatures haven?t yet tipped beyond the realm of natural variation means it?s still too soon to say with certainty that the changes there are driven by people. Others are more confident. Since places all over the globe are simultaneously being pushed near or past normal variability, it strengthens the case that Antarctica has also been influenced by anthropogenic climate change, Steig says. But that doesn?t mean human activity is the only, or even the strongest, force shaping the region?s climate. ?It?s the details of what that [human] impact will be and how large it will be relative to other things ? that?s really the important question,? he says. Disentangling the anthropogenic and natural forces responsible for Antarctica?s recent temperature changes is now the focus of intense scrutiny.
Devil in the climate details
During the last few years, researchers have identified several drivers of Antarctica?s warming. The most surprising influence: the tropics. Through statistical analyses and climate simulations, Steig and colleagues have linked West Antarctica?s rising temperatures to unusual warmth in the central tropical Pacific relative to nearby parts of the ocean. When the sea surface heats up, warm air rises, boosting activity in the atmosphere above. The atmospheric changes alter circulation in such a way that more heat is transported to the South Pacific near West Antarctica, Steig and colleagues reported in 2011 in Nature Geoscience.
That means the fate of West Antarctica depends on how the tropical Pacific responds to global warming. Unfortunately, the ocean?s climate future is just as murky as Antarctica?s. It?s possible, Steig notes, that as global temperatures continue to climb, the central tropical Pacific may or may not keep warming relative to other parts of the tropics. So far, climate simulations have predicted both scenarios. ?The uncertainty about future changes in the tropics translates to uncertainty in changes all over the globe,? Steig says. But, he adds, ?If I had to bet, I?d say it will keep warming in West Antarctica.?
If Steig is right, parts of the Antarctic Peninsula will continue to warm, too. The western side of the peninsula faces the South Pacific, and winter, fall and spring temperatures there also appear to be governed by changes in the tropical Pacific, Steig and his University of Washington colleague, Qinghua Ding, reported in May in the Journal of Climate.
Summer warming on the eastern side of the peninsula, however, ?seems to be a different beast,? Steig says. Stronger westerly winds seem to push warm air over the peninsula to its eastern side, says climate scientist Gareth Marshall of the British Antarctic Survey. As the air sinks, it heats the surface. This process is probably what caused the catastrophic collapse of the Larsen B Ice Shelf, he says.
Summer?s stronger westerly winds are one phenomenon that scientists have definitively tied to human activity. The seasonal depletion of the ozone layer above Antarctica, and to a lesser extent the influx of greenhouse gases, have lowered atmospheric pressure over the South Pole relative to the mid-latitudes. This has boosted the westerly winds and shifted them farther south toward the Pole. With global restrictions on ozone-destroying chlorofluorocarbons, scientists expect the ozone hole to recover during the 21st century. The recovery could weaken the winds and halt the peninsula?s warming ? unless rising greenhouse gases compensate for the ozone hole?s disappearance. ?Which of these two effects will win out adds another layer of uncertainty? to Antarctica?s future, Marshall says.
That uncertainty extends to East Antarctica, where the westerly winds have a very different effect. Here the westerlies act as a barrier to warm air. But in the last decade, Marshall and his colleagues have uncovered an unexpected relationship between East Antarctica?s temperatures and the winds encircling the continent. Before 2000, when atmospheric pressure above East Antarctica was low relative to the mid-latitudes and the westerlies were enhanced, temperatures remained cool, the researchers noted in February in the Journal of Climate. During most of the first decade of the 21st century, the team found the opposite effect: summer and fall warming in East Antarctica despite strong westerly winds. Marshall suspects some type of natural climate variability accounts for the reversal. Better predictions for the future will rely on figuring out the source of this climate variability and how it interacts with anthropogenic activity to modify the region?s temperatures. ?What it does really is make life more complicated,? he says.?
Time will tell
The only way to unravel Antarctica?s complicated climate and predict its future will be to gather more observations over time. But polar researchers aren?t waiting to consider the consequences of warming. If the trend continues in West Antarctica and the Antarctic Peninsula, the continent could see more ice shelf breakups like Larsen B. The ice shelves that ring the continent act like dams to keep Antarctica?s ice sheets in place. If those dams break, glaciers from the continent?s interior can surge into the ocean and raise sea level.
Melting on the tops of ice shelves is not the only concern. Scientists estimate that the oceans around Antarctica have warmed at a rate twice the global average over the last few decades. The warmer water can melt ice shelves from below. In June, researchers reported in Science that this is indeed happening: Slightly more than half of Antarctica?s ice shelf losses from 2003 to 2008 were the result of ocean warming. The basal melting isn?t limited to West Antarctica and the Antarctic Peninsula; it?s occurring in East Antarctica, too.
Along with sea level rise, scientists fear that climate change will interfere with the ocean?s ability to store carbon dioxide. The ocean surrounding Antarctica soaks up about 40 percent of the oceans? anthropogenic carbon and transports much of it to the deep sea, where the element can stay put for centuries. Earlier this year, researchers documented that the fiercer winds around the continent are, in some parts of the southern oceans, bringing deep water to the surface faster than they once did, raising concerns about the release of sequestered carbon. Another worry, Schneider notes, is that stronger winds could also disrupt the ocean currents in the Antarctic that circulate heat to other parts of the world.
Although the ramp-up in research hasn?t yet provided a detailed climate forecast for Antarctica, the work does reveal that the remote landmass is not as isolated as once thought. Its fate is linked with every corner on Earth.
Citations
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E.J. Steig et al. Recent climate and ice-sheet changes in West Antarctica compared with the past 2,000 years. Nature Geoscience. Vol. 6, May 2013, p. 372. doi:10.1038/ngeo1778. [Go to]
N.J. Abram et al. Acceleration of snow melt in an Antarctic Peninsula ice core during the twentieth century. Nature Geoscience. Vol. 6, May 2013, p. 404. doi:10.1038/ngeo1787. [Go to]
D.H. Bromwich et al. Central West Antarctica among the most rapidly warming regions on Earth. Nature Geoscience. Vol. 6, February 2013, p. 139. doi:10.1038/ngeo1671. [Go to]
G.J. Marshall, A. Orr and J. Turner. A predominant reversal in the relationship between the SAM and East Antarctic temperatures during the 21st century. Journal of Climate. Published online February 4, 2013. doi:10.1175/JCLI-D-12-00671.1. [Go to]
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D.P. Schneider, C. Deser and Y. Okumura. An assessment and interpretation of the observed warming of West Antarctica in the austral spring. Climate Dynamics. Vol. 38, January 2012, p. 323. doi: 10.1007/s00382-010-0985-x. [Go to]
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[14] G.J. Marshall et al. The impact of a changing Southern Hemisphere Annular Mode on Antarctic Peninsula summer temperatures. Journal of Climate. Vol. 19, October 2006, p. 5388. doi:10.1175/jcli3844.1. [Go to]
Source: http://www.sciencenews.org/view/feature/id/351507/title/Taking_Antarcticas_temperature
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