Global Warming, Antarctica
Above Antarctica’s Amundsen Sea, two enormous glaciers, Pine Island and Thwaites, are poised like corks in a bottle, disintegrating as relatively warm water laps at their bases and draining ice from the West Antarctic ice sheet. Both glaciers are sitting on a ridge of land that slopes below sea level, making them an unstable gateway to the entire West Antarctic ice sheet, which becomes more vulnerable as water temperatures rise. Jon Gertner wrote in the New York Times Sunday Magazine (2015) that glaciologist Eric Rignot foresees a time, in 30 to 40 years, when “people will be accustomed to watching Thwaites and Pine Island disintegrate constantly, iceberg by iceberg, into the ocean. And by then, he adds, their collapse ‘will be a part of everyday life.’ ”
A “disaster scenario,” as described by Richard Alley, a glaciologist at Pennsylvania State University, has the Pine Island glacier retreating enough to “make a hole in the side of the ice sheet.… The remaining ice would drain through that hole” ( Melting 1998 ). Once enough ice had drained through the hole, the West Antarctic ice sheet might eventually collapse, raising average sea levels around the world 15 to 20 feet in a few years. Such an increase in mean sea level would flood roughly 30 percent of Florida and Louisiana; 15 percent to 20 percent of the District of Columbia, Maryland, and Delaware; and 8 percent to 10 percent of the Carolinas Page 191 | Top of Articleand New Jersey. Inundation of coastal areas would have a similar impact around the world ( Schneider and Chen 1980 ). Among the flooded areas would be the centers of some of the world’s great urban and commercial centers—from New York City to Mumbai (Bombay), Calcutta, and Manila.
During 1998, Eric Rignot wrote in Science that West Antarctica’s Pine Island glacier was retreating at 1.2 kilometers a year (plus or minus 0.3 kilometers) and that its ice was thinning 3.5 meters per year (plus or minus 0.9 meter). “The fast recession of the Pine Island glacier, predicted to be a possible trigger for the disintegration of the West Antarctic ice sheet, is attributed to enhanced basal melting of the glacier’s floating tongue by warm ocean waters,” Rignot wrote ( Rignot 1998 , 549).
This glacier is widely believed to be “the ice sheet’s weak point” ( Kerr 1998 , 499). Although the accelerated melting of this glacier does not portend an immediate disintegration of the West Antarctic ice sheet, Alley wrote that “most models indicate [that the retreat] would speed up if it kept going” ( Kerr 1998 , 499). One observer was quoted in this context as stating that a quick collapse of the West Antarctic ice sheet would “back up every sewer in New York City” ( Kerr 1998 , 500). Rignot speculated that warmer ocean waters were causing the bottom of the Pine Island glacier to rapidly melt. “This is one of the most sensitive ice sheets to climatic change. For many, many years we have neglected the importance of bottom melting,” Rignot said ( Melting 1998 ).
“The sudden appearance of thousands of small icebergs suggests that the shelves have essentially broken up in place and then flushed out by storms or currents afterward,” said Scambos ( Britt 1999 ). The Larsen and Wilkins ice shelves have been melting since the 1950s, and scientists had expected them to fall apart. The disintegration, however, occurred more quickly than anticipated. “We have evidence that the shelves in this area have been in retreat for 50 years, but those losses amounted to only about 7,000 square kilometers,” said David Vaughan, a researcher with the Ice and Climate Division of the British Antarctic Survey. “To have retreats totaling 3,000 square kilometers in a single year is clearly an escalation. Within a few years, much of the Wilkins ice shelf will likely be gone” ( Britt 1999 ).
Hemorrhaging Ice at “An Alarming Rate”
By 2012, the Pine Island glacier (PIG) was “hemorrhaging” ice at “an alarming rate” as relatively warm water eroded its mass from 3,000 feet below. AutoSub3, a robot submarine, explored the bottom side of the ice shelf in 2009, mapping the sea floor with sonar. The research vessel Nathaniel B. Palmer gathered data from the sub that revealed how the underside of the glacier had lost 19 cubic miles of ice during 2009 alone. The melting glacier was flowing into the ocean more quickly. From 1974 to 2009, the PIG thinned by 230 feet and accelerated more than 70 percent ( Fox 2012 ).
Radar images taken from satellite observations of the Pine Island glacier during the 1990s indicate that the glacier has been shrinking rapidly. Its shrinking is important “because it could lead to a collapse of the West Antarctic ice sheet,” said Eric Rignot, who led the study. “We are seeing a … glacier melt in the heart of Antarctica” ( Melting 1998 ). “The continuing retreat of the Pine Island glacier could be a Page 192 | Top of Articlesymptom of the WAIS [West Antarctic Ice Sheet] disintegration,” said Craig Lingle, a glaciologist at the University of Alaska–Fairbanks, who is familiar with the study ( Melting 1998 ).
By 2008, two-thirds of ice-mass loss from the West Antarctic ice sheet was stemming from the Pine Island glacier and its environs, where release of ice had doubled during the decade ending in 2008 ( Gillet-Chaulet and Durand 2010 , 794). By 2009, the Pine Island glacier was losing ice four times as quickly as a decade previously, according to satellite imagery. The disclosure alarmed scientists and led them to cut their estimates for the demise of this glacier to just one century rather than 600 years.
During October 2009, scientists flying over Antarctica found a deep-water channel beneath the Pine Island glacier that is probably a path for relatively warm water to melt ice from below, A major reason why it was losing more than 19 cubic miles of ice per year. Using satellite images, scientists spotted a series of large surface undulations on the ice shelf. Next they matched the undulations with the timing of warm water pulses in the waters adjacent to the ice shelf. When surface winds are strong, they stir the Southern Ocean and lift the warm water onto the continental shelf where the additional heat contributes to melt ( “Unstable” 2010 ). A channel of relatively warm water running below the glacier also may be accelerating the melting. The channel conducts ocean water to the grounding line, melting the ice shelf from below.
In the meantime, late in 2009 an unmanned submarine found that an undersea ridge to which the Pine Island glacier may have been frozen in the past was now well below the underside of the ice, as relatively warm seawater continued to eat away at its base ( Kerr 2010 ).
A modeling study published in 2010 argued that the Pine Island glacier had passed its tipping point, which could bring on collapse of parts of the West Antarctic ice sheet. The study by Richard Katz and colleagues at the University of Oxford projected that the glacier could lose half its mass in less than a century. Katz and M. Grae Worster wrote in Britain’s Proceedings of the Royal Society (2010) , “Our results indicate that unstable retreat of the grounding line over retrograde beds is a robust feature of models that evolve based on force balance at the grounding line. We conclude, based on our simplified model, that unstable grounding-line recession may already be occurring at the Pine Island glacier” ( Katz and Worster 2010 ).
Warming Water Erodes Ice
Warming water in the Amundsen Sea continues to erode the glacier from below, “pushing the grounding line higher up the continental shelf” ( Barley 2010 ). This model may understate the speed at which glacier’s grounding line was retreating. “Ours is a simple model of an ice sheet that neglects some important physics,” said Richard Katz. “The take-home message is that we should be concerned about tipping points in West Antarctica and we should do a lot more work to investigate” ( Barley 2010 ).
NASA’s Earth Observatory reported on October 19, 2012, that five days earlier “scientists flying over Antarctica’s Pine Island glacier ice shelf … made a startling discovery: a massive rift running about 29 kilometers (18 miles) across a part of the glacier’s floating tongue. The rift was 80 meters (260 feet) wide on average, and 50 to 60 meters (170 to 200 feet) deep.” “When the crack reaches the other side of the ice shelf,” said the report, “it will send a huge new iceberg drifting into Pine Island Bay” ( “A Growing Rift” 2012 ).
Rifts in the Pine Island glacier form roughly every five years. “What makes this one remarkable is that it will lead to calving of a significantly larger iceberg than PIG has produced in the last few decades,” said Joseph MacGregor, a research scientist at the University of Texas–Austin. “It is likely that the front of PIG will be farther back than any time in the recent past after the iceberg calves,” he said ( “A Growing Rift” 2012 ).
Another massive iceberg calved in July 2013 adjacent to open water in Pine Island Bay and the Amundsen Sea ( “New Ice Island” 2013 ). Yet another one calved between November 9 and 11 the same year, developing from rifts that scientists had been watching for two years. Although the term iceberg may evoke an image of a chunk of ice, this one was really the size of a small island—35 kilometers by 20 kilometers (21 by 12 miles), or 700 square kilometers—roughly the size of Singapore ( “Major Iceberg” 2013 ). As a whole, the PIG had been moving into the sea at roughly 4 kilometers a year, so the new “iceberg” was not a surprise, but it was unusually large, providing more evidence that “warmer seawater below the shelf will cause the ice grounding line to retreat and the glacier to thin and speed up” ( “Major Iceberg” 2013 ).
Pine Island Glacier Melt Rates Vary
Although the Pine Island glacier has thinned and accelerated rapidly over the last several decades, melting can vary immensely from year to year. The PIG has been melting at an average rate of 100 meters (330 feet) a year (contributing by itself 7 percent of Earth’s recent sea level rise). Why is the Pine Island glacier melting so quickly? The NASA Earth Island Observatory explained that it
flows out of Antarctica’s Hudson Mountains and floats over the ocean. Scientists think that the glacier is shrinking because the ocean water that flows under the glacier is warming, increasing melt at the base.… Sea ice abuts the floating glacier tongue except in three places along the front of the glacier. These ice-free areas are called polynyas [and are] present in these three locations when sea ice is present. The polynyas most likely form where warm ocean currents rise toward the ocean surface. ( “Polynyas” 2011 )
Within the overall pattern, however, melting decreased 50 percent between January 2010 and January 2012, with “large fluctuations in the ocean heat available in the adjacent bay and enhanced sensitivity of ice-shelf melting to water temperatures at intermediate depth, as a seabed ridge blocks the deepest and warmest waters from reaching the thickest ice” ( Dutrieux et al. 2014 , 174).
Is West Antarctican Ice Melt Irreversible?
By 2014, several scientists regarded the dissolution of the West Antarctic ice sheet as highly likely, probably beginning in the 22nd century, triggering a world sea level rise of 16 to 18 feet. This collapse was forecast in 1978 by glaciologist John H. Mercer (who died in 1987), who was an outlier at the time. In the decades since, his ideas have gathered credibility. By 2014, scientists were placing a date on the ice sheet’s dissolution—200 to 900 years ( Rignot et al. 2014 ).
A year after the studies projected the inevitability of the collapse of the West Antarctic ice sheet, another study published in the Proceedings of the National Academy of Sciences late in 2015 mapped it out. The paper concluded, “If the Amundsen Sea Sector of the ice sheet is destabilized—something that is now well underway—then Page 195 | Top of Articlethe entire marine part of West Antarctica will be discharged into the ocean.” The projected timetable for this discharge is “centuries to millennia” ( Feldmann and Levermann 2015 ). They left refinement of the timing to future studies, warning that it could happen more quickly. Their models project that at current rates of ice melt, the ice sheet’s fate could be sealed within 60 years. By that time, melting may be self-sustaining, with relatively warm water eroding any ice reaching the ocean. They wrote:
The Antarctic ice sheet is losing mass at an accelerating rate, and playing a more important role in terms of global sea-level rise. The Amundsen Sea sector of West Antarctica has most likely been destabilized. Although previous numerical modeling studies examined the short-term future evolution of this region, here we take the next step and simulate the long-term evolution of the whole West Antarctic ice sheet. Our results show that if the Amundsen Sea sector is destabilized, then the entire marine ice sheet will discharge into the ocean, causing a global sea-level rise of about three meters. We thus might be witnessing the beginning of a period of self-sustained ice discharge from West Antarctica that requires long-term global adaptation of coastal protection. ( Feldmann and Levermann 2015 )
Resting atop a deep marine basin, the West Antarctic ice sheet has long been considered prone to instability. Using a numerical model, we investigated the sensitivity of Thwaites glacier to ocean melt and whether its unstable retreat is already under way. Our model reproduces observed losses when forced with ocean melt comparable to estimates. Simulated losses are moderate (<0.25 mm per year at sea level) over the 21st century but generally increase thereafter. Except possibly for the lowest-melt scenario, the simulations indicate that early-stage collapse has begun. Less certain is the time scale, with the onset of rapid (>1 mm per year of sea-level rise) collapse in the different simulations within the range of 200 to 900 years.
Like a Cork in a Bottle
Some scientists compare the “tongue” of a glacier (where the body of ice meets the sea) to a cork in a bottle. “The tongue of the glacier or the cork in the bottle [does] not represent that much,” said Claudio Teitelboim, director of the Center for Scientific Studies, a private Chilean institution that cooperates with NASA to survey the ice fields of Antarctica and Patagonia. “But once the cork is dislodged, the contents of the bottle flow out, and that can generate tremendous instability” ( Rohter 2005 ).
Glaciers flowing into Antarctica’s Amundsen Sea, which help drain the West Antarctic ice sheet, were thinning twice as fast near the coast by 2004 as they had during the 1990s. Warmer seawater erodes the bond between coastal ice and the Page 196 | Top of Articlebedrock below, “like weakening the cork in a bottle,” said Robert H. Thomas, a glacier expert for NASA in Wallops Island, Virginia. “You start to let stuff out” ( Revkin 2004 ). In 2004, Thomas and several coauthors wrote in Science, “Recent aircraft and satellite laser altimeter surveys of the Amundsen Sea sector of West Antarctica show that local glaciers are discharging about 250 cubic kilometers of ice per year to the ocean, about 60 percent more than is accumulated within their catchment basins” ( Thomas et al. 2004 , 255). Currently, such a discharge could raise world sea levels about 0.2 millimeters per year—not a startling amount. However, the long-term implications of such ice flow may be more ominous: “Most of these glaciers flow into floating ice shelves over bedrock up to hundreds of meters deeper than previous estimates, providing exit routes for ice from further inland if ice-sheet collapse is underway” ( Thomas et al. 2004 , 255).
West Antarctica’s ice sheets (areas of floating ice at the edge of the ice shelf) have been thinning at accelerating rates since at least the middle 1990s, increasing the chance that at least some of them will collapse and contribute to worldwide sea level rise, according to data analyzed by Fernando Paolo of the Scripps Institution of Oceanography in San Diego, California, and his colleagues. The ice sheets in some areas have declined by as much as 18 percent in slightly less than 20 years ( Paolo et al. 2015 ). “Eighteen percent over the course of 18 years is really a substantial change,” said Paolo. “Overall, we show not only that the total ice shelf volume is decreasing, but we see an acceleration in the last decade” ( Aguillera 2015 ).
The researchers explained the importance of this decline in ice thickness: “The floating ice shelves surrounding the Antarctic ice sheet restrain the grounded ice-sheet flow. Thinning of an ice shelf reduces this effect, leading to an increase in ice discharge to the ocean.… West Antarctic losses increased by 70 percent in the last decade, and earlier volume gain by East Antarctic ice shelves ceased” ( Paolo et al. 2015 ). The rapid acceleration in melting after 2003 followed a decade of relative stability. The unstable sectors of the West Antarctic ice sheet could lose half their volume in 200 years at current melting rates.
A major portion of the ice shelves on the southern Antarctic peninsula have destabilized since 2009. As B. Wouters and colleagues reported in Science, “Ice mass loss of the marine-terminating glaciers has rapidly accelerated from close to balance in the 2000s to a sustained rate of −56 ± 8 gigatons per year, constituting a major fraction of Antarctica’s contribution to rising sea level. The widespread, simultaneous nature of the acceleration, in the absence of a persistent atmospheric forcing, points to an oceanic driving mechanism” ( Wouters et al. 2015 , 899).
Late in May 2014, two groups of scientists “reported that Thwaites glacier, a keystone holding the massive West Antarctic ice sheet together, is starting to collapse. In the long run, they say, the entire ice sheet is doomed. It would release enough meltwater to raise sea levels by more than three meters” ( Sumner 2014 , 683). With his colleagues, Anders Leverman of the Potsdam Institute for Climate Page 197 | Top of ArticleImpact Research in Germany asserted “that even if emissions were to stop tomorrow, we have probably locked in several feet of sea level rise over the long term” ( Gillis 2013 ). “The surprises keep coming,” said Andrew J. Monaghan, a scientist at the National Center for Atmospheric Research in Boulder, Colorado, a participant in the study. “When you see this type of warming, I think it’s alarming” ( Gillis 2012 ).
Cautions and Debates
Eric Rignot of the University of California–Irvine, coauthor of one paper, suggested that one-third of West Antarctica could be gone within 100 to 200 years. Rignot noted that the scientific community “still balks at this”—particularly the 100-year projection—but that he thinks observational studies are showing that ice sheets can melt at a faster pace than model-based projections have considered ( Mooney and Warrick 2014 ).
In 2014, with lead author J. Mouginot and B. Scheuchi, Eric Rignot wrote:
We combine measurements of ice velocity from Landsat feature tracking and satellite radar interferometry, and ice thickness from existing compilations to document 41 years of mass flux from the Amundsen Sea Embayment … of West Antarctica. The total ice discharge has increased by 77 [percent] since 1973. Half of the increase occurred between 2003 and 2009. Grounding-line ice speeds of Pine Island glacier stabilized between 2009 and 2013, following a decade of rapid acceleration, but that acceleration reached far inland and occurred at a rate faster than predicted by advective processes. Flow speeds across Thwaites glacier increased rapidly after 2006, following a decade of near stability, leading to a 33[-percent] increase in flux between 2006 and 2013. Haynes, Smith, Pope, and Kohler glaciers all accelerated during the entire study period. The sustained increase in ice discharge is a possible indicator of the development of a marine ice sheet instability in this part of Antarctica. ( Mouginot et al. 2014 , 1576)
Benjamin Strauss of Climate Central, estimated that “12.8 million Americans live on land less than 10 feet above their local high-tide line” ( Mooney and Warrick 2014 ). Given population trends over the last few centuries, those figures may me much higher by the time the ice melts. Other bodies of ice—in Greenland, East Antarctica, and mountain glaciers—will also be melting at the same time.
Writing in the Washington Post, Chris Mooney and Jody Warrick (2014) described other experts’ cautions about these studies:
Other scientists urged caution in interpreting the findings, saying it is not clear whether the recent accelerated melting is an anomaly or a persistent phenomenon that will continue into the future. Ocean circulation patterns in the south polar region are still not fully understood, and it is possible that the migration of warmer water into the Amundsen Sea is unrelated to the overall Page 198 | Top of Articleclimate warming trend, said Olga Sergienko, a glaciologist Princeton University’s Cooperative Institute for Climate Science who was not involved in the studies. “This represents only about 20 years of observation, and on the time scale of ice sheets that’s just a blink,” said Sergienko, who also is with the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J. ( Mooney and Warrick 2014 )
Antarctica: Scientific Issues
Warmth can erode ice in many ways. The most familiar to us is warm air on a sunny day—direct sunshine augmenting the atmosphere’s warmth. Direct sunlight plays only a minor role in melting Antarctic ice, however. More often relatively warm water erodes an ice shelf from below. Pools of liquid water also can form on top of ice sheets and work their way into them. Melt water and rainfall also may drain into crevasses in ice, provoking vertical fractures. Heavy ice near the top of a sheet also may break apart, shearing off huge chunks. These last two are being studied as ways in which enough of the East Antarctic ice sheet could have eroded to become the main source for high sea levels during warm periods over the last 25 million years. In some periods, ice has melted relatively quickly, raising sea levels 20 meters or more.
David Pollard and colleagues have studied such periods, and found that:
In response to atmospheric and ocean temperatures typical of past warm periods, floating ice shelves may be drastically reduced or removed completely by increased oceanic melting, and by hydrofracturing due to surface melt draining into crevasses. Ice at deep grounding lines may be weakened by hydrofracturing and reduced buttressing, and may fail structurally if stresses exceed the ice yield strength, producing rapid retreat. Incorporating these mechanisms in our ice-sheet model accelerates the expected collapse of the West Antarctic ice sheet to decadal time scales, and also causes retreat into major East Antarctic subglacial basins, producing ~17 m global sea level rise within a few thousand years. The mechanisms are highly parameterized and should be tested by further process studies. But if accurate, they offer one explanation for past sea level high stands, and suggest that Antarctica may be more vulnerable to warm climates than in most previous studies. ( Pollard et al. 2015 , 112)
The speed at which Antarctic ice may melt depends not only on how much temperatures rise but also on the ways in which ice moves within the ice cap. Jonathan L. Bamber, David G. Vaughan, and Ian Joughin have been studying these “rivers” of subsurface Antarctic ice. “It has been suggested,” they write, “that as much as 90 percent of the discharge from the Antarctic ice sheet is drained through a small number of … ice streams and outlet glaciers fed by relatively stable and inactive catchment areas.” Their research suggests that “each major drainage basin is fed by complex systems of tributaries that penetrate up to Page 199 | Top of Article1,000 kilometers from the grounding line to the interior of the ice sheet” ( Bamber et al. 2000 , 1248). Such “complex flows” are noted throughout the Antarctic ice sheet by these researchers.
Bamber et al. assert that “this finding has important consequences for the modeled or estimated dynamic response time of past and present ice sheets to climate forcing” ( Bamber et al. 2000 , 1248). The researchers also find evidence of similar ice-sheet dynamics in Greenland, although they are smaller in scale. “This evidence,” they write, “challenges the view that the Antarctic plateau is a slow-moving and homogenous region” ( Bamber et al. 2000 , 1250). These researchers also contend that the dynamics of large ice flows are too complex for current models to predict, so climate modelers have little idea how global warming will affect the largest of Earth’s remaining ice masses.
Increasing Snowfall over Interior Antarctica
Global warming can work in contradictory ways. Witness the fact that sea level rise may be slowed by increasing snowfall over Antarctica as provoked by rising temperatures. The eastern half of Antarctica has been gaining weight, more than 45 billion tons a year, according to one scientific study. Data from satellites bouncing radar signals off the ground show that the surface of eastern Antarctica appears to be slowly growing higher—by 1.8 centimeters a year—as snow and ice pile up ( Chang 2005 ). As temperatures rise, so does the amount of moisture in the air, causing snowfall to increase in cold areas such as inland eastern Antarctica. “It’s been long predicted by climate models,” said Dr. Curt H. Davis, a professor of electrical and computer engineering at the University of Missouri. In the meantime, however, another study found that changes in snowfall had been insignificant since the 1950s ( Monaghan et al. 2006 ).
Satellite radar altimetry measurements suggest that the East Antarctic ice sheet interior north of 81.6° south increased in mass by 45 ± 7 billion tons per year from 1992 to 2003. Comparisons with contemporaneous meteorological model snowfall estimates suggest that the gain in mass was associated with increased precipitation. A gain of this magnitude is enough to slow sea level rise by 0.12 ± 0.02 millimeters per year ( Davis et al. 2005 ). The accumulation occurring across 2.75 million square miles of eastern Antarctica corresponds to a gain of 45 billion tons of water a year or the removal of the top 0.12 millimeter of the world’s oceans. According to Davis, Antarctica “is the only large terrestrial ice body that is likely gaining mass rather than losing it” ( Chang 2005 ).
The data, from two European Space Agency satellites, cover 1992 to 2003, but because the satellites do not pass directly over the South Pole, they did not provide any information for a 1,150-mile-wide circular area there. Assuming that snow was falling there at the same rate seen in the rest of Antarctica, the total gain in snowfall would correspond to a 0.18-millimeter-a-year drop in sea levels ( Chang 2005 ). R.A. Winkelmann and colleagues wrote in Nature (2012 , 239), “Snowfall and discharge are not independent, but … future ice discharge will increase by up to three times as a result of additional snowfall under global warming.”
Chemicals, Climate Change, and the Food Chain
As glaciers melt along the fringes of Antarctica, trace amounts of DDT are showing up in Adélie penguins. Although the amounts currently are too little to harm the birds, they suggest, according to one report, that “the presence of the chemical could be an indication that other frozen pollutants will be released because of climate change, says Heidi Geisz, a marine biologist at Virginia Institute of Marine Science in Gloucester” ( Callaway 2008 ). She says that other chemical pollutants such as polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) also could be released.
Scientists collected three decades of data indicating that climate change is contributing to declining populations of Antarctic fur seals in the southern Atlantic Ocean by reduced availability of prey, causing significant declines in in birth weight. “Our results provide compelling evidence that selection due to climate change is intensifying, with far-reaching consequences for demography as well as phenotypic and genetic variation,” wrote Jaume Forcada and Joseph Ivan Hoffman (2014 , 462).
“Antarctica’s fate is not as simple as that of an ice cube melting in the sun, scaled up a trillionfold,” Jane Qiu wrote in Science (2012) .
Changing wind patterns are an unsung force shaping Antarctica’s future. Retreating sea ice and stronger winds have caused seawater to mix more deeply, a process that churns sunlight-dependent phytoplankton into the ocean’s depths. As a result, phytoplankton biomass has declined by 12 percent over the past 30 years. Higher on the food chain, that means fewer krill and fish larvae. These creatures are also getting hammered by the loss of sea ice, which hides them from predators. The complex interplay between air, sea, and ice has emerged as a central theme underlying climate change in Antarctica. Shifting wind patterns and corresponding ocean changes can explain climate responses across the continent. ( Qiu 2012 , 879)
Climate, Stratospheric Ozone Levels, and the Circumpolar Vortex
Changes in stratospheric ozone chemistry also may have aided the growth of sea ice around Antarctica since the late 1980s, according to a report in Geophysical Research Letters in April 2009 by scientists from British Antarctic Survey (BAS) and NASA ( Turner at al. 2009 ). The research indicates that “the ozone hole delay[ed] the impact of greenhouse gas increases on the climate of the continent” ( “Increasing” 2009 ). This has produced an average increase of 100,000 square kilometers a decade since the 1970s. The increase in sea ice has been used by climate contrarians to refute effects of global warming—that is, as a counterpoint to rapid melting of sea ice in the Arctic.
Professor John Turner of BAS and lead author of the report said,
Our results show the complexity of climate change across the Earth. While there is increasing evidence that the loss of sea ice in the Arctic has occurred due to human activity, in the Antarctic human influence through the ozone Page 201 | Top of Articlehole has had the reverse effect and resulted in more ice. Although the ozone hole is in many ways holding back the effects of greenhouse gas increases on the Antarctic, this will not last, as we expect ozone levels to recover by the end of the 21st century. By then there is likely to be around one-third less Antarctic sea ice. ( “Increasing” 2009 )
Although sea ice has increased slightly around the coast of East Antarctica, it has decreased more rapidly in West Antarctica on the Antarctic peninsula, which has warmed by almost 3°C since the 1960s. Even farther west, sea ice cover over the Ross Sea has increased. Depletion of ozone “has strengthened surface winds around Antarctica and deepened the storms in the South Pacific area of the Southern Ocean that surrounds the continent. This resulted in greater flow of cold air over the Ross Sea (West Antarctica), leading to more ice production in this region,” according to this research ( “Increasing” 2009 ).
The work of David W. J. Thompson and Susan Solomon may be “the strongest evidence yet” that a shift in the Antarctic Oscillation “could explain a number of different components of [Antarctic] climate trends,” according to David Karoly, a meteorologist at Monash University in Clayton, Australia ( Kerr 2002 ). The researchers linked cooling in the stratosphere induced by depleted ozone levels with acceleration of winds. “During the summer–fall season,” Thompson and Solomon have written, “the trend toward stronger circumpolar flow has contributed substantially to the observed warming over the Antarctic peninsula and Patagonia and to the cooling over eastern Antarctica and the Antarctic plateau” ( Thompson and Solomon 2002 , 895).
Writing in the May 3, 2002 edition of Science, Thompson, a professor of atmospheric science at Colorado State University, and Solomon, a senior scientist at the National Oceanic and Atmospheric Administration in Boulder, Colorado, asserted that ozone depletion over the Antarctic may help explain both contradictory trends. “Ozone seems to be capable of tickling the Southern Hemisphere patterns,” Thompson said ( Chang 2002 ). Thompson and Solomon assert that a vortex of winds blowing around Antarctica traps cold air at the South Pole and has strengthened in the past few decades, keeping the cold air even more confined. The Antarctic peninsula lies outside the wind vortex and thus escapes the cooling effect. Ozone depletion may be a key causal factor in strengthening the wind pattern, according to Thompson and Solomon. “That’s where we speculate,” Dr. Thompson said, “and the emphasis is on the word ‘may’ ” ( Chang 2002 ).
Scientists already knew that ozone depletion has cooled the upper atmosphere. Thompson and Solomon’s research indicates that parts of Antarctica the troposphere, the lowest six miles of the atmosphere, also has cooled. “It’s a lot of food for thought in there,” said Dr. John E. Walsh, a professor of atmospheric science at the University of Illinois ( Chang 2002 ). Walsh said the data tying the cooling to stronger winds were convincing. “My one reservation,” he said, “is the link to the ozone” ( Chang 2002 ). He noted that the ozone hole was usually largest in November or December but that the greatest cooling had been some six months later. Thompson agreed that ozone depletion could not explain the whole climactic picture Page 202 | Top of Articleand said other influences such as ocean currents probably played important roles, too. “I seriously doubt it’s the only player,” he said. “I think it’s one of many” ( Chang 2002 ).
The idea that stratospheric ozone depletion has been a factor in driving a stronger circumpolar vortex (with cooling inside the vortex and warming outside) has been gaining support. In 2003, Nathan P. Gillett and David W. J. Thompson published results of a modeling study supporting this effect during the spring and summer. “The results,” they wrote in Science, “provide evidence that anthropogenic emissions of ozone-depleting gases have had a distinct impact on climate not only at stratospheric levels but at Earth’s surface as well” ( Gillett and Thompson 2003 , 273; Karoly 2003 ).
Mark P. Baldwin and colleagues wrote in Science, “The resulting ozone ‘hole’ leads to a relative reduction in solar heating and a stronger vortex. Observations and recent model simulations show that the strengthening of the polar vortex during spring leads to lower surface temperatures over Antarctica and higher temperatures in the mid-latitudes of the Southern Hemisphere that persist into summer” ( Baldwin et al. 2003 , 317).
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See also: Adaptation, Animal, Amphibians, and Warming Habitats ; Animal Life, Antarctic ; Ice Melt, Antarctica ; Ice Shelves, Antarctic; Inland Cooling, Antarctica ; Ocean Circulation ; Oceans’ Absorption of Heat ; Sea Level Rise ; Temperatures, Greenhouse Gas Levels and
Gale Document Number: GALE|CX7352100085