Antarctica, A Barometer Of Global Warming
James N. Barnes
Being the largest areas covered in snow and ice on Earth, the Antarctic and the Arctic are extremely important to the balance of the global climate, and both are remarkably vulnerable to global climate change. Scientific research carried out in the Antarctic provides crucial information about the impacts of human activities in fueling global climate change. Year by year as that scientific evidence becomes more compelling, the need for a paradigm shift away from our present carbon-based energy system becomes manifestly clear.
The Antarctic Peninsula has experienced a major warming over the last 50 years. Winter temperatures have increased at 15 times the rate of global warming, making the Antarctic Peninsula, together with the Arctic, the regions that are warming fastest on Earth. Over the past 61 years, 87% out of the 244 marine glacier fronts on the Antarctic Peninsula and associated islands have retreated, and the clear boundary between mean advance and retreat has migrated progressively southward. Sea ice has decreased in both concentration and duration around the Antarctic Peninsula and in the southern Bellingshausen Sea. The troposphere over the Antarctic continent (the lowest layer of the atmosphere, which extends to approximately 8 km above ground) also has warmed significantly over the past 30 years, accompanied by a concurrent cooling in the stratosphere (the layer of the atmosphere above that of the troposphere). This phenomenon is consistent with what would be expected as a result of increasing greenhouse gases.
The latest evidence shows that parts of the Antarctic are melting and eroding much more quickly than predicted, and in some remarkable ways. Temperatures along the Antarctic Peninsula have risen as fast as anywhere on earth over the past few decades. These rapid changes already are adversely impacting the marine ecosystem along the west coast of the northern Antarctic Peninsula. It is possible that changes here foreshadow changes in the future for other parts of the ocean south of the Polar Front, but the time frame for these wider changes is anything but clear.
It is fitting that the International Polar Year (2007-2008) has commenced a two-year research program in the Antarctic and Arctic. The work accomplished should further our understanding of the complex interactions of polar climate with the ecosystem.
Antarctica is composed of two geologically distinct regions, East Antarctica and West Antarctica, separated by the Transantarctic Mountains. Together the West and East Antarctic ice sheets constitute Earth’s largest reservoir of fresh water. The presence of these massive ice sheets and the polar location make Antarctica a powerful heat sink that strongly affects the climate of the whole Earth. The white surface of snow and ice reflect 90% of the sun’s radiation. The annual sea-ice cover around the continent, which in winter reaches an area greater than that of the continent itself, seasonally modulates exchanges of heat, moisture and gases between the atmosphere and ocean. As the sea ice forms each winter, the salt it rejects sinks to the sea floor to form cold, dense, oxygen-rich Antarctic Bottom Water that flows north under the world’s oceans, contributing significantly to the “ocean conveyor” and ventilating deep-sea life.
The Antarctic ice sheets contain sufficient ice to raise world-wide sea level by more than 60 meters if melted completely. The most immediate threat to the inhabited world comes from the melting of the West Antarctic ice sheet (WAIS), which is a marine-based ice sheet that rests on a bed far below sea level and has the potential for rapid shrinkage. Based on the latest evidence, the WAIS is thought by some in the scientific community to be capable of catastrophic disintegration due to global warming, which could raise sea levels by 5 meters in a few centuries. That would inundate large portions of the world’s low-lying countries and coastal cities. It appears to be losing mass now at a rate that contributes a significant portion of current global sea level rise. The East Antarctic ice sheet, on the other hand, which is independent and not seabed grounded, appears to be thickening slightly, but this is not enough to offset losses from West Antarctica. The fact that rates of discharge from some Antarctic ice streams draining WAIS have increased markedly since the 1990s suggests that the mass balance may also be rapidly changing.
Winter temperatures over the Antarctic continent, and principally in West Antarctica, as well as the Southern Ocean, have risen by more than 2°C in the past 30 years. Warming also in the middle troposphere (the layer of the atmosphere at 5 km above ground) over the Antarctic is the largest regional warming on Earth at this level. In particular, the northerly region of the Antarctic Peninsula has experienced rapid warming over the last 50 years and glaciers are shrinking rapidly. In one study on trends in 244 marine glacier fronts on the Antarctic Peninsula and associated islands over the past 61 years, 87% were shown to have retreated, and a clear boundary between mean advance and retreat has migrated progressively southward. The complete disintegration of Antarctic Peninsula glaciers would raise sea level by about 0.5 meter.
Ice cores from the Antarctic continent have provided one of the key pieces of information in understanding humans’ role in global climate change. Ice cores from Vostok and Dome C reveal that today’s concentration of greenhouse gases in the atmosphere has not been exceeded during the past 650,000 years and likely not during the past 20 million years. They show that the rate of increase of greenhouse gases in the atmosphere over the past century is unprecedented, at least during the past 20,000 years. Using these data, researchers have found a close correlation between global temperature and greenhouse gas concentrations in the atmosphere over the past 420,000 years, supporting the link between human use of fossil fuels and global climate change. This is already evident from isotopic measurements showing a significant fossil fuel imprint on carbon in the earth’s atmosphere.
British Antarctic Survey (BAS) 2005 studies of the melting of Antarctic Peninsula glaciers indicate that they are melting much faster than previously predicted. Some researchers have concluded that the melting of these glaciers is already likely contributing a non-negligible part to sea level rise. In the Amundsen Sea sector of West Antarctica, accelerating glacier melt is now discharging enough excess ice to raise sea level more than 0.2 mm per year. This is the sector widely believed to be the most vulnerable portion of the WAIS, with the potential for a further rapid acceleration of ice discharge.
The stability of Antarctic ice shelves in a warming climate has long been discussed, and the recent collapse of a significant part of the Larsen Ice Shelf off the northern Antarctic Peninsula has led to a refocus on the implications of ice shelf decay for the stability of Antarctica’s grounded ice. Records from six marine sediment cores in the vicinity of the Larsen Ice Shelf demonstrate that the recent collapse of the Larsen B Ice Shelf is unprecedented during the Holocene – since the end of the last ice age more than 12,000 years ago. This research implies that the Larsen B Ice Shelf has been thinning throughout the Holocene, and that the recent prolonged period of warming in the Antarctic Peninsula region, in combination with the long-term thinning, has led to its collapse.
State-of-the-art climate models project further decline in Antarctic sea ice through the 21st century as a result of global warming. Krill and other organisms depend upon a reliable seasonal pack ice cycle of advance and retreat to reproduce. Long and extended pack ice coverage during winter months provides refuge for krill against predation and a favorable habitat for the ice-algae upon which krill forage. Ice-algae are a critical food source for juvenile krill during the winter and early spring. A study conducted in 2004 found that, coincident with a decline in sea ice, krill populations in certain areas of the South West Atlantic have declined significantly since the 1970s (up to 80% in one region). It was also found that sufficient winter ice in the Antarctic Peninsula and Southern Scotia Arc, which are major krill spawning and nursery areas, affects krill density across the whole ocean basin, including areas north of the Seasonal Ice Zone. The loss of sea ice is likely behind the decline as well as declines in pack-ice dwelling fish species, such as the silverfish Pleuragramma antarcticum, another critical species in neritic areas of the Southern Ocean.
One species to feel the effects directly is the Adélie penguin, Pygoscelis adeliae. Quoting a recent article featuring the work of David Ainley and William Fraser:
Along the tip of the Antarctic Peninsula, which reaches farther north than anywhere else on the continent, average annual temperatures have risen 4.5 degrees in just the last 30 years. By comparison, the Earth’s annual temperature has increased by 1.4 degrees in the last century. In this vulnerable area, entire colonies of Adélie penguins have disappeared because, researchers believe, the ice no longer extends far enough into the sea to allow the birds to reach their winter feeding grounds. Biologist William Fraser monitors a 50-square-mile area where 56,000 Adélies have perished. “For our region I work in, the Adélies will be locally extinct within a decade,” Fraser said. “One of the colonies we worked on for 30 years went extinct last year, from 1,000 breeding pairs to zero.” “Climate change is very serious stuff, and that’s the message Adélie penguins have been telling us,” Ainley said. “Humans have to learn lessons from what these penguins are going through.”
* 이 글은 그린리포트 2007 11-12월호에 실렸습니다.