Ecological Responses to Climate Change on the Antarctic Peninsula
The peninsula is an icy world that's warming faster than anywhere else on Earth, threatening a rich but delicate biological community
Climatic Regime Change
The Antarctic Peninsula is the long, curving arm that reaches north from the Antarctic mainland and extends its fingers toward the tip of South America. Forty million years ago, the peninsula was an isthmus connecting the two continents. Then tectonic activity carried Antarctica farther toward the South Pole, opening up the Drake Passage, which is now a thousand kilometers of open water between Cape Horn and the northern extremity of the peninsula.
The creation of the Drake Passage removed the last land barrier to ocean circulation at latitude 60 degrees south. The result was the formation of the Antarctic Circumpolar Current (ACC), which flows from west to east, or clockwise as seen from the South Pole. The ACC is the strongest and fastest of all ocean currents, transporting a volume of water equivalent to 30,000 times the flow at Niagara Falls.
The development of this circumferential current had a profound effect on the Antarctic climate and biota. The ACC isolated the continent from warmer waters and more temperate atmospheric conditions to the north, cooling both the air and the sea. It was probably at about the time the current became established that the permanent ice cap grew to cover almost the entire continent. (Antarctic ice now sequesters almost two-thirds of the planet's freshwater.)
Within the context of this frigid world, the Antarctic Peninsula is unusual in several respects. In part the difference is just a matter of latitude: From its base at 75 degrees south, the peninsula extends 1,500 kilometers north to beyond the Antarctic Circle, reaching 63 degrees at the tip.
Naturally, the climate moderates somewhat with distance from the pole, but there are other factors working as well. Around most of the continent, the ACC is separated from direct contact with the continental shelves and coastal waters by circulatory gyres and by embayments such as the Ross Sea. On the western shore of the peninsula, however, the ACC impinges directly on the continental shelf break, a narrow zone where the sea floor plunges steeply from a depth of about 700 meters to 3,000 meters. The ACC wells upward here and floods onto the continental shelf, which has a mean depth of approximately 400 meters. (The Antarctic shelf is about twice as deep as shelves elsewhere, being depressed like the rest of the continent by the weight of the ice it bears.)
The upwelling of the ACC brings both warmth and nutrients to the coastal waters of the western peninsula. Surface waters there range in temperature from a winter low of just above –2 degrees Celsius (the freezing point of seawater) to a summer high of about +1 degree. The temperature of the ACC water is +2 degrees, with little seasonal variation. Although +2 is far from balmy by the standards of summer beachgoers, the slight warming has a tremendous biological impact.
The peninsula also differs from the rest of Antarctica in its response to recent global climatic trends. Whereas the continent proper has not warmed appreciably in the past century, there has been a 3.4-degree increase in the mean annual temperature along the peninsula. And, as already noted, the average midwinter temperature has climbed 6 degrees since 1950. If the trend continues, the average midwinter temperature will rise above the freezing point of seawater by the middle of this century. After that, sea ice will not form in most years, leading to a regime change in the ecosystem. Already, in the past quarter-century, the mean extent of sea ice coverage along the western peninsula has declined by 40 percent, and the average annual duration of sea ice cover has shortened by 80 days.
The glaciers and ice shelves along the peninsula are also in retreat. Glacial ice is formed not from seawater but from accumulated precipitation on land, which then flows into the sea to form a shelf. Over the past 50 years, rapid warming has triggered the loss of eight peninsular ice shelves. For example, in 2002 a section of ice the size of Rhode Island broke away from the Larsen B ice shelf on the eastern side of the peninsula. On the western shore, a 400-square-kilometer chunk of the Wilkins ice shelf collapsed just this past March.