Difference between revisions of "Antarctica"

Due to its location at the [[South Pole]], Antarctica receives relatively little solar radiation. This means that it is a very cold continent where water is mostly in the form of [[ice]]. Precipitation is low (most of Antarctica is a desert) and almost always in the form of snow, which accumulates and forms a giant ice sheet which covers the land. Parts of this ice sheet form moving glaciers known as ice streams, which flow towards the edges of the continent. Next to the continental shore are many ice shelves. These are floating extensions of outflowing glaciers from the continental ice mass. Offshore, temperatures are also low enough that ice is formed from seawater through most of the year. It is important to understand the various types of Antarctic ice to understand possible effects on sea levels and the implications of global cooling.

Sea ice extent expands annually in the Antarctic winter and most of this ice melts in the summer. This ice is formed from the ocean water and floats in the same water and thus does not contribute to rise in sea level. The extent of sea ice around Antarctica has remained roughly constant in recent decades, although the thickness changes are unclear.

Melting of floating ice shelves (ice that originated on the land) does not in itself contribute much to sea-level rise (since the ice displaces only its own mass of water). However it is the outflow of the ice from the land to form the ice shelf which causes a rise in global sea level. This effect is offset by snow falling back onto the continent. Recent decades have witnessed several dramatic collapses of large ice shelves around the coast of Antarctica, especially along the Antarctic Peninsula. Concerns have been raised that disruption of ice shelves may result in increased glacial outflow from the continental ice mass.

Some of Antarctica has been warming up; particularly strong warming has been noted on the Antarctic Peninsula. A study by Eric Steig published in 2009 noted for the first time that the continent-wide average surface temperature trend of Antarctica is slightly positive at >0.05 °C (0.09 °F) per decade from 1957 to 2006. This study also noted that West Antarctica has warmed by more than 0.1 °C (0.2 °F) per decade in the last 50 years, and this warming is strongest in winter and spring. This is partly offset by autumn cooling in East Antarctica. There is evidence from one study that Antarctica is warming as a result of human carbon dioxide emissions, but this remains ambiguous. The amount of surface warming in West Antarctica, while large, has not led to appreciable melting at the surface, and is not directly affecting the West Antarctic Ice Sheet's contribution to sea level. Instead the recent increases in glacier outflow are believed to be due to an inflow of warm water from the deep ocean, just off the continental shelf. The net contribution to sea level from the Antarctic Peninsula is more likely to be a direct result of the much greater atmospheric warming there.

In 2002 the Antarctic Peninsula's Larsen-B ice shelf collapsed. Between 28 February and 8 March 2008, about 570 km2 (220 sq mi) of ice from the Wilkins Ice Shelf on the southwest part of the peninsula collapsed, putting the remaining 15,000 km2 (5,800 sq mi) of the ice shelf at risk. The ice was being held back by a "thread" of ice about 6 km (4 mi) wide, prior to its collapse on 5 April 2009. According to NASA, the most widespread Antarctic surface melting of the past 30 years occurred in 2005, when an area of ice comparable in size to California briefly melted and refroze; this may have resulted from temperatures rising to as high as 5 °C (41 °F).

A study published in Nature Geoscience in 2013 (online in December 2012) identified central West Antarctica as one of the fastest-warming regions on Earth. The researchers present a complete temperature record from [[Westarctica]]'s [[Byrd Station]] and assert that it "reveals a linear increase in annual temperature between 1958 and 2010 by 2.4±1.2 °C".

Some scientific studies suggest that ozone depletion may have a dominant role in governing climatic change in Antarctica (and a wider area of the Southern Hemisphere). Ozone absorbs large amounts of ultraviolet radiation in the stratosphere. Ozone depletion over Antarctica can cause a cooling of around 6 °C in the local stratosphere. This cooling has the effect of intensifying the westerly winds which flow around the continent (the [[polar vortex]]) and thus prevents outflow of the cold air near the South Pole. As a result, the continental mass of the East Antarctic ice sheet is held at lower temperatures, and the peripheral areas of Antarctica, especially the [[Antarctic Peninsula]], are subject to higher temperatures, which promote accelerated melting. Models also suggest that the ozone depletion/enhanced polar vortex effect also accounts for the recent increase in sea ice just offshore of the continent.