Not All Ice Melts
Global warming means melting of polar ice and rising seas. Well not always it seems. Melting may not be the destroyer of all ice. Melting ice shelves may actually spur the growth of sea ice in Antarctica. While Arctic sea ice has dwindled, the extent of Antarctic sea ice has expanded by nearly 2 percent per decade since 1985. As the oceans have warmed in the same time period, deep ocean currents have carried heat to the deep waters surrounding Antarctica. The warmth may be melting the base of ice shelves which then crack and break off. On the surface it will look like the Antarctica ice is expanding.
Ocean warming may be a major driver of sea-ice expansion in the Antarctic, researchers reported in Nature Geoscience. While sea ice at the North Pole has shrunk substantially over the past three decades, scientists have struggled to explain why sea ice near the South Pole has grown in extent over the same period.
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. Recent decades have witnessed several dramatic collapses of large ice shelves around the coast of Antarctica, especially along the Antarctic Peninsula. 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.
"The paradox is that global warming leads to more cooling and more sea ice around Antarctica," says Richard Bintanja, a climate researcher at the Royal Netherlands Meteorological Institute in Utrecht. Bintanja and his colleagues show that enhanced selective melting of the Antarctic ice sheet — which is losing mass at a rate of 250 gigatonnes yearly — has probably been the main factor behind the small sea-ice expansion in the region.
Its authors analyzed satellite and buoy observations of ocean temperature and salinity for the period 1985—2010. They then compared observed changes in those data with the output of a global climate model that simulated how losing 250 gigatonnes of melt water from the Antarctic ice sheet each year would affect ocean conditions. In the model, the melt water formed a cool freshwater cap that facilitated the expansion of sea ice, leading the researchers to identify this as the most likely cause.
There are other plausible explanations for Antarctic sea-ice expansion, however. "The mechanism could be completely true, but this study does not demonstrate that increased melting has made a significant contribution to the increase in sea-ice cover," says Paul Holland, an ocean modeller at the British Antarctic Survey in Cambridge, UK.
Wind effects move the ice and direct warming or cooling air. Winds can have a significant effect on sea ice extent. Using satellite data for sea-ice motion in 1992—2010, Holland and his colleague Ron Kwok, a climate researcher at NASA's Jet Propulsion Laboratory in Pasadena, California, showed that in certain regions of Antarctica, such as the Weddell Sea, sea-ice changes are almost entirely due to the physical force of the winds. In other areas, such as the King Håkon Sea, they result from the combined effects of wind force and temperature.
Bintanja says that wind effects are important locally but that melt water influences sea-ice expansion regionally. Holland counters that ice melt is not uniform around the Antarctic coastline — as assumed by the authors of the latest study — but is concentrated in certain locations. Holland says that both wind patterns and melt water may be expanding the sea-ice near the South Pole.
For further information see Ice Melting.
Antarctica image via Wikipedia.