SINGAPORE — When sea ice covering the Arctic Ocean fell to a record-low level last month, much of the analysis in Asia and the Pacific focused on the opening of new and shorter commercial shipping routes to Europe, and increased access to Arctic offshore oil and gas resources.
These could be very positive developments for the region, especially for the energy-short trading economies of Northeast Asia led by Japan, China and South Korea.
But the dramatic decline in both the extent of Arctic sea ice and its thickness since the start of reliable satellite measurements in 1979 may also signal an alarming acceleration in global warming and climate change caused mainly by burning fossil fuels and clearing forests.
Sea ice is an extensive layer of frozen ocean water that cools the polar zones — the Arctic in the Northern Hemisphere and Antarctica in the Southern Hemisphere. It also helps to moderate the global climate.
Sea ice has a bright surface. As a result, about 80 percent of the sunlight that strikes it is reflected back into space. However, as sea ice melts in the summer, it exposes the dark ocean surface that then absorbs approximately 90 percent of the sunlight. The ocean warms and Arctic temperatures rise further.
Yet there is a seeming paradox between what is happening to sea ice in the Arctic, where it is shrinking fast, and Antarctica, where it has been expanding steadily in recent years.
Climate-change skeptics claim that the growth in Antarctica offsets the retreat of sea ice in the Arctic, demonstrating that nature has a self-righting mechanism. Skeptics question dire warnings from many climate scientists of a warmer world by the end of the century that will set the stage for a long period of catastrophic extreme weather and rising sea levels as the great land-based ice sheets start to melt, first Greenland in the Arctic, and then on a much larger scale in Antarctica.
Research suggests that the two polar zones are reacting differently to measured warming of the atmosphere and seas in both places because of geography. Antarctica is a vast continent encircled by water, whereas the Arctic is an ocean surrounded by exposed land, which reacts more quickly to warming atmospheric temperatures than Antarctica's seas or its thick ice sheets.
Wind and ocean currents around Antarctica isolate it from global weather patterns, keeping it very cold. By contrast, the Arctic Ocean is closely linked with the land climate systems around it, making it more sensitive to change.
Arctic sea-ice loss is cited as one of the most striking and earliest examples of climate change and what it will mean for the future of life on Earth. That it is happening in the Northern Hemisphere, where most of the world's people live, accentuates the potential severity.
The latest polar measurements unveiled by the U.S. National Snow and Ice Data Center (NSIDC) earlier this month showed that Arctic summer sea ice extent reached its lowest point this year on Sept. 16, when it covered 3.41 million sq. km, nearly 3.3 million sq. km below the 1979 to 2000 average.
Ten days later, Antarctica's winter sea ice reached its maximum extent of 19.44 million sq. km, slightly higher than the previous record in 2006. Arctic summer sea ice decline has been far faster than the growth of Antarctic winter sea ice. But the relevant comparison is really summer minimums and winter maximums in each hemisphere.
From 1979-1983 in the Arctic, the summer minimum of sea ice covered an average of just over 51 percent of the ocean. It fell to just 24 percent of the ocean surface this year. In Antarctica, the comparable figures were 13.8 percent and 14.6 percent of the ocean, meaning that summer sea ice shrinkage in the northern hemisphere is greatly outstripping growth in the southern hemisphere.
The same sharp contrast is evident in the winter sea ice maximum of both polar zones between 1979 and 2012. The decline in Arctic winter ice extent is eight times as fast as the increase in Antarctic winter ice.
That still leaves the question why Antarctic sea ice is increasing when local and global temperatures are warming?
Scientists will not know the answers until more research is done. But they think it is partly the result of wind and ocean movements. NSIDC director Mark Serreze also attributes a role to the hole in the ozone layer above Antarctica. He explains that because of this, the stratosphere above Antarctica is very cold.
"Ozone in the stratosphere absorbs UV (ultraviolet) light, and less absorption (by) ozone makes the stratosphere really cold," Serreze says. This cold air descends to the surface, keeping the sea ice extensive.
In the rapidly warming Arctic, the Greenland ice sheet has been melting, leading to substantial net ice loss in recent years. The worry is that if global warming continues on the track predicted by many climate scientists, Antarctica's sea ice and its vast land-based ice sheet will eventually follow.
After all, research has shown that about 52 million years ago, when the concentration of carbon dioxide — the main greenhouse gas from human activity today — was more than twice its current level, Antarctica had palm trees and other tropical vegetation. Summer temperatures on the coast ranged between 20 and 27 degrees Celsius.