Study Reveals Surprising Trends in Polar Regions
A 30-year satellite record of sea ice in the two polar regions reveals that while the Northern Hemisphere Arctic ice has melted, Southern Hemisphere Antarctic ice has actually increased in more recent years. However, due to dramatic losses of Antarctic sea ice between 1973 and 1977, sea ice in both hemispheres has shrunk on average when examined over the 30-year time frame.
This figure compares the averages of Arctic sea ice for the month of September from 1973-1976 (left) to the averages for the month of September from 1999-2002 (right).
The study, which appeared in a recent issue of the Geophysical Research letters, presents the longest continuous record of sea ice for both hemispheres based primarily on satellites. The longer time frame highlights some new information about sea ice trends over time.
One interesting trend is that since 1979, the Arctic has been losing ice at a rate of 360,000 square kilometers (139,000 square miles) per decade.
"If you compare the rate of loss in the Arctic for the last two decades, it is 20 percent greater than the rate of loss over the last three decades," said Don Cavalieri, lead author of the study, and a senior researcher at NASA's Goddard Space Flight Center.
Cavalieri believes the poles are responding differently to variations in the atmosphere and climate. "What remains is to sift out and understand how these variations are driving the sea ice in each hemisphere."
In the Antarctic, there was a dramatic loss of sea ice cover from 1973 to 1977, and since then the ice has gradually spread in area.
"The increase has been slow enough that it does not totally wipe out the earlier decreases," said Claire Parkinson, senior researcher at NASA's Goddard Space Flight Center, and a co-author of the paper. Another co-author is Konstantin Y. Vinnikov, of the department of meteorology at the University of Maryland, College Park.
Overall, from 1972 to 2002, the Antarctic ice declined on average by 150,000 square kilometers per decade (almost 58,000 square miles).
"Trying to explain why these things happen becomes tricky," said Parkinson. "The temperature connection where warmer temperatures lead to greater melt is reasonably direct, but far from the complete story. Winds and waves move ice around, and consequently the ice can move to places where it is warm enough that it wouldn't have formed."
The study uses satellite data from NASA's Nimbus 5 Electrically Scanning Microwave Radiometer (ESMR), NASA's Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR), and the Defense Meteorological Satellite Program Special Sensor Microwave Imagers (SSMIs). For the year and a half in between 1977 and 1978, when a major gap occurred in the satellite sea ice data, the researchers used data and maps from the National Ice Center.
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NASA's Goddard Space Flight Center