Washington, Feb 11: Earth has been shedding sea ice at an average annual rate of 35,000 square kilometres since 1979, the equivalent of losing an area larger than the US state of Maryland every year, a new NASA study has found.
"Even though Antarctic sea ice reached a new record maximum this past September, global sea ice is still decreasing," said Claire Parkinson, author of the study and climate scientist at NASA's Goddard Space Flight Centre in Greenbelt.
"That's because the decreases in Arctic sea ice far exceed the increases in Antarctic sea ice," said Parkinson.
Parkinson used microwave data collected by NASA and Department of Defence satellites for her study published in the Journal of Climate.
She added Arctic and Antarctic sea ice extents month by month from November 1978 to December 2013 to determine the global ice extent for each month.
Her analysis shows that over the 35-year period, the trend in ice extents was downward in all months of the year, even those corresponding to the Arctic and Antarctic sea ice maximum extents.
Furthermore, the global ice decrease has accelerated: in the first half of the record (1979-96), the sea ice loss was about 21,500 square kilometres per year.
This rate more than doubled for the second half of the period (1996 to 2013), when there was an average loss of 50,500 square kilometres per year.
"This doesn't mean the sea ice loss will continue to accelerate," Parkinson added.
"After all, there are limits. For instance, once all the Arctic ice is gone in the summer, the Arctic summertime ice loss can't accelerate any further," said Parkinson.
Sea ice has diminished in almost all regions of the Arctic, whereas the sea ice increases in the Antarctic are less widespread geographically.
Although the sea ice cover expanded in most of the Southern Ocean between 1979 and 2013, it decreased substantially in the Bellingshausen and Amundsen seas.
These two seas are close to the Antarctic Peninsula, a region that has warmed significantly over the last decades.
In her study, Parkinson also shows that the annual cycle of global ice extents is more similar to the annual cycle of the Antarctic ice than the Arctic ice.
The global minimum ice extent occurs in February of each year, as does the Antarctic minimum extent, and the global maximum sea ice extent occurs in either October or November, one or two months after the Antarctic maximum.
This contrasts with the Arctic minimum occurring in September and the Arctic maximum occurring in March.