Accelerated sea ice loss in the Arctic over the last 35 years cannot be compensated for by gains seen in Antarctica, Nasa scientists have said.
Before and after images showing the loss and gains in the Arctic sea and Antarctica since 1979 show that as a whole, the planet has lost 13,500sq/m of sea ice every year – an area larger than the state of Maryland.
Researchers at Nasa's Goddard Space Flight Centre said decreases in Arctic sea ice far exceed the increases seen in Antarctic sea ice. "Even though Antarctic sea ice reached a new record maximum this past September, global sea ice is still decreasing," said study author Claire Parkinson.
"One of the reasons people care about sea ice decreases is that sea ice is highly reflective whereas the liquid ocean is very absorptive. So when the area of sea ice coverage is reduced, there is a smaller sea ice area reflecting the sun's radiation back to space. This means more retention of the sun's radiation within the Earth system and further heating."
Scientists used microwave data on sea ice every month from November 1978 to December 2013. Their findings, published in the journal Climate, showed there was a downward trend in all months of the year.
The study also showed global ice decrease accelerated in the second half of the study period, with the rate more than doubling from 1996 onwards. However, Parkinson said this trend will not necessarily continue: "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."
Parkinson also found the global cycles of sea ice are far more similar to that of the Antarctic ice than the Arctic. Global peaks are seen in October or November, while lows take place in February.
Globally, the average minimum ice cover was 7.03sq/m, with the maximum reaching 10.27sq/m.
She said she does not believe Antarctic sea ice expansion will ever accelerate and overturn the downward global trend. "I think that the expectation is that, if anything, in the long-term the Antarctic sea ice growth is more likely to slow down or even reverse."
Explaining why it is important to realise sea ice gains do not negate losses, she said: "When I give public lectures or talk with random people interested in the topic, often somebody will say something in the order of 'well, the ice is decreasing in the Arctic but it's increasing in the Antarctic, so don't they cancel out?'. The answer is no, they don't cancel out."