What's been happening to the Arctic ice sheet since 1979?

The answer to that question is conveyed pretty dramatically by this September 2012 satellite photo from NASA, which they captioned "Arctic Sea Ice Hits Smallest Extent In Satellite Era":

Some highlights from NASA's commentary:

Satellite data reveal how the new record low Arctic sea ice extent, from Sept. 16, 2012, compares to the average minimum extent over the past 30 years (in yellow). Sea ice extent maps are derived from data captured by the Scanning Multichannel Microwave Radiometer aboard NASA's Nimbus-7 satellite and the Special Sensor Microwave Imager on multiple satellites from the Defense Meteorological Satellite Program. Credit: NASA/Goddard Scientific Visualization Studio

The frozen cap of the Arctic Ocean appears to have reached its annual summertime minimum extent and broken a new record low on Sept. 16, the National Snow and Ice Data Center (NSIDC) has reported. Analysis of satellite data by NASA and the NASA-supported NSIDC at the University of Colorado in Boulder showed that the sea ice extent shrunk to 1.32 million square miles (3.41 million square kilometers).

The new record minimum measures almost 300,000 square miles less than the previous lowest extent in the satellite record, set in mid-September 2007, of 1.61 million square miles (4.17 million square kilometers). For comparison, the state of Texas measures around 268,600 square miles. [....]

Arctic sea ice cover naturally grows during the dark Arctic winters and retreats when the sun re-appears in the spring. But the sea ice minimum summertime extent, which is normally reached in September, has been decreasing over the last three decades as Arctic ocean and air temperatures have increased. This year's minimum extent is approximately half the size of the average extent from 1979 to 2000. This year's minimum extent also marks the first time Arctic sea ice has dipped below 4 million square kilometers. [....]

"Climate models have predicted a retreat of the Arctic sea ice; but the actual retreat has proven to be much more rapid than the predictions," said Claire Parkinson, a climate scientist at NASA Goddard Space Flight Center, Greenbelt, Md. "There continues to be considerable inter-annual variability in the sea ice cover, but the long-term retreat is quite apparent."

"The core of the ice cap is the perennial ice, which normally survived the summer because it was so thick", said Joey Comiso, senior scientist with NASA Goddard. "But because it's been thinning year after year, it has now become vulnerable to melt".

=>  As it happens, in 2013 the area covered by Arctic sea ice at the end of the summer rebounded  from the 2012 low point (though it was still one of the lowest measures on record), leading to the usual flurry of excited claims by climate-change denialists that this proved global warming isn't happening after all. That was predictable, but tiresome. To reiterate a point from last week's post on how to use and misuse climate-change data , one of the most common fallacies peddled (or sincerely believed) by climate-change deniers is confusing short-term fluctuations with long-trends. Over the past several decades, the unambiguous long-term trend in the extent of Arctic sea ice has been down, down, down.

And that long-term picture is even more alarming if one considers the total volume of Arctic sea ice as well as its surface area. Over time, as noted in the NASA commentary quoted above, the Arctic ice cap has been getting steadily thinner, and thus increasingly vulnerable to further melting. The pattern of long-term shrinkage in the volume of Arctic sea ice is captured in this graph, based on estimates from the Polar Science Center at the University of Washington.

=> To sum it up:

Playing out over a generation, the decline of the Arctic ice cap has been one of the most striking effects of global warming, a change in the planetary aspect so large it would have been visible to an observer on the moon. “We could be looking at summers with essentially no sea ice on the Arctic Ocean only a few decades from now,” said Mark Serreze, director of the National Snow and Ice Data Center in Boulder, Colo., in a statement.

—Jeff Weintraub