Earth's crisp white skullcap of Arctic sea ice is emerging from winter much the worse for wear.
Scientists monitoring the ice's annual growth and contraction say the frigid sheath ended winter with the fifth-smallest geographic reach since 1979, when satellites first began tracking sea-ice trends. All six below-average winters have occurred between 2004 and 2009.
This year, winter ice also enters a new melt season with record-low levels of thick older ice, the kind that has has survived several summers. This is the ice that persists the longest to help cool the planet during summer; it reflects sunlight back into space during the Arctic's long hours of daylight – think 186 "days" of sunlight at the North Pole.
And it's the ice that provides the foundation for further thickening when sea ice expands again the following winter.
From 1981 to 2000, multiyear ice made up an average of 30 percent of the Arctic's ice cover at winter's end. Coming out of this winter, only 9.8 percent of the ice was of the multiyear persuasion.
You can read more about it here.
The culprit, say researchers with the National Snow and Ice Data Center and NASA's Jet Propulsion Laboratory, is global warming. (No Virginia, it actually hasn't ended yet; you can get started on some explanations here and here).
In addition, some naturally shifting climate patterns superimposed atop the long-term warming trend have contributed to the thinner, less extensive sea ice.
During the late 1980s through the mid 1990s, these natural variations got stuck in a phase that generated winds to drive large amounts of ice through the Fram Strait and out into the Greenland Sea, explains Walter Meier, a researcher with the National Snow and Ice Data Center in Boulder, Colo. In addition, warmer water from the Atlantic and Pacific has been moving north into the Arctic Basin.
"Since that period, we haven't seen a persistently strong Arctic Oscillation," he said during a briefing this morning. He was referring to the natural climate pattern involved. More recently, this pattern has entered a neutral or slightly weak phase. But, he adds, "we're still losing the summer ice and losing the thicker ice cover."
The persistently strong pattern toward the end of the last decade "may have played a role in triggering the loss of ice cover," he continued. But "it certainly is not the overriding factor in terms of long-term loss."
Going into this past winter, researchers say, things appeared to be looking up at the top of the world. The winter began with a larger inventory of two-year ice than the year before. But during the winter, much of that "tweenage" ice blew out through the Fram Strait toward lower latitudes and melted. This boosted the proportion of single-year ice in the Arctic.
The multitrillion dollar question: What happens next?
In a research paper published in the current issue of Geophysical Research Letters, they estimate that by 2037, the Arctic Ocean will be virtually ice-free in September. And it's not out of the question to expect a nearly ice-free September by 2028.
They base their work on modeling exercises in which they picked a handful of models (six) out of the entire batch the Intergovernmental Panel on Climate Change used for its 2007 reports on the state of climate science. These six did the best job of reconstructing the Arctic's past climate, particularly the coming and going of sea ice with the seasons.
Then they combined two emissions scenarios – business as usual and another close relative – from the IPCC reports. (Worldwide emissions of CO2 are exceeding business as usual by a long shot.)
They used 2007's summer sea ice levels as a starting point. Then they ran each model several times, and with slightly different starting conditions to mimic the climate's natural variations. Then they averaged the results to filter out natural variations and spot the projected trend.
Others have suggested the summer ice could vanish by 2013 or 2014.
And Dr. Meier's view?
For his part, Dr. Meier is reluctant to estimate the time when the Arctic Ocean will be ice-free in the summer.
"I certainly wouldn't want to put money on a [given] year," he says.
Ice-free summers by 2013 or 2014 "seems fairly unlikely," he continues. "But it's not totally outside the realm of possiblity."
That's a change from five years ago, he adds. "Such a suggestion would have been laughed out of the room. Now I think you have to say: 'Well, that's really unlikely to happen; but if things happen just the right way, it could.' "
More summers like 2007's could accelerate the loss; summers like last year's could slow the process a bit.