As the long summer’s day turns to night across the high Arctic, the diminished polar ice cap is beginning to re-freeze. It has a head start this year, because ice now covers more of the Arctic Ocean than last year at this time.
There was more ice last year than the year before, too.
This will be the first time since 1990-1992 that Arctic sea ice coverage, at its minimum size for the year, will have increased in back-to-back years. You might be surprised to hear this, thanks in part to news stories that reported less ice than usual at the end of last winter, along with a dramatic decline in thicker, multi-year ice. The New York Times reported just last Friday that two German freighters are completing the first-ever commercial Arctic transit, delivering construction supplies from South Korea to Rotterdam via the Siberian coast.
The Arctic is the most damning evidence of climate change, and the shrinkage of summer sea ice is Exhibit A. If you compare satellite-generated views from early September 1980 with the same date this year, you get the picture. Three decades ago, sea ice lurked just offshore of Alaska and eastern Russia, thickly clogged the narrow channels of the Canadian archipelago and central Siberia, and extended halfway down the east coast of Greenland.
This year an Inupiat hunter or a polar bear would have had a long paddle to reach any ice from the Alaskan mainland. The Russian coastline is nearly ice-free, to the benefit of those German freighters, and an intrepid navigator might just about be able to find a path through the Canadian islands.
But if we compare this year’s image to one from two years ago, the perspective is a bit different. In 2007, which was the year of the greatest ice retreat since satellite records have been kept, the North Pole itself was briefly exposed to open water. The famed Northwest Passage through Canada’s waters is wide open. This was the summer that Arctic navigation became big international news.
Figures from the National Snow and Ice Data Center at the University of Colorado put the average ice extent in August 2009 at 2.42 million square miles. That is about 17 percent more than during the record-low season of 2007, and about 4 percent more than last year.
Ice, it turns out, is a complicated subject. Knowing the area that is ice-covered does not tell us how much ice actually is present. To know that, we must have a good idea of the ice thickness, the amount of open water between the floes, and the extent and depth of any meltwater puddles that might sit on top of the ice. This information is not reliably available for the entire ice cap, although a decision by U.S. intelligence agencies this summer to release declassified spy-satellite photos will help.
Then there is the question of why the ice cover is changing, and especially what has caused the sharp decline of summertime ice in the current decade. Global warming usually is blamed, and the “global warming” label usually carries an explicit or implicit “human-induced” modifier. But it is not necessarily so.
Ice does not vanish from the Arctic Ocean only when it melts away. If the winds blow in certain ways, especially but not only in the warmer months, ice can leave the Arctic for more southern latitudes. If the wind blows from other directions, the Eurasian and North American land masses can keep it hemmed in.
The Arctic Oscillation is an alternating weather pattern that can persist for years, then switch and persist for years again. In the early 1990s the AO favored the exit of ice from the Arctic Ocean. This helped flush older, thicker ice out of the Arctic. It was replaced with newer, thinner ice, which is more prone to summer melt, especially with the long-term trend toward higher temperatures. The AO has been mainly neutral during the past decade.
The trend toward less ice was reinforced each autumn because water releases heat as it turns to ice. If there is more open water at the start of the freeze-up season, more heat must be released into the air as the water freezes. This helps warm the early Arctic winter. If there is less open water to freeze this year than last year, less heat will be released this autumn. That does not mean this will necessarily be a colder Arctic winter than last year, because other atmospheric factors will be involved, but it will contribute to an earlier and more vigorous freeze-up.
Alarm about global warming leads many people, including some scientists who should know better, to blame every mild winter, summer drought or major hurricane on climate change, as though such things have not always occurred. The shrinkage of summer ice in the northern polar seas has likewise been blamed on climate change. Most likely, there is a link — but either other factors were at play in the dramatic changes of the past decade, or the past two years are signaling that the planet is getting colder.
I’ll bet on the “other factors” answer. Though I do not doubt that the climate is warming, as it has since the peak of the last Ice Age, I am skeptical of our ability at this stage to accurately predict the future, or even to fully measure current conditions. Ice, it turns out, is a complicated subject.