As public conversation about global warming has centered for several weeks on its possible role in a dreadful hurricane season, climate scientists have been tracking a much clearer and more alarming impact.
This is the time of year when Arctic sea ice shrinks to its annual minimum, and satellite measurements indicate that the ice cover has fallen below a “tipping point” from which it may not recover. By the end of the century, and possibly much earlier, the polar region seems likely to be ice-free through the summer months — a condition that has not occurred for roughly a million years.
The consequences of such change are enormous, and reach far beyond the sea-level changes from which Minnesotans may feel comfortably distant. Sea ice is a great stabilizer of the global climate, a sort of flywheel that smooths and slows the impact of changes occurring elsewhere in the system. Without it, those unhappy trends will widen and accelerate.
Here’s the problem: Because of its high reflectivity, white sea ice absorbs far less of the sun’s heat than open water. The same is true of snow- and glacier-covered land masses, like much of Greenland. When the ice retreats, more of the polar ocean is exposed to sunlight, which means both the seawater and the air above it get warmer.
Thus begins an unfortunate feedback loop: the warmer the ocean and the atmosphere, the more sea ice melts at the edges; the more the ice retreats, the warmer the ocean and atmosphere become. The ice still rebounds in the cold months, but once it shrinks beyond a certain point during the summer, it can’t fully recover.
Since the sea ice is a floating part of the ocean, its melting has no major direct impact on sea levels. Indirectly, however, the consequences look dire: Warmer sea and air temperatures in the polar region will melt more ice cover on land masses — Greenland, the Arctic regions of Canada, Scandinavia and Russia — and this does put more water into play. Indeed, it has been calculated that melting Greenland’s glaciers alone would cause extensive flooding in every coastal city on the planet. Also, melting permafrost will unlock massive amounts of additional carbon dioxide now stored in frozen soil, adding to the globe-warming emissions from burning coal, oil and wood.
Often scientists find it a tricky undertaking to separate the signals of climate change from natural cycles, but with sea ice the distinctions are easier to draw. Ice cores and other physical evidence have established the natural patterns of the last million years, and they show nothing like the marked monthly declines that have been measured for the last quarter-century.
The Arctic has lost about 20 percent of its summer sea ice during that period, and the pace has picked up. The summer ice cover was at its smallest in recorded history in 2002, failed to recover in 2003 or 2004, and this summer set new lows for June, July and August. Sometime next week scientists will report their findings for September, when the ice reaches its annual minimum, and the bad news is likely to continue.
What’s to be done? There appears to be no way of restoring the lost sea ice. However, some might take that as all the more reason to get serious about what humans can do: ratchet back the emissions from cars, power plants and other industrial sources that keep moving all the planet’s climate systems toward a point of no return.
© Copyright 2005 Star Tribune