I first went to the Arctic in the summer of 1970, aboard the Canadian oceanographic ship Hudson, which was carrying out the first circumnavigation of the Americas. The ship was ice-strengthened and needed to be. Along the coasts of Alaska and the Northwest Territories, Arctic Ocean ice lay close in to land, leaving a gap of only a few miles to do our survey. Sometimes ice went right up to the coast. That was considered normal.

Today a ship entering the Arctic from the Bering Strait in summer finds an ocean of open water in front of her. Water extends far to the north, stopping only a few miles short of the pole. From space the top of the world now looks blue instead of white. Things are worse than appearances would suggest, however. What ice is still left is thin—average thickness dropped 43 percent between 1976 and 1999, sonar measurements show. By 2015, at this rate, summer melting will outstrip the accumulation of new ice in winter, and the entire ice cover will collapse. Once summer ice goes away entirely, the physics of latent heat will make it very difficult, if not impossible, to get it back. We will have entered what Mark C. Serreze, director of the National Snow and Ice Data Center at the University of Colorado at Boulder, calls the Arctic “death spiral.”

Once ice yields to open water, the albedo—the fraction of solar radiation reflected back into space—drops from 0.6 to 0.1, which will accelerate warming of the Arctic. According to my calculations, the loss of the remaining summer ice will have the same warming effect on the earth as the past 25 years of carbon dioxide emissions. Because a third of the Arctic Ocean is composed of shallow shelf seas, surface warming will extend to the seabed, melt offshore permafrost and trigger the release of methane, which has a much greater greenhouse warming effect than CO2. A Russian-U.S. expedition led by Igor Semiletov has recently observed more than 200 sites off the coast of Siberia where methane is welling up from the seabed. Atmospheric measurements also show that methane levels are rising, most likely largely from Arctic emissions.

To avoid the consequences of a collapse of summer ice, we need to bring back the ice we have lost. That will require more than merely slowing the pace of warming—we need to reverse it.

Reducing carbon emissions and replacing fossil fuels with renewables, including nuclear power, are the most sensible long-term solutions, of course. But these measures are not going to save the Arctic ice. After decades of our trying, CO2 levels in the global atmosphere continue to rise at a more than exponential rate.

It is time to consider a radical course: geoengineering. By this I mean techniques to artificially lower surface temperatures by blocking the sun. One proposal entails “whitening” low-level clouds by injecting fine sprays of water into them; another involves releasing solid sulfates into the atmosphere from balloons, causing radiation-reflecting aerosols to form. A simpler step would be to paint roofs and pavements white. Such measures are sticking-plaster solutions. They would have to be continuously applied, given that any cessation would bring warming back at an accelerated rate. Nor do they counter direct CO2 effects such as ocean acidification. But they might buy us time.

Is there a geoengineering technique that would cool the entire planet? Is there a way to cool only the Arctic in summer, to keep sea ice from disappearing? What effect would cloud whitening or chemical release over the Arctic have on precipitation patterns and on temperature? Finding out will require much research and modeling. This must be done urgently. We can no longer afford the luxury of talking about reducing CO2 emissions by some conveniently distant date in the future. We need action now.

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