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A Slippery Slope

More than 20 years ago, Hansen also explained why global warming has lagged the greenhouse buildup. In 1985, he suggested that it should take between 50 and 100 years for the excess energy reaching the planetary surface to have its full effect on temperature, because the energy will first go to heating the oceans; only when they begin to warm will the atmosphere follow suit. Just last year, when studies demonstrating a global rise in ocean temperatures confirmed his thinking, Hansen began referring to the heating of the oceans as the “smoking gun” of global warming.

Another factor, which Hansen and GISS modeling specialist Andy Lacis have termed a “Faustian bargain,” also suppresses atmospheric warming. In 1990, Hansen and Lacis showed that traditional air pollution has produced a mighty parasol effect. We send dust and aerosols into the air from tailpipes and smokestacks, by burning the wood and dung that provide heat and light to hundreds of millions of the world’s very poor, and through slash-and-burn agriculture and other land use practices that have exposed vast tracts of dried-out, eroded soil to the blowing wind. The dimming of incident sunlight caused by reflection from these airborne particles now offsets about half the warming of the industrial age.

To continue offsetting our growing greenhouse emissions, we would have to maintain the rapid growth of traditional, noxious air pollution. But the United States and Europe have begun controlling it, and the dismal air quality in Beijing and Mumbai is convincing the Chinese and Indians that they must, too. Faust’s payment to the greenhouse is now coming due.

Owing to greenhouse changes we have already incurred, Hansen told his audience in San Francisco, Earth’s temperature will rise about 0.5 ºC in the next 50 years even if we stop burning fossil fuels today. We’re on a slippery slope: we could cross a threshold that leads to a drastically different planet, half a century before knowing that we’ve done so. Hansen believes we are horrifyingly close to such a threshold, and that we will cross it if we don’t change our greenhouse ways within the next few years.

Earth is now passing upward through the highest temperatures of the past 12,000 years, and the half a degree that is already in the pipeline will bring temperatures within half a degree of the high points they have reached only a few times in the past two million years. During a warm period about 120,000 years ago, for example, sea levels were probably five or six meters higher than they are today.

Running future emissions scenarios on a GISS computer model, Hansen finds that if we remain on the path he calls “business as usual,” temperatures will rise between two and three degrees this century, making Earth as warm as it was about three million years ago, when the seas were between 15 and 35 meters higher than they are today. There go many major cities and the dwellings of about half a billion people.

Evidence suggests that the seas could rise in a matter of decades or centuries; recent events in Greenland and Antarctica indicate that the process may already have begun. The last great ice sheet collapse, about 14,000 years ago, sent the seas up a total of 20 meters, at the rate of one meter every 20 years for 400 years. Just the first meter would obliterate New Orleans, force tens of millions of people in Bangladesh to emigrate, and inundate rice-growing river deltas throughout Asia, a major food source for our species.

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