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This article is from the In-Depth Report Earth 3.0: Solutions for Sustainable Progress
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Global Warming: Beyond the Tipping Point

The world's most outspoken climatologist argues that today's carbon dioxide levels are already dangerously too high. What can we do if he is right?



FRED CONRAD New York Times/Redux Pictures (Hansen)

The basic proposition behind the science of climate change is so firmly rooted in the laws of physics that no reasonable person can dispute it. All other things being equal, adding carbon dioxide (CO2) to the atmosphere—by, for example, burning millions of tons of oil, coal and natural gas—will make it warm up. That, as the Nobel Prize–winning chemist Svante Arrhenius first explained in 1896, is because CO2 is relatively transparent to visible light from the sun, which heats the planet during the day. But it is relatively opaque to infrared, which the earth tries to reradiate back into space at night. If the planet were a featureless, monochromatic billiard ball without mountains, oceans, vegetation and polar ice caps, a steadily rising concentration of CO2 would mean a steadily warming earth. Period.

But the earth is not a billiard ball. It is an extraordinarily complex, messy geophysical system with dozens of variables, most of which change in response to one another. Oceans absorb vast amounts of heat, slowing the warm-up of the atmosphere, yet they also absorb excess CO2. Vegetation soaks up CO2 as well but eventually re­releases the gas as plants rot or burn—or, in a much longer-term scenario—drift to the bottom of the ocean to form sedimentary rock such as limestone. Warmer temperatures drive more evaporation from the oceans; the water vapor itself is a heat-trapping gas, whereas the clouds it forms block some of the sun’s warming rays. Volcanoes belch CO2, but they also spew particulates that diffuse the sun’s rays. And that’s just a partial list.

Because including all these factors in calculations about the effects of CO2 increase is hugely difficult, it is no surprise that climate scientists are still struggling to understand how it all will likely turn out. It is also no surprise, given his track record as something of a climate change agitator, that James Hansen, director of the NASA Goddard Institute for Space Studies, has been circulating a preprint of a journal paper saying that the outcome is likely to turn out worse than most people think. The most recent major report from the Intergovernmental Panel on Climate Change in 2007 projects a temperature rise of three degrees Celsius, plus or minus 1.5 degrees—enough to trigger serious impacts on human life from rising sea level, widespread drought, changes in weather patterns, and the like.

But according to Hansen and his nine co-authors, who have submitted their paper to Open Atmospheric Science Journal, the correct figure is closer to six degrees C. “That’s the equilibrium level,” he says. “We won’t get there for a while. But that’s where we’re aiming.” And although the full impact of this temperature increase will not be felt until the end of this century or even later, Hansen says, the point at which major climate disruption is inevitable is already upon us. “If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted,” the paper states, “CO2 will need to be reduced from its current 385 ppm [parts per million] to at most 350 ppm.” The situation, he says, “is much more sensitive than we had implicitly been assuming.”

As with many of Hansen’s assertions, this one pushes the science further than some of his colleagues would be willing to go. Back in 1998, for example, Hansen was arguing that the human impact on climate was unquestionable, even as other leading climate scientists continued to question it. He was subsequently proved right, not only about the human influence but about the approximate pace of future temperature rise. But just as in 1998, the underlying motivation for his claims, if not all of his conclusions, is shared pretty much universally.

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