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Greenhouse gases and our perception of risk

By Moises Velasquez-ManoffStaff writer of The Christian Science Monitor / June 3, 2009

On Monday, Columbia University economist Jeffrey Sachs  told an audience at the Asia Society in New York City that, given China's dependence on coal to fuel economic growth – and its determination to grow – the United States has no option but more nuclear energy and carbon sequestration. (Here's Grist's account.)

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In some ways, that seems fair enough. According to the World Resources Institute, the US is responsible for 29 percent of all human-emitted greenhouse gases (GHGs) for the past 150 years. That's a larger share than any other single country, and more than triple what China, the second largest climate offender, has contributed so far. (See Greenpeace's new report "America's Share of the Climate Crisis.")

The by-country breakdown raises the question, though: Why can't nations be responsible just for what they've emitted historically? Why can't we just clean up our own respective messes?

Because Earth's climate doesn't work like a bank. Customers can't deposit and withdraw GHGs at will and expect the whole structure to remain stable. Who has contributed what, when matters less than how much there is total, and at what point the whole system will go over some tipping point.

On this last point – the risk that GHG build-up could set off relatively rapid and rather unpleasant changes – James Hansen, head of NASA's Goddard Institute for Space Studies, warns that we've already gone too far.

Some years ago, scientists thought stabilizing atmospheric CO2 at 550 ppm would work. Then the figure became 450 ppm. Now Dr. Hansen argues that it's got to be 350 ppm. We're currently at 385 parts per million. We need to backtrack, he says. (Here's more on the science and the "350" rallying cry.)

Hansen's argument [PDF] is essentially about avoiding bad gambles. He says that the specter of what we don't know – how and when Earth's climate system goes over these thresholds – combined with what we do know (that it has gone over these thresholds in the past and that these changes correlate with GHG concentrations) represent an unacceptable risk.

He notes that climate is already changing –  a poleward shift of midlatitude deserts and the melting of mountain glaciers  – and that many of the changes are coming faster than scientists predicted.

"Impacts of this climate shift support the conclusion that 385 ppm CO2 is already deleterious," he wrote [PDF] last year.

Harvard economist Martin Weitzman breaks the unknown down into economic terms. He says that, even when the probability of a catastrophe is small, if the catastrophe is potentially great – like sea level rise inundating coastal cities and a mass extinction of some life-forms that fail to adjust – then the best investment is acting now to avoid that not-very-likely but potentially game-changing outcome.

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