Scientific American: What Is the Right Number to Combat Climate Change? [View All]

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http://www.scientificamerican.com/article.cfm?id=right-number-to-combat-climate-change

November 30, 2009

What Is the Right Number to Combat Climate Change?

Is there a safe level of carbon dioxide in the atmosphere to prevent "dangerous anthropogenic interference" in the climate?

By David Biello

This December, world leaders will meet in Copenhagen to add more hot air to efforts to combat climate change. That is so because although the impacts humanity would like to avoid—fire, flood and drought, for starters—are clear, the right numbers to halt global warming are not. Despite decades of effort, scientists do not know precisely what temperatures or greenhouse gas concentrations in the atmosphere constitute a danger.

When it comes to defining the climate's sensitivity to forcings like rising atmospheric carbon dioxide levels, "we don't know much more than we did in 1975," says climatologist Stephen Schneider of Stanford University, who first defined the term climate sensitivity in the 1970s. "What we know is if you add watts per square meter to the system it's going to warm up."

Greenhouse gases add those watts by acting as a blanket, trapping the sun's heat; they have warmed Earth by roughly 0.75 degree Celsius over the last century. Scientists can measure how much energy greenhouse gases now add (roughly three watts per square meter), but what eludes precise definition is how much other factors—the response of clouds to warming, the cooling role of aerosols, the heat and gas absorbed by oceans, human transformation of the landscape, even the natural variability of solar strength—diminish or strengthen that effect. "We may have to wait 20 or 30 years before the data set in the 21st century is good enough to pin down sensitivity," says climate modeler Gavin Schmidt of the NASA Goddard Institute for Space Studies (GISS).

Despite all these variables, scientists from Svante Arrhenius to those on the United Nations Intergovernmental Panel on Climate Change have noted that doubling preindustrial concentrations of CO2 in the atmosphere from 280 parts per million (ppm) would likely result in a world with average temperatures roughly 3 degrees C warmer.

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