Climate Numerology; January 2010; Scientific American Magazine; by David Biello; 2 Page(s)

Last December world leaders met in Copenhagen to add more hot air to the climate debate. That is because although the impacts humanity would like to avoid—fire, flood and drought, for starters—are pretty clear, the right strategy to halt global warming is not. Despite decades of effort, scientists do not know what “number”—in terms of temperature or concentrations of greenhouse gases in the atmosphere—constitutes a danger.

When it comes to defining the climate’s sensitivity to forcings such as 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.”