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.”
Greenhouse gases add those watts by acting as a blanket, trapping the sun’s heat. They have warmed the earth by roughly 0.75 degree Celsius over the past 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 play a role—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. “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.
Despite all these variables, scientists have noted for more than a century that doubling preindustrial concentrations of CO2 in the atmosphere from 280 parts per million (ppm) would likely result in global average temperatures roughly three degrees C warmer.
But how much heating and added CO2 are safe for human civilization remains a judgment call. European politicians have agreed that global average temperatures should not rise more than two degrees C above preindustrial levels by 2100, which equals a greenhouse gas concentration of roughly 450 ppm. “We’re at 387 now, and we’re going up at 2 ppm per year,” says geochemist Wallace Broecker of Columbia University. “That means 450 is only 30 years away. We’d be lucky if we could stop at 550.”
Goddard’s James Hansen argues that atmospheric concentrations must be brought back to 350 ppm or lower—quickly. “Two degrees Celsius [of warming] is a guaranteed disaster,” he says, noting the accelerating impacts that have manifested in recent years. “If you want some of these things to stop changing—for example, the melting of Arctic sea ice—what you would need to do is restore the planet’s energy balance.”
Other scientists, such as physicist Myles Allen of the University of Oxford, examine the problem from the opposite side: How much more CO2 can the atmosphere safely hold? To keep warming below two degrees C, humanity can afford to put one trillion metric tons of CO2 in the atmosphere by 2050, according to Allen and his team—and humans have already emitted half that. Put another way, only one quarter of remaining known coal, oil and natural gas deposits can be burned. “To solve the problem, we need to eliminate net emissions of CO2 entirely,” Allen says. “Emissions need to fall by 2 to 2.5 percent per year from now on.”
Climate scientist Jon Foley of the University of Minnesota, who is part of a team that defined safe limits for 10 planetary systems, including climate, argues for erring on the side of caution. He observes that “conservation of mass tells us if we only want the bathtub so high either we turn down the faucet a lot or make sure the drain is bigger. An 80 percent reduction [in CO2 by 2050] is about the only path we go down to achieve that kind of stabilization.”