MEEHL/NCAR

How to best curb greenhouse gas emissions is a hotly debated topic. But new research suggests that putting the brakes on greenhouse gas levels is not enough to slow down climate change because the ocean responds so slowly to perturbations. The study results, published today in the journal Science, indicate that even if greenhouse gas levels had stabilized five years ago, global temperatures would still increase by about half a degree by the end of the century and sea level would rise some 11 centimeters.

"Many people don't realize we are committed right now to a significant amount of global warming and sea level rise because of the greenhouse gases we have already put into the atmosphere," says study author Gerald Meehl of the National Center for Atmospheric Research (NCAR) in Boulder, Colo. "The longer we wait, the more climate change we are committed to in the future." Meehl and his NCAR colleagues ran two coupled climate models that link major components of our planet's climate and incorporate their interactions. The researchers then analyzed scenarios in which greenhouse gases continue to accumulate in the atmosphere at low, moderate and high rates. The highest rates of accumulation led to model results that included a 3.5 degrees Celsius increase in global temperatures and a 30 centimeter rise in global sea level.

But even without additional greenhouse gas contributions, they found, global temperature would continue to rise because of a characteristic known as thermal inertia. Water in the oceans heats and cools more slowly than air does because of its greater density, leading to a delayed response. In addition, carbon dioxide and other greenhouse gases have long atmospheric lifetimes and can affect temperatures for years after first being introduced into the atmosphere. The authors conclude that "at any given point in time, even if concentrations are stabilized, there is a commitment to future climate changes that will be greater than those we have already observed."