A report released by researchers at Tufts University, commissioned by the Natural Resources Defense Council (NRDC), presents two ways of estimating the costs of inaction on climate change, both leading to staggering bottom lines. A comprehensive estimate, based on state-of-the-art computer modeling, finds that doing nothing on global warming will cost the United States economy more than 3.6 percent of GDP - or $3.8 trillion annually (in today's dollars) - by 2100.