Global warming comes with a big price tag for every country around the world. The 80 percent reduction in U.S. emissions that will be needed to lead international action to stop climate change may not come cheaply, but the cost of failing to act will be much greater. New research shows that if present trends continue, the total cost of global warming will be as high as 3.6 percent of gross domestic product (GDP). Four global warming impacts alone—hurricane damage, real estate losses, energy costs, and water costs—will come with a price tag of 1.8 percent of U.S. GDP, or almost $1.9 trillion annually (in today’s dollars) by 2100. We know how to avert most of these damages through strong national and international action to reduce the emissions that cause global warming. But we must act now. The longer we wait, the more painful—and expensive—the consequences will be.