Many people are concerned that an increasing concentration of greenhouse gases in the atmosphere – due primarily to such human activities as burning fossil fuels for energy – is causing the Earth to warm, with potentially harmful results. In response, many developed countries agreed to the Kyoto Protocol on Climate Change, committing them to limit and eventually reduce their greenhouse gas emissions. The United States chose not to participate, in part because the agreement exempts such developing countries as China and India, although they have the world's fastest-growing economies and emissions.