Earth’s climate is warming as a result of anthropogenic emissions of greenhouse gases, particularly carbon dioxide (CO2) from fossil fuel combustion. Anthropogenic emissions of non-CO2 greenhouse gases, such as methane, nitrous oxide and ozone-depleting substances (largely from sources other than fossil fuels), also contribute significantly to warming. Some non-CO2 greenhouse gases have much shorter lifetimes than CO2, so reducing their emissions offers an additional opportunity to lessen future climate change.