evidence: Greenhouse gases are gases—like carbon dioxide (CO2), methane, and nitrous oxide—that keep the Earth warmer than it would be without them. The reason they warm the Earth has to do with the way energy enters and leaves our atmosphere. When energy from the sun first reaches us, it does so mainly as light. But when that same energy leaves the Earth, it does so as infrared radiation, which we experience as heat. Greenhouse gases reflect infrared radiation, so some of the heat leaving the Earth bounces off the greenhouse gases in our atmosphere and comes back to the Earth’s surface. This is called the "greenhouse effect," in a comparison to the heat-trapping glass on a greenhouse. [...] For climate change, the most important greenhouse gas is carbon dioxide, which is why you hear so many references to "carbon" when people talk about climate change. There are three main reasons CO2 is so central to the global warming happening today. First, there is just so much of it: we now add over 35 billion tons of CO2 to the atmosphere every year, mostly by burning carbon-rich fuel like coal and oil that had previously been trapped in the ground. Second, it lasts a long time in the atmosphere. The CO2 we emit today will stay above us reflecting heat for hundreds of years. This means that, even if we stop all new CO2 emissions tomorrow, it will take many lifetimes before the warming effect of our past emissions fades away.