The greenhouse effect is a process where greenhouse gases in the atmosphere absorb and emit thermal radiation from the surface, trapping heat and warming the lower atmosphere. Some of the re-radiated energy is sent back towards the surface, causing the surface and lower atmosphere to be warmer than they would be from solar heating alone. Major greenhouse gases that contribute to this effect include water vapor, carbon dioxide, methane, and ozone. Global warming is believed to be caused by a strengthening of the greenhouse effect due to increased levels of greenhouse gases from human activity like fossil fuel burning and deforestation.