The sun is the source of energy for Earth (1). Some of the sun’s energy is reflected back to space (2), but the rest is absorbed by the atmosphere, land, and ocean and re-emitted as longwave radiation (radiant heat). Some of this radiant heat is absorbed and then re-emitted by greenhouse gases in the lower atmosphere, trapping heat in the lower atmosphere and reducing how much is radiated to outer space. This process is known as the greenhouse effect (3). Changes to the amount of incoming solar radiation (1), the amount of reflected sunlight (2), and the heat-trapping capacity of the atmosphere (3) cause climate warming or cooling. Factors that drive such changes are called climate drivers or climate forcing agents.
Upper panel: National Academy of Sciences and the Royal Society (2014).