The Earth’s energy budget describes how energy received from the sun by the Earth is distributed and absorbed across the components of the Earth (e.g. by the atmosphere, ocean, land or ice).
Ultimately, this incoming energy heats the planet and is balanced by the Earth emitting a similar amount of radiant energy back to space. The budget describes where energy is absorbed in the Earth system (heating up that part of the system) and where radiant energy is lost (cooling the system). Changes in the Earth’s energy budget, for example due to increasing amounts of atmospheric greenhouse gases, is the main driver of global warming.
The Earth’s energy budget is determined by the amount of sunlight (solar radiation) entering the atmosphere and the amount of thermal infrared energy (terrestrial radiation) leaving the atmosphere along with reflected sunlight.
If the amount of energy entering the atmosphere is equal to the amount of energy leaving, then the average global temperature will remain constant.
If the energy entering is less than the energy leaving, there will be global cooling.
If the energy entering is greater than the energy leaving, there will be global warming.
Solar energy largely comprises of ultraviolet, visible and near-infrared radiation. As sunlight travels down through the atmosphere, some radiation is reflected back into space by clouds, gas molecules and particulates and some is absorbed by the atmosphere (such as ultraviolet by ozone), but more than half reaches the surface of the Earth. At the surface, some radiation is reflected, but most is absorbed by the ground, heating it up.
The surface then radiates thermal energy (at significantly longer wavelengths than the sun’s radiation) into the atmosphere. Some of this energy escapes directly into space and some is absorbed by clouds, but most is absorbed by gases such as carbon dioxide and water vapour, heating the atmosphere up. The atmosphere then re-radiates the energy in all directions, so some escapes into space, but some is radiated back towards the surface. This is known as the natural Greenhouse Effect and is essential for keeping the Earth at a habitable temperature. Increasing the concentration of gases such as carbon dioxide, however, leads to an enhanced greenhouse effect which gives rise to global warming, as the amount of thermal radiation escaping to space is reduced and a new balance is reached through the surface warming and emitting more radiation.