To maintain a steady global temperature, Earth must emit the same amount of energy back into space as it receives from solar radiation. Earth's radiation balance is the accounting of the average annual energy gain from solar radiation versus the average annual loss from infrared radiation that leaves the Earth system. When the energy gain and energy loss are in balance, Earth neither heats up nor cools down.
Many physical factors of the Earth and its atmosphere play roles in Earth's radiation balance. Without the atmosphere, for instance, the average surface temperature on Earth would be about 34°C colder than it is at present. The warming of Earth that results from retention of heat in its atmosphere is called the greenhouse effect. Changes to the atmosphere, therefore, can change the temperature of Earth.
Human activities are influencing Earth's radiation balance in the following ways:
Burning fossil fuels and clearing forests add CO2 (a greenhouse gas) to the atmosphere, which means that more radiation is re-radiated back to Earth.
Expansion of livestock and wetland crop production adds CH4 and N2O (also greenhouse gases).
Deposition of dust and dark-colored soot particles ("black carbon") energy absorbed by snow and ice, thereby increasing the melting of persistent ice fields such as the Greenland ice sheet and Arctic sea ice, as well as advancing the timing of spring snowmelt in some temperate regions.
Other human activities have cooling effects rather than warming effects. The amount of incoming solar radiation reflected by the Earth system increases, for example, as humans inject more aerosols (tiny particulates and water droplets) into the atmosphere in the form of smoke and air pollution, and as they clear land. However, when climate scientists incorporate all of these effects into quantitative computer models of Earth's climate, they conclude that the net effect of human activities is climate warming, and that the rate of warming predicted by the models is in close agreement with the actual warming observed in long-term temperature records.