Global average temperature is determined by Earth’s energy balance. Earth’s energy balance may be altered in three ways. First, the intensity of the sun’s energy may increase or decrease. Second, the reflection of incoming solar radiation by clouds or ice may increase or decrease, causing either more or less radiation to be reflected to space rather than to Earth’s surface. And third, the amount of outgoing infrared radiation from Earth’s atmosphere to space may increase or decrease.

Any factor that causes a change to Earth’s energy balance is known as a radiative forcing or a forcing. A radiative forcing is expressed in W/m². A positive forcing, such as that produced by increasing concentrations of greenhouse gases, tends to warm the Earth’s surface. A negative forcing, such as that produced by airborne particulates that reflect solar energy, tends to cool the Earth’s surface. Forcings may also be either natural- or human-caused (also known as anthropogenic).

Earth’s climate has changed throughout geologic history, and most all of the changes have occurred due to natural factors. Some of these factors include Earth’s orbital changes, volcanic eruptions, solar activity, plate tectonics, and El Niño-Southern Oscillation (ENSO). Climate also changes in response to the concentration of greenhouse gases in the atmosphere. The concentration of the atmospheric greenhouse gases can increase or decrease due to natural phenomena and human activity.