Earth’s temperature is a balancing act
Earth’s temperature depends on the balance between energy entering and leaving the planet’s system . When incoming energy from the sun is absorbed by the Earth system, Earth warms. When the sun’s energy is reflected back into space, Earth avoids warming. When energy is released back into space, Earth cools. Many factors, both natural and human, can cause changes in Earth’s energy balance, including:
Line graph with a line that show the observed temperature increases, a blue band that show how the temperature would have changed over the past century due to only natural forces, and a red band that shows the combined effects of natual and human forces. The blue band that shows natural forces starts and ends the 20th century just above 56 degrees Fahrenheit. The actual observed global average temperatures closely follows the model projections that use both human and natural forces - beginning in 1900 at just above 56 degrees Fahrenheit and ending in 2000 around 58 degrees Fahrenheit. View enlarged image
Models that account only for the effects of natural processes are not able to explain the warming over the past century. Models that also account for the greenhouse gases emitted by humans are able to explain this warming.
Source: USGRCP (2009)
Changes in the greenhouse effect, which affects the amount of heat retained by Earth’s atmosphere
Variations in the sun’s energy reaching Earth
Changes in the reflectivity of Earth’s atmosphere and surface