So if more solar radiation is being absorbed by the Earth than Earth is radiating away, Earth will warm up and radiate more, until it radiates as much as it is absorbing. That is what is currently happening, because the increased amount of carbon dioxide prevents Earth from radiating enough energy away at normal temperatures. Earth currently receives 0.58 watts per square meter more energy than it emits. This imbalance is caused mainly by the excess carbon dioxide, and has already resulted in a temperature increase over the last 100 years that is a 2% increase beyond the natural greenhouse effect. Because there is still an energy imbalance, we know Earth will keep on getting warmer until it can radiate as much energy as it receives.
A large part of the energy from the Sun that reaches Earth is absorbed at the surface because the air is transparent to sunlight. Earth transfers this absorbed sunlight energy to its atmosphere in several ways, but Earth can only release this energy back into space as radiated electromagnetic energy. Because Earth is much less warm than the Sun, Earth can only release this energy as infrared radiation. The air is not transparent to all infrared radiation. Energy that is transferred to the air that is not radiated directly into space causes an increase in temperature of the air.
Water vapor and carbon dioxide in the air absorb infrared radiation, and cause the temperature of the air at Earth’s surface (and the surface itself) to be 33 degrees Celsius warmer than it would be if there were no water vapor or carbon dioxide in the atmosphere. If there were no water vapor or carbon dioxide, the oceans would be totally frozen and there would be no life on Earth. This 33 degree increase is known as the natural greenhouse effect, because the details of the warming process are nearly identical to how a greenhouse works.
Water vapor and carbon dioxide absorb infrared light due to molecular absorption. Specific molecules vibrate in ways that absorb some “colors” (frequency ranges) of electromagnetic radiation, but not others. Water vapor has two absorption bands that block the lowest and highest frequencies of Earth’s radiation. Any energy released from Earth must escape in a middle zone of frequencies between the two frequency “walls” due to water vapor, or radiate from higher in the atmosphere. Because Earth cannot radiate freely, it warms up until it can radiate the same amount of radiation that it receives. Water vapor causes about 60% of this warming.
