Clouds affect Earth's climate by absorbing and radiating solar energy. Photo courtesy of kevindooley/Flickr.
The study of clouds, where they occur, and their characteristics, play a key role in the understanding of climate change. Low, thick clouds primarily reflect solar radiation and cool the surface of the Earth. High, thin clouds primarily transmit incoming solar radiation; at the same time, they trap some of the outgoing infrared radiation emitted by the Earth and radiate it back downward, thereby warming the surface of the Earth. The Earth's climate system constantly adjusts in a way that tends toward maintaining a balance between the energy that reaches the Earth from the sun and the energy that goes from Earth back out to space. Scientists refer to this as Earth's radiation budget.
The components of the Earth system that are important to the radiation budget are the planet's surface, atmosphere, and clouds. Most of the sun's energy is in the form of visible radiation. Visible radiation and radiation with shorter wavelengths, such as ultraviolet radiation are labeled shortwave. The sun's energy is emitted in all directions, with only a small fraction being in the direction of the Earth. Both the amount of energy and the wavelengths at which energy is emitted by any system are controlled by the average temperature of the system's radiating surfaces, plus the emission properties.
Energy goes back to space from the Earth system in two ways: reflection and emission. Part of the solar energy that comes to Earth is reflected back out to space in the same, short wavelengths in which it came to Earth. The fraction of solar energy that is reflected back to space is called the albedo.
As you learned in the previous week, different parts of the Earth have different albedos. For example, ocean surfaces and rain forests have low albedos, which means that they reflect only a small portion of the sun's energy. Deserts, ice, and clouds, however, have high albedos; they reflect a large portion of the sun's energy. Over the whole surface of the Earth, about 30 percent of incoming solar energy is reflected back to space. Because a cloud usually has a higher albedo than the surface beneath it, the cloud reflects more shortwave radiation back to space than the surface would in the absence of the cloud, thus leaving less solar energy available to heat the surface and atmosphere. Hence, this cloud albedo forcing, taken by itself, tends to cause a cooling of the Earth's climate.
Another part of the energy going to space from the Earth is the electromagnetic radiation emitted by the Earth. The solar radiation absorbed by the Earth causes the planet to heat up until it is emitting as much energy back into space as it absorbs from the sun. Most of this emitted radiation is at longer wavelengths. Unlike solar radiation, which is mostly at wavelengths visible to the human eye, the Earth's longwave radiation is mostly at infrared wavelengths, which are invisible to the human eye.
When a cloud absorbs longwave radiation emitted by the Earth's surface, the cloud re-emits a portion of the energy to outer space and a portion back toward the surface. The energy re-emitted back toward the Earth will increase the temperature of the Earth's surface and atmosphere. This process is called cloud greenhouse forcing and tends to cause a heating of the Earth's climate.
A significant fraction of the longwave radiation emitted by the surface is absorbed by trace gases in the air. This heats the air and causes it to radiate energy both out to space and back toward the Earth's surface. The energy emitted back to the surface causes it to heat up more, which then results in greater emission from the surface. This heating effect of air on the surface, called the atmospheric greenhouse effect, is due mainly to water vapor in the air, but also is enhanced by carbon dioxide, methane, and other infrared-absorbing trace gases.
The balance of the opposing cloud albedo and cloud greenhouse forcings determines whether a certain cloud type will add to the air's natural warming of the Earth's surface or produce a cooling effect. High thin cirrus clouds tend to enhance the heating effect, and low thick stratocumulus clouds have the opposite effect. The overall effect of all clouds together is that the Earth's surface is cooler than it would be if the atmosphere had no clouds.Source: Clouds and Radiation. Earth Observatory. Retrieved from http://earthobservatory.nasa.gov/Features/Clouds/ on November 30, 2010.