Division of Energy

Global Climate Change: Clouds and Climate

Clouds and oceans are the two largest and least understood components of the climate system. What effect do clouds have on climate? What effect will clouds have on climate change?

Radiation

Energy may be transferred from one object to another without the space between them being heated. This is the mechanism where earth receives energy from the sun as radiation. Radiation travels in the form of waves that release energy when they are absorbed by an object. Waves of radiation can be distinguished by wavelength, which is the distance between two successive crests of the wave.

The solar radiation reaching earth is in three wavelength spectra. The sun emits 44 percent of its radiation in the visible spectrum, seven percent in the ultraviolet spectrum (short wave), and 49 percent in the infrared spectrum (long wave).

The Balancing Act

The Earth and all things continually radiate energy. If an object radiates more energy than it absorbs, it cools; if it absorbs more energy than it emits, it gets warmer. When an object emits and absorbs energy at equal rates, its temperature remains unchanged. Absorption and emission are in balance.

The region near the equator (low latitudes) gains more solar radiation than is lost. In contrast, the poles (high latitudes) lose more energy than they receive. To maintain an energy balance, the atmosphere and the oceans transfer the excess energy from the equator to the poles. Any alteration of the energy-transfer system will change the climate.

Selective Absorbers

The Earth absorbs all radiation spectra that strikes it and then emits infrared radiation. The earth's atmosphere, however absorbs and emits infrared radiation. Additionally, the atmosphere absorbs some wavelengths of radiation but is transparent to others. Thus, the atmosphere is a selective absorber. The atmosphere's absorption of infrared radiation emitted from the earth and the resultant warming of the air is the greenhouse effect (see fact sheet #1 in this series). Gases in the atmosphere also are selective absorbers. Ozone selectively absorbs ultraviolet radiation. Carbon dioxide and water vapor are strong, selective absorbers of infrared radiation.

Atmospheric Window

Water vapor and carbon dioxide, however, do not absorb all wavelengths in the infrared spectrum. A certain wavelength range of emitted infrared radiation is not absorbed and passes upward through the atmosphere into space. This wavelength range is called the atmospheric window.

Enhancement of the Greenhouse Effect

At night, clouds can enhance the greenhouse effect. Tiny cloud droplets are selective absorbers in that they are good absorbers of infrared radiation but poor absorbers in the visible spec-trum of solar radiation. They even absorb wavelengths in the atmospheric window. Water vapor and the tiny water droplets of clouds have differing abilities to absorb infrared radiation. Thus, clouds can enhance the greenhouse effect by closing the atmospheric window. This process keeps calm, cloudy nights warmer than calm, clear ones.

If clouds remain into the next day, they prevent much of the solar radiation from reaching the ground by reflecting it back to space. They are poor absorbers of visible solar radiation. Since the radiation does not reach and warm the ground, cloudy calm days normally are cooler than clear, calm days.

Clouds and Climate Change

Satellite data suggest that clouds at all levels appear to cool the earth's climate since they reflect and radiate away more energy than they retain. This data would support the negative-feedback theory where global warming would increase the amount of water vapor in the atmosphere, which in turn may bring about an increase in global cloudiness, offsetting further warming. The negative feedback theory's validity depends upon the types of clouds formed. Low clouds (surface to 6,500 feet) and middle clouds (6,500 to 23,000 feet) are mostly water. They reflect more solar radiation than they absorb infrared radiation. Low and middle clouds therefore have a cooling effect.

On the other hand, high clouds (16,000 to 43,000 feet) are mostly ice which absorbs more infrared radiation than water. Since high clouds are thin, they also allow sunlight to pass through to warm the ground. High clouds then have a warming effect.

The formation of predominantly high clouds would support the positive feedback theory. Global warming would increase the amount of water vapor in the atmosphere, which may bring about an increase in global cloudiness, absorbing more infrared radiation and enhancing the Greenhouse Effect.

The percentage of radiation reflected from a surface is called the albedo. Thick clouds have a higher albedo than thin clouds. Clouds reflect upwards about 20 percent of the total incoming solar radiation, a cooling process. They absorb about 3 percent of incoming solar radiation and absorb infrared radiation from the Earth, a warming process.

Summary

Current research supports the strong influence of clouds upon climate. Researchers at the British Meteorological Office altered the representation of clouds in their climate model. Initially, the model projected a global temperature rise of about 5 degrees Celsius, accompanying a doubling of atmospheric carbon dioxide. However, when water clouds replaced ice clouds in the simulation, the projected temperature rise was less than 2 C.

A study conducted in 1989 used 14 climate models to simulate global climate response when current values of carbon dioxide were doubled. All models were in agreement under clear skies. But when clouds were incorporated into the models, the models did not agree, and, in fact, varied greatly over a wide range.

There are many variables that can alter the effect of clouds on the climate system. Will cumuliform or stratiform clouds develop? Will low, middle, or high clouds form? What will be the thickness of clouds formed? And what is the percentage of the sky covered? Just how the climate will respond to changes in cloudiness will depend on the type of clouds that form and their physical properties, such as liquid water (or ice) content and droplet size.