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4.2.7. Variation of clouds and aerosols

Aerosols are small solid or liquid particles suspended in the atmosphere (moving together with air). They reflect and absorb parts of solar radiation, and the fraction varies according to the physical properties and shapes of the particles. If they mainly reflect solar radiation, the energy absorbed by the earth including the atmosphere decreases, so they act to decrease temperature. If they mainly absorb solar radiation, temperature of the layer where the aerosols exist will rise, but temperature of the surface below will lower. On the other hand, aerosols absorb and emit terrestrial radiation, so they also have greenhouse effect. It acts to lower the temperature of the layer where the aerosols exist and to raise the temperature at the surface and in the lower atmosphere.

Clouds consist of liquid or solid H2O particles suspended in the air. So they are covered by the definition of aerosols, but usually they are discussed separately. Clouds also have both the effect of reflecting solar radiation and the greenhouse effect. Greenhouse effect may be dominant only in the case of thin cumulus clouds in the upper troposphere. In other cases the reflection effect is dominant. If temperature rises globally, it is certain that the fraction of cumulus-type clouds developing vertically increases, and that the cloud top height becomes higher. But it is not sure whether the `cloud amount' (fraction of area covered by clouds to total surface area) increases or decreases. Cloud will make a negative feedback to temperature if it increases, and negative if it decreases.

For cloud particles to form, aerosols that can act as condensation nuclei are necessary. If the amount or properties of condensation nuclei change, cloud water content and cloud droplet size will change, and they affect radiative balance. Some scientists consider this as an important factor of climatic variation.

Beside clouds, such aerosols as those come from volcanic eruption, sand dusts, and soot may affect climate through radiative balance. Among various particles ejected from volcanoes, solid particles such as volcanic ashes fall in a short time scale and the effect is limited to the region near the volcano. Global effects of volcanic eruption are caused by sulfate acid liquid droplets, and particles of sulfates and nitrates residing in the stratosphere. They are originally ejected as gas and formed into particles in the atmosphere. They reside in the atmosphere in a time scale of a year. Since they have strong reflectance to solar radiation, they act to lower temperature at the surface and in the lower atmosphere.


next up previous
Next: Factors of climatic variation Up: Factors of climatic variation Previous: Variation of greenhouse gas

2003-09-10, 2004-05-10