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Aerosols and Global Dimming

The effect of aerosols on climate is an area of active research. As is the case with most aspects of climate science, the issues involved are complex. The effect of aerosols on clouds and climate is not as simple as more aerosols mean more clouds and greater albedo and hence less light at the surface and thus cooling. Climate scientists must determine what types of clouds are produced at what altitudes by various combinations of aerosols. Some aerosol-induced clouds have especially small cloud droplets, and are thus particularly good at reflecting sunlight. Others have larger droplets and tend to create rain, which alters the water cycle. Though increased cloudiness blocks incoming sunlight in the day, it also blocks outgoing infrared radiation at night, enhancing the greenhouse effect. Though black carbon prevents sunlight from reaching Earth's surface, the light it absorbs is reradiated as heat into the atmospheric layers in which it is present. Scientists still have much to learn about how alterations in energy flow at various elevations impacts the overall character of those atmospheric layers. (reference)

Albedo: The term albedo (Latin for white) is commonly used to applied to the overall average reflection coefficient of an object. For example, the albedo of the Earth is 0.39 (Kaufmann) and this affects the equilibrium temperature of the Earth. The greenhouse effect, by trapping infrared radiation, can lower the albedo of the earth and cause global warming. (reference)

Greenhouse effect: The greenhouse effect is a naturally occurring process that aids in heating the Earth's surface and atmosphere. It results from the fact that certain atmospheric gases, such as carbon dioxide, water vapor, and methane, are able to change the energy balance of the planet by absorbing longwave radiation emitted from the Earth's surface. Without the greenhouse effect life on this planet would probably not exist as the average temperature of the Earth would be a chilly -18 Celsius, rather than the present 15 Celsius. (reference)