indirect effect
Indirect effects of aerosols include their influence on the radiation
balance and hydrology through their impact on cloud microphysical processes
(first indirect effect) and amount (second indirect effect). There is also a
semi-direct effect, in which the heating by aerosol particles due to absorption
of solar radiation results in a decrease of cloud amount.
Most elementary school students learn that clouds form when enough water vapor condenses. That’s true, but aerosols play a critical role in the process. In fact, most clouds owe their existence to aerosols that serve as the tiny “seeds,” called cloud condensation nuclei.
Natural aerosols—often sulfates, sea salt or ammonium salts—are the most common condensation nuclei in pristine environments. Polluted air, in contrast, usually contains much higher concentrations of water-soluble particles, which means pollution-rich clouds tend to have more numerous, but smaller, droplets. The small droplets make polluted clouds look brighter than they would otherwise be. Just as many bits of crushed ice give light more surfaces to reflect off—appearing brighter than a solid cube of ice--if the water in a cloud is divided into a larger number of smaller droplets, it will scatter more light and become more reflective.
Brighter clouds, in turn, block sunlight from reaching Earth’s surface, shading the planet and producing net cooling. This cloud brightening effect—called the “cloud albedo effect”--may have a big impact on the climate, though only in recent years has it been possible to start quantifying the effect.
Most elementary school students learn that clouds form when enough water vapor condenses. That’s true, but aerosols play a critical role in the process. In fact, most clouds owe their existence to aerosols that serve as the tiny “seeds,” called cloud condensation nuclei.
Natural aerosols—often sulfates, sea salt or ammonium salts—are the most common condensation nuclei in pristine environments. Polluted air, in contrast, usually contains much higher concentrations of water-soluble particles, which means pollution-rich clouds tend to have more numerous, but smaller, droplets. The small droplets make polluted clouds look brighter than they would otherwise be. Just as many bits of crushed ice give light more surfaces to reflect off—appearing brighter than a solid cube of ice--if the water in a cloud is divided into a larger number of smaller droplets, it will scatter more light and become more reflective.
Brighter clouds, in turn, block sunlight from reaching Earth’s surface, shading the planet and producing net cooling. This cloud brightening effect—called the “cloud albedo effect”--may have a big impact on the climate, though only in recent years has it been possible to start quantifying the effect.
(left) Clouds with low
aerosol concentration and a few large droplets do not scatter light well, and allow much of the Sun's light to pass through and reach the surface.
(right) The high
aerosol concentrations in these clouds provide the nucleation points necessary for the formation of many small liquid water droplets. Up to 90% of visible radiation is reflected back to space by clouds without reaching Earth's surface.
aerosol concentration and a few large droplets do not scatter light well, and allow much of the Sun's light to pass through and reach the surface.
(right) The high
aerosol concentrations in these clouds provide the nucleation points necessary for the formation of many small liquid water droplets. Up to 90% of visible radiation is reflected back to space by clouds without reaching Earth's surface.
Current estimates suggest the cooling driven by aerosol indirect effects is less than half as much as the warming caused by greenhouse gases when averaged over the globe. But these indirect effects are highly uncertain and vary considerably in space and time. Therefore, on smaller space and time scales, the climate effects of aerosols can be significant.
The details of aerosol indirect effects are only partially understood, as most instruments cannot measure aerosols within clouds. Climatologists consider the role of clouds to be the largest single uncertainty in climate prediction. Less than a third of the models participating in the Fourth Intergovernmental Panel on Climate Change (IPCC) included indirect aerosol effects, even in a very limited way, and those considered only sulfate aerosols.
The details of aerosol indirect effects are only partially understood, as most instruments cannot measure aerosols within clouds. Climatologists consider the role of clouds to be the largest single uncertainty in climate prediction. Less than a third of the models participating in the Fourth Intergovernmental Panel on Climate Change (IPCC) included indirect aerosol effects, even in a very limited way, and those considered only sulfate aerosols.