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According to an article in Scientific American, "Climate Change: Larch in the Lurch," Another potential positive feedback event is occurring in Siberia. As temperatures increase, larch, deciduous needled trees, retreat north and are replaced by evergreens such as spruce or fir. When larch lose their needles in winter, it exposes more snow on the ground to sunlight than the evergreens do. This means that less light is reflected back into space as the larch are replaced by fir and spruce. The light coming from the sun is the energy that the Earth receives that keeps the planet warm enough for life to exist. But there is a somewhat delicate balance. The amount of heat that is retained is determined by how much of the light is absorbed and reflected. As the amount of light that is absorbed increases, the Earth tends to get warmer. The more light that is reflected, the cooler the Earth will tend to be. The overall reflectivity of the Earth is known as the albedo. Larch trees allowing more snow to be exposed during the winter increases the albedo. Fir and Spruce trees covering up the snow reduces the albedo. Warming temperatures tend to reduce the area covered by larch trees and increase the area covered by fir and spruce. Fewer larch and more evergreens mean a lower albedo, more light is absorbed which leads to more warming. This is what is known as a positive feedback loop. An initial warming leads to more warming. It tends to accelerate the rate of temperature increase of the Earth. It makes global warming happen faster. There are negative feedback loops as well. These tend to decrease the rate of warming as temperatures rise. Clouds have both positive and negative feedback effects. Sunlight is reflected from the tops of clouds, as everyone knows. As temperatures rise, more water evaporates and creates more clouds. More clouds mean more light is reflected, i.e., the Earth's albedo increases, tending to slow the warming. However, clouds also trap more of the light energy that is transmitted through the clouds which increases the warming effect of the sunlight. For very thick clouds, the amount of light that is reflected exceeds the amount of light that is trapped, so overall, thick clouds had a cooling effect. With very thin clouds, such as high cirrus clouds or contrails, the effect is not as clear. Measurements seem to indicate that the overall effect is a positive feedback. It thus is important to know the proportion of thick and thin clouds to determine whether the overall effect of increasing cloud cover due to warming is positive or negative. This remains an area of active research.
Aerosols also affect whether clouds have a net positive or negative feedback effect. Aerosols consist of tiny particulates, one to ten microns (micrometers - millionths of a meter), that are emitted into the atmosphere. The first effect of aerosols is to provide "seeds" for condensation of water droplets (technically, these are called nucleation sites). The more particulates there are, the number of droplets in the clouds increase and the size of the droplets decrease; both of these effects increase the reflectivity (albedo) of the clouds and cause a net cooling effect. However, particulates also absorb sunlight, warming the air, raising the dew point and causing the clouds to dissipate and a net warming effect. Modeling based on measurements made over the Amazon have shown that neither effect always predominates. Instead, there is an ebb and flow of which effect predominates at any time. At lower levels of aerosols, the first effect, increased reflection and cooling, tends to predominate while at higher aerosol concentrations, the evaporation of clouds and warming is predominant. However, this is just one study and more work still needs to be done. This study does lend a cautionary note to those who wish to engage in geoengineering and inject aerosols, usually sulfur compounds are recommended. Without further study and a better understanding of the interplay of the many factors influencing the atmosphere, the risk of unintended consequences remains uncomfortable high.
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