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Aerosols and Clouds: Global Dimming

Aerosols from human activities like burning fossil fuels and biofuels have caused global dimming by reflecting sunlight since the 1960s. Aerosols also indirectly influence clouds in ways that reflect more sunlight. However, aerosol levels have declined globally since 1990, reducing their masking of greenhouse gas warming. Black carbon deposited on snow and ice absorbs sunlight and increases melting. Humans have converted over a third of the Earth's land to agriculture, reducing forests, which affects climate through changes in albedo, evaporation, and wind patterns. Deforestation is a major cause of land surface change impacting global warming.

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51 views2 pages

Aerosols and Clouds: Global Dimming

Aerosols from human activities like burning fossil fuels and biofuels have caused global dimming by reflecting sunlight since the 1960s. Aerosols also indirectly influence clouds in ways that reflect more sunlight. However, aerosol levels have declined globally since 1990, reducing their masking of greenhouse gas warming. Black carbon deposited on snow and ice absorbs sunlight and increases melting. Humans have converted over a third of the Earth's land to agriculture, reducing forests, which affects climate through changes in albedo, evaporation, and wind patterns. Deforestation is a major cause of land surface change impacting global warming.

Uploaded by

asif
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Aerosols and clouds

Air pollution, in the form of aerosols, not only puts a large burden on human health, but also affects the
climate on a large scale.[77] From 1961 to 1990, a gradual reduction in the amount of sunlight reaching the
Earth's surface was observed, a phenomenon popularly known as global dimming,[78] typically attributed to
aerosols from biofuel and fossil fuel burning. [79] Aerosol removal by precipitation gives tropospheric aerosols
an atmospheric lifetime of only about a week, while stratospheric aerosols can remain in the atmosphere for
a few years.[80] Globally, aerosols have been declining since 1990, meaning that they no longer mask
greenhouse gas warming as much.[81]

In addition to their direct effects (scattering and absorbing solar radiation), aerosols have indirect effects on
the Earth's radiation budget. Sulfate aerosols act as cloud condensation nuclei and thus lead to clouds that
have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with
fewer and larger droplets.[82] This effect also causes droplets to be more uniform in size, which reduces the
growth of raindrops and makes clouds more reflective to incoming sunlight. [83] Indirect effects of aerosols are
the largest uncertainty in radiative forcing. [84]

While aerosols typically limit global warming by reflecting sunlight, black carbon in soot that falls on snow or
ice can contribute to global warming. Not only does this increase the absorption of sunlight, it also increases
melting and sea-level rise.[85] Limiting new black carbon deposits in the Arctic could reduce global warming by
0.2 °C (0.36 °F) by 2050.[86]

Land surface change

Deforestation in Brazil in 2016

Humans change the Earth's surface mainly to create more agricultural land. Today, agriculture takes up 34%
of Earth's land area, while 26% is forests, and 30% is uninhabitable (glaciers, deserts, etc.). [87] The amount of
forested land continues to decrease, largely due to conversion to cropland in the tropics. [88] This deforestation
is the most significant aspect of land surface change affecting global warming. The main causes of
deforestation are: permanent land-use change from forest to agricultural land producing products such as
beef and palm oil (27%), logging to produce forestry/forest products (26%), short term shifting cultivation
(24%), and wildfires (23%).[89]

In addition to affecting greenhouse gas concentrations, land-use changes affect global warming through a
variety of other chemical and physical mechanisms. Changing the type of vegetation in a region affects the
local temperature, by changing how much of the sunlight gets reflected back into space (albedo), and how
much heat is lost by evaporation. For instance, the change from a dark forest to grassland makes the surface
lighter, causing it to reflect more sunlight. Deforestation can also contribute to changing temperatures by
affecting the release of aerosols and other chemical compounds that influence clouds, and by changing wind
patterns.[90] In tropic and temperate areas the net effect is to produce a significant warming, while at latitudes
closer to the poles a gain of albedo (as forest is replaced by snow cover) leads to an overall cooling effect. [90]
Globally, these effects are estimated to have led to a slight cooling, dominated by an increase in surface
albedo.[91]

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