7. Human-made tropospheric aerosols
Description
This article is from the Climate Change FAQ, by Jan Schloerer jan.schloerer@medizin.uni-ulm.de with numerous contributions by others.
7. Human-made tropospheric aerosols
Aerosols are tiny (0.001 to 10 micrometres) airborne particles. In the
troposphere, the lower about 10 to 15 km of our atmosphere, human-made
aerosols have greatly increased since about 1850. They present a large
source of uncertainty in assessing human influences on climate.
`Fine' aerosol particles with sizes between about 0.1 and 1 micrometre
can influence climate in two ways. Under clear sky they scatter and
absorb solar radiation; some of the scattered sunlight goes back to
space (the direct effect). Acting as cloud condensation nuclei, they
may enhance reflectivity and life-time of clouds (indirect effect).
Sulfur dioxide from fossil fuel burning, yielding sulfate particles
after oxidation, is presently the largest source of fine human-made
aerosols. Another large source is organic and elemental carbon from
burning of tropical forests and savannahs. Globally averaged, fine
human-made tropospheric aerosols may currently cancel about 50 % of
the warming effect of human-made greenhouse gases. So far, though, the
uncertainty range is large, stretching from roughly 10 to 100 %. [IPCC
94, sections 3, 4.4, 4.7] [IPCC 95, sections 2.3, 2.4.2] [Schwartz]
Moreover, global averages are misleading. Even if the global averages
of aerosol and greenhouse gas forcing cancel, their different distri-
butions may cause climatic changes. With life-spans of up to over
100 years, human-made greenhouse gases are fairly evenly distributed.
Most tropospheric aerosols are washed out after about a week, they are
unevenly distributed. Human-made sulfate aerosols occur mainly down-
wind of northern industrialized areas. Most biomass smoke rises from
tropical land areas during the dry season. Cutting back sulfur dioxide
emissions or biomass burning reduces the aerosol load quickly, leaving
over the more longlived greenhouse gases. [Andreae] [IPCC 94 and 95]
By the way, roughly one third of the tropospheric sulfate load has
natural precursors, mainly oceanic dimethyl sulfide (DMS) and volcanic
sulfur dioxide. Violent volcanic eruptions, like Pinatubo 1991, give
rise to stratospheric sulfate aerosols which, being more long-lived
than their tropospheric cousins, tend to warm the stratosphere and to
cool the troposphere and surface for a few years. [IPCC 94, p 135-7,
141-4, 186-9] [IPCC 95, p 115-6, 148, 504-6]
'Coarse' aerosols with particle sizes between 1 and 10 micrometres
include mineral dust raised by wind blowing over dry soils. Human
influences like over-cultivation and soil erosion may have up to
doubled the flux of mineral dust. Mineral dust is most abundant over
North Africa, the Arabian Sea, and South Asia. It scatters sunlight
and absorbs outgoing terrestrial infrared. One study suggests that
these two effects largely cancel at the top of the atmosphere. If so,
mineral dust has little effect on earth's overall radiation balance,
although it regionally cools the surface and warms the air, which in
turn may affect atmospheric circulation. However, as with sulfate
aerosols and biomass smoke, there are large uncertainties. [Andreae]
[Sokolik] [Tegen]
Pinning down aerosol effects more precisely will be tough. Aerosols
are hard to measure. Size, shape, composition and regional distribu-
tion of the particles vary. So do their effects on climate. Aerosols
can cause not just local but also distant responses, because heat or
rather, in the case of many aerosols, coolness is transported by the
atmosphere and ocean. Assessing the climatic effects of aerosols
involves modeling of regional climates and of clouds, both of which
are not yet very reliable. [Andreae] [IPCC 94/95] [Peter] [Schwartz]
[Sokolik] [Wielicki, p 2127-29, 2146]
Continue to:
My Books
