Environmental Engineering Reference
In-Depth Information
COOLING OR WARMING?
(1968)—this would be enough to account for the
global cooling which took place between 1940
and 1960. Results such as these were based on
the observation that global cooling often
followed major volcanic eruptions. Particulate
matter produced by human activity was
considered equivalent to volcanic dust and,
therefore, capable of contributing directly to
global cooling. In addition, by elevating
background turbidity levels, it allowed smaller
volcanic eruptions to be more effective in
producing climatic change (Bryson 1968). This
comparison of aerosols of human and volcanic
origin has been questioned, however (Kellogg
1980; Toon and Pollack 1981). Most particulate
matter injected into the atmosphere during
human activities does not rise beyond the
tropopause. As a result its residence time is
limited, and its impact is confined to an area
commonly between 1,000 and 2,000 km
downwind from its source (Kellogg 1980). Most
sources of anthropogenic aerosols are on land,
and there the addition of particles to the
boundary layer tends to reduce the combined
albedo of the surface and the lower atmosphere.
The reduced reflection of incoming radiation then
promotes warming. The opposite effect is
experienced over the oceans, where the combined
albedo is increased, producing greater reflectivity
and therefore cooling (Bolle
et al.
1986).
Differential changes such as these might in time
alter local circulation patterns through their
influence on atmospheric stability.
Little anthropogenically produced
particulate matter enters the stratosphere at
present, but, should that change, the effects
would be greater and more prolonged than
those produced by the tropospheric aerosols
(Bolle
et al.
1986). Stratospheric aerosols alter
the energy budget in two ways. They scatter,
reflect and, to a lesser extent, absorb incoming
solar radiation which reduces the amount of
energy reaching the earth's surface and
contributes to cooling. They also absorb
outgoing terrestrial infrared radiation. The net
result is a warming of the stratosphere.
Information on the warming ability of
In the mid-1970s, increasing atmospheric
turbidity associated with human activity was
considered to be one of the mechanisms capable
of inducing global cooling (Calder 1974; Ponte
1976). The processes involved seemed plausible
and logical, at least in qualitative terms. The
introduction of pollutants into the atmosphere,
at a rate greater than they could be removed by
natural processes, would allow the progressive
build up of aerosols until sufficient quantities had
accumulated to cause a rise in global turbidity
levels. The net result would be a reduction in
insolation values at the earth's surface as more
of the direct beam solar radiation was scattered
or reflected by the particulate matter in the
atmosphere. The ability of some of the aerosols
to act as condensation nuclei would also tend to
increase cloudiness and further reduce the receipt
of insolation. It has been estimated that natural
aerosols in the troposphere probably reduce
global surface temperatures by about 1.5°C
(Toon and Pollack 1981), and results obtained
by atmospheric modelling techniques suggest that
a doubling of the atmospheric aerosol content
would reduce surface temperature by up to 5°C
(Sellers 1973). Global cooling following major
volcanic eruptions would tend to support such
estimates, and other natural events, such as forest
fires, have been linked with regional cooling.
Wildfires in Alberta, Canada in 1982 reduced
average daytime temperatures in the north-
central United States by 1.5-4.0°C, and fires in
China in 1987 were followed by reductions of
2.0-6.0°C in daytime temperatures in Alaska
(Appleby and Harrison 1989). The local cooling
in the Middle East at the height of the Kuwait
oil fires also fits this pattern.
No realistic value for the impact of
anthropogenic aerosols on global temperatures
is available. It has been estimated that a 3 to 4
per cent increase in global turbidity levels would
be sufficient to reduce the mean temperature of
the earth by 0.4°C, and, according to proponents
of anthropogenically produced dust as a factor
in climatic change—such as Reid Bryson
