Environmental Engineering Reference
In-Depth Information
ing effect,
by absorbing and releasing heat into the tro-
posphere, or a
cooling effect,
by reflecting more sunlight
back into space.
An increase in thick and continuous clouds at low
altitudes can
decrease
surface warming by reflecting
and blocking more sunlight. In contrast, an increase in
thin and discontinuous cirrus clouds at high altitudes
can warm the lower troposphere and
increase
surface
warming.
In addition, infrared satellite images indicate that
the wispy condensation trails (contrails) left behind by
jet planes might have a greater impact on the tempera-
ture of the troposphere than scientists once thought.
NASA scientists found that jet contrails expand and
turn into large cirrus clouds that tend to release heat
into the upper troposphere. If these preliminary re-
sults are confirmed, emissions from jet planes could be
responsible for as much as half of the tropospheric
warming in the northern hemisphere.
Some studies suggest that larger amounts of CO
2
in
the atmosphere could increase the rate of photosynthe-
sis in some areas with adequate water and soil nutri-
ents. This would remove more CO
2
from the tropos-
phere and help slow global warming.
However, recent studies indicate that this effect
would be temporary for two reasons.
First,
the increase
in photosynthesis would slow as the plants reach matu-
rity and take up less CO
2
from the troposphere.
Second,
carbon stored by the plants would be returned to the at-
mosphere as CO
2
when the plants die and decompose
or burn.
Science: Effects of a Warmer Troposphere
on Methane Emissions
Warmer air can release methane gas stored in bogs,
wetlands, and tundra soils and make the air even
warmer.
Global warming could be accelerated by an increased
release of methane (a potent greenhouse gas; see
Table 16-1) from two major sources: bogs and other
wetlands and ice-like compounds called
methane hy-
drates
trapped beneath arctic permafrost. Significant
amounts of methane would be released into the tro-
posphere if the permafrost in tundra and boreal forest
soils melts, as is currently occurring in parts of
Canada, Alaska, China, and Mongolia. The resulting
tropospheric warming could lead to more methane re-
lease and still more warming.
Science: Effects of Outdoor Air Pollution
Aerosol pollutants and soot produced by human
activities can warm or cool the atmosphere, but such
effects will decrease with any decline in outdoor air
pollution.
Aerosols (microscopic droplets and solid particles) of
various air pollutants are released or formed in the
troposphere by volcanic eruptions (Figure 16-1) and
human activities. They can either warm or cool the air
depending on factors such as their size and the reflec-
tivity of the underlying surface.
Most tropospheric aerosols, such as sulfate parti-
cles produced by fossil fuel combustion, tend to cool
the atmosphere and thus temporarily slow global
warming. However, a recent study by Mark Jacobson
of Stanford University indicated that tiny particles
of
soot
or
black carbon aerosols
—produced mainly from
incomplete combustion in coal burning, diesel en-
gines, and open fires—may be the second biggest
contributor to global warming after the greenhouse
gas CO
2
.
Climate scientists do not expect aerosol pollutants
to counteract or enhance projected global warming
very much in the next 50 years for two reasons.
First,
aerosols and soot fall back to the earth or are washed
out of the lower atmosphere within weeks or months,
whereas CO
2
and other greenhouse gases remain in
the atmosphere for decades to several hundred years.
Second,
aerosol inputs into the atmosphere are being
reduced—especially in developed countries.
16-4 POSSIBLE EFFECTS
OF A WARMER WORLD
Science: Projected Effects
of a Warmer Troposphere
A warmer climate would have beneficial and harmful
effects, but poor nations in the tropics would suffer
the most.
A warmer global climate could have a number of
harmful and beneficial effects (Figure 16-10, p. 376) for
humans, other species, and ecosystems, depending
mostly on location and the rate at which the climate
changes.
Some areas will benefit because of less severe win-
ters, more precipitation in some dry areas, less precipi-
tation in wet areas, and increased food production.
Also, some plant and animal species adapted to higher
temperatures may be able to expand their populations
and range.
Other areas will suffer harm from excessive heat,
lack of water, and decreased food production. Wildfires
are likely to increase in forest and grassland areas where
the climate becomes drier. Also, tree deaths would in-
crease from larger disease and pest populations that
would thrive in areas with a warmer climate. Many
Science: Effects of Higher CO
2
Levels
on Photosynthesis
Increased CO
2
in the troposphere could increase plant
photosynthesis, but several factors can limit or offset
this effect.
