Page 356 - Earth's Climate Past and Future

332 PART V • Historical and Future Climate Change

Ozone is naturally converted back to oxygen (O ) in the Ozone also occurs naturally in much greater abun-
2
atmosphere by a similar process, but in this case the dance in the lower troposphere. It originates from both
radiation source can be light in either ultraviolet or vis- natural and anthropogenic processes, including biomass
ible wavelengths: burning and oil production in refineries. At these lower
levels in the atmosphere, ozone generally plays a posi-
O + Radiation → O + O
3 2 tive environmental role by cleansing carbon monoxide
(UV or visible) (CO) and sulfur dioxide (SO ) from the air.
2
O + O → 2O At high concentrations, however, ozone is toxic to
3 2
plants and an irritant to human eyes and lungs. In the
With visible radiation far more abundant than ultravio- lowermost atmosphere, ozone trends have moved in the
let radiation, ozone is naturally destroyed much faster opposite direction during the industrial era. Human
than it is produced. As a result, ozone is a short-lived activities have caused large ozone increases that have
gas. In addition, the rate of conversion back to O
2 produced periodic smog alerts in many large cities.
increases when certain chemicals are present to speed up Slow-moving air masses settle over urban areas and
the reaction. Chlorine reacts with ozone and destroys it, allow concentrations of ozone and other pollutants
forming chlorine monoxide (ClO): to build to dangerous levels in summer. Tropospheric

Cl + O → ClO + O ozone acts as another greenhouse gas, and its buildup in
3 2
the troposphere has added about 10% to the industrial
Chlorine then reacts with free oxygen molecules and is era warming of Earth’s surface.
liberated from ClO:
ClO + O → Cl + O IN SUMMARY, the increase in CFC concentrations
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during the late twentieth century has added to the
These liberated chlorine atoms then begin a new cycle greenhouse-gas warming, but the destruction of
of ozone destruction. This cycle is important to humans stratospheric ozone has cooled the planet slightly.
because ozone in the stratosphere forms a natural protec- Increases in tropospheric ozone have contributed to
tive barrier that shields life forms from levels of ultravio- the warming.
let radiation that would otherwise produce cell mutations
including skin cancers.
In the last century, human activities have greatly 18-7 Sulfate Aerosols
accelerated the natural destruction of ozone by adding
extra chlorine to the stratosphere. Measurements from Industrial era smokestacks emit the gas sulfur dioxide
1960 to 1990 showed that the amount of ozone in a col- (SO ) as a by-product of smelting operations in furnaces
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umn of air over Antarctica had decreased considerably and from burning coal. SO reacts with water vapor and
2
in the region where stratospheric chlorine is unusually is transformed into sulfate particles, called sulfate
abundant (Figure 18–9A, B). aerosols. Because these aerosols stay within the lower
The largest decreases occurred high in the Antarc- several kilometers of the atmosphere, their primary
tic stratosphere during the spring season. Isolation impact on climate is regional in scale.
of Antarctic polar air from the rest of Earth’s atmos- Until the 1950s, smokestacks in Europe and North
phere through the winter allows CFCs to accumulate America were small and most SO emissions stayed
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to high levels that rapidly destroy ozone when solar close to ground level, producing thick industrial hazes
radiation increases early in the following spring. The and sulfur-rich acidic air around cites. Building facades
region over Antarctica in which stratospheric ozone and cemetery monuments made of limestone and mar-
is much less abundant than elsewhere is called the ble were deeply etched by these acidic hazes. In the
ozone hole. 1970s, taller smokestacks were built to disperse SO
2
This clear connection between CFCs and ozone emissions higher in the atmosphere (up to 3 km). This
depletion caused so much alarm that the world’s nations effort dramatically improved air quality in many cities,
signed a treaty in Montreal in 1987 to reduce and but it created a different problem in more distant areas.
ultimately eliminate the use of CFCs. Production imme- The sulfate particles that are now being sent higher
diately began to decline (Figure 18–9C), and the con- in the atmosphere are carried by fast-moving winds
centrations of the type of CFCs that industries found across broad areas. Although sulfates stay in the atmos-
easiest to replace stabilized and began a slow decline. phere for only a few days before rain removes them,
Other CFCs that are still in widespread use have contin- they can be carried 500 or more kilometers downwind
ued to increase, but at slower rates. Stratospheric ozone from source regions. Today large plumes of sulfate
levels have stopped falling, but have not yet begun a aerosols are carried far from sources in Eastern Europe,
significant recovery toward natural levels. east-central North America, and China (Figure 18–10).