Page 371 - Earth's Climate Past and Future

CHAPTER 19 • Future Climatic Change 347

Even after human CO emissions begin to fall, the amount than CO . Most of the land that can be used to
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slowly decreasing yearly CO emissions continue to grow rice is already in irrigation, and future increases in
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push the atmospheric concentration to even higher lev- the extent of these CH -emitting “rice wetlands” are
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els. Not until about a century after the peak in CO likely to be negligible. As the number of humans on
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emissions does the rate of additional input by humans Earth levels out after mid-century (Figure 19–1), the
drop below the rate of removal by the ocean. At this number of CH -emitting livestock that people tend
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point, the CO concentration begins to decline. should also stabilize.
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Over even longer intervals, other considerations Unlike CO , methane stays in the atmosphere for
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come into play. Over the course of hundreds to thou- only a decade before being oxidized to other gases. One
sands of years, the acidity produced by the CO interesting question is whether a warmer future world
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absorbed in the deep ocean dissolves some of the CaCO will cause large Arctic reservoirs of now-frozen methane
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on the seafloor (Figure 19–4B). This process neutralizes to melt. If that happens, unknown amounts of CH gas
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CO in the ocean. The ultimate fate of our excess CO could be added to the greenhouse effect as a positive
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pulse will be a slow-acting chemistry experiment in the feedback (Box 19–1).
deep ocean. Future additions of SO and carbon aerosols to the
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atmosphere by humans are also difficult to predict. The
positive impact of environmental cleanup efforts in some
19-3 Other Human Effects on the Atmosphere
industrialized nations will be countered to an unknown
Other emissions by humans may also be important. extent by increased burning of sulfur-rich coal in nations
Methane production is likely to increase, but by a smaller still undergoing industrialization. A net increase in SO
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BOX 19-1 CLIMATE INTERACTIONS AND FEEDBACKS
Will Frozen Methane Melt?

ethane exists as a gas in the atmosphere, but in
Increase in
MEarth’s colder regions it also occurs in a frozen form greenhouse gases
known as methane clathrate, a mixture of methane and Global
warming
slushy ice. Clathrates occur in deep-ocean sediments
along continental margins, where the pressure pro- Increased
warming
duced by overlying water and sediments makes CH stable
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at temperatures well above freezing (5°C or more).
Clathrates also occur in the Arctic, both in shallow ocean
sediments and below permafrost on land. The volume of
CH stored in these reservoirs is enormous, far exceed- Melting
4 of frozen
ing the volume in all wetlands and livestock reservoirs
CH 4
combined.
Without major changes in climate, most methane
clathrate will remain trapped in its present form, but Increase in
with the large future warming projected for north polar CH 4 gas
regions, the question is whether or not it will remain
trapped in the slushy ice. Permafrost is expected to con-
Methane clathrate feedback Future warming of the deep
tinue to melt from the top down, but will this warming
coastal ocean and melting of polar permafrost could release
reach deep enough to tap the methane clathrates and frozen methane cause additional global warming.
liberate some of the trapped CH to the atmosphere?
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Because it will take centuries for the higher surface warming of the poles or the deeper ocean causes the
temperatures to penetrate far into permafrost and ocean release of even a small fraction of the vast mass of frozen
sediments, most (but not all) scientists doubt that large methane, it could provide a significant positive feedback
amounts of CH will be released. Still, if future greenhouse to the initial greenhouse-gas warming.
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