Page 381 - Earth's Climate Past and Future

CHAPTER 19 • Future Climatic Change 357

many millennia. Part of the anthropogenic warming now-overdue glaciation is underway (Chapter 15). In the
will persist as long as this tail end of the excess CO future, humankind will probably be weighing the merits
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pulse lingers in the atmosphere. of engineering a warmer or a colder Earth.
One technological idea for avoiding adding huge
amounts of CO to the atmosphere is to pipe the excess Epilogue
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industrial CO into the ocean or into old oil fields. As
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with many technological solutions, cost is a major issue. Humans have been responsible for three major alter-
Many CO -emitting sites lie far from the ocean or from ations of Earth’s natural state. The first was the mass
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oil reservoirs, and some of the CO would have to be extinction of large mammals by late Stone Age peoples
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piped hundreds to thousands of miles at considerable cost. near 50,000 and again near 12,500 years ago. The sec-
In addition, CO sent directly into the ocean will increase ond was the clearing of the forests of southern Eurasia
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the acidity of the ocean even faster than present rates. during the last several millennia to open up land for
The atmospheric chemist Paul Crutzen has sug- agriculture. The third and largest has been the wide
gested that sulfate aerosols could be used to offset some range of industrial era changes that have further altered
or all of the CO warming. He proposed that very large Earth’s surface during the past two centuries. Humans
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amounts of sulfate aerosols be added to the stratosphere now move more debris (rock and sediment) than the
to simulate the effects of volcanically emitted aerosols in combined action of all Earth’s rivers, glaciers, and winds.
blocking solar radiation and cooling climate. By adding Estimated future changes in climate will be on a scale
enormous amounts of aerosols, we might be able to cool comparable to the largest natural changes of the past.
Earth’s climate even in the absence of effective action in The projected 5°C warming from the 4 × CO emissions
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reducing emission of CO and other greenhouse gases. scenario would be equivalent to the amount of cooling at
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Because gravity pulls stratospheric aerosols down into the most recent glacial maximum 20,000 years ago. The
the troposphere within a few years (where they are soon projected 2.5°C warming from the 2 × CO emissions
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removed by rainfall), sustaining a high enough concentra- scenario would amount to half of the glacial maximum
tion to offset global warming would require a constant cooling. We are now ~0.7°C of the way into this huge
supply of aerosols, by one estimate half a million tons of new experiment in transforming our planet. Unless tech-
aerosols per year. Several means of delivering the aerosols nology or extreme conservation efforts intervene, Earth
have been proposed: balloons, artillery shells fired into is headed toward a warmer future at rates that are
space, a fleet of high-flying planes larger than any modern unprecedented in its 4.5-billion-year history.
commercial airliners. By one estimate, the cost would be
hundreds of millions of dollars per year, but even this
price would be far lower than the expense in sequestering Key Terms
CO after burning. One of several possible drawbacks is methane clathrate (p. 347) ocean acidification (p. 351)
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the fact that the additional sulfates would add to the acid-
ification of the ocean and of terrestrial environments.
Another recent suggestion is to use genetic manipu- Review Questions
lation to develop microbes that feed on CO and turn it
2 1. What factors will determine how much CO
into some form of carbon other than greenhouse gases. 2
If such a transformation turns out to be possible, we humans add to the atmosphere in the future?
could get rid of one greenhouse gas and turn it into 2. Where will all the excess carbon eventually go?
another one that is a source of energy.
In the distant future, most of the pulse of excess CO 3. In what way will the future CO warming be like
2
2 and unlike past CO warmings?
will have disappeared into the ocean, and climate will have 2
cooled back toward its natural level (see Figures 19–3 and 4. In a 2 × CO world, how many trends under
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19–5) but probably not to a glaciation. Part of the excess way in recent decades are likely to continue?
CO pulse will linger in the atmosphere for tens of thou-
2 5. In a 4 × CO world, where will ice of any kind still
sands of years and keep climate too warm for new ice 2
to accumulate on North America or Eurasia. Ongoing be found on Earth?
methane emissions from irrigated areas and landfills will 6. Will future temperature changes be readily apparent
also prevent cooling to the point of glaciation. to the average person? Why or why not?
In a world where climate can be manipulated,
humankind will face the question of what “natural” is. By 7. What are the disadvantages in drastically reducing
consensus, “natural” might be the climate that existed our industrial emissions of sulfur and carbon?
just prior to the industrial era. An equally viable view is 8. Yes or no: Do you think that future global
that the true natural state would be one in which the warming is a major concern?