Page 375 - Earth's Climate Past and Future

CHAPTER 19 • Future Climatic Change 351

FIGURE 19-9 2 × CO world The
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2 × CO world likely to exist by the year
2
2100 will in many ways be similar to the
world that existed 10 million years ago,
with less sea ice and permafrost in
polar regions, far fewer mountain
? glaciers, and greener scrub vegetation
? ? in some semiarid desert regions.

?
?

?

Mountain glaciers and Greenland and West Antarctic
permafrost melting ice sheets melting
Sea ice retreating East Antarctic ice sheet growing?
Forests moving north
Greener deserts
More evaporation

most glaciers should completely disappear in a 2 × CO will be affected in future centuries and millennia by
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world. The melting ice will contribute to a further ocean acidification. The ocean has an “alkaline” pH
global rise in sea level in the future. that ranges from 7.8 to 8.5, compared to the 7.0 pH
The disappearance of mountain glaciers will have boundary between acid and alkaline conditions. Because
consequences for water supplies in some arid regions. In the pH scale is logarithmic, a shift from 8.0 to 7.0 indi-
many arid areas, winter rains deliver water during a time cates a tenfold increase in relative acidity (or decrease in
when crops cannot be grown. Water during the early alkalinity).
part of the growing season comes from melting of win- Over the several thousand years of deforestation and
ter snows on nearby mountains. Later in the growing the past few hundred years of other carbon emissions,
season, runoff from melting glaciers becomes a major humans have already added ~120 billion tons of carbon
source of water. In a 2 × CO world, higher evaporation to the ocean, and the average pH has shifted by 0.1 unit.
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will put more stress on water supplies. Mountain snow By 2100, the pH will have shifted by another 0.2 unit or
packs will be thinner because more winter precipitation more, in effect doubling the acidity level of the ocean. As
will instead fall as rain. In addition, runoff from moun- this happens, snaillike organisms called pteropods with
tain glaciers will end when the glaciers disappear. As a carbonate shells made of aragonite (a form of CaCO )
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result, mountains will store and deliver less summer sea- will have increasing trouble producing shells, as will
son water for irrigation and other human uses. some species of corals. Coral reefs will also face more
Fauna and flora on mountainsides will also be frequent episodes of “bleaching,” with the unusual
affected by warming. To remain in an optimal tempera- warmth during large El Niño years killing temperature-
ture regime, both will have to shift to higher elevations, sensitive species.
and the transition will be easier for relatively mobile life The slowest-responding parts of the climate system
forms than less mobile ones. In some cases, the warm- are the ice sheets on Greenland and Antarctica. Prior to
ing may push the preferred environment “off the top” ~7 million years ago, no ice sheet existed on Greenland,
of the mountains and cause species extinctions. apparently because temperatures were too warm to per-
The ocean (especially the deep ocean) is one of the mit ice accumulation at near-polar latitudes. In the future,
slower-responding parts of the climate system, but it we face a different situation: we start off in a world with