Page 378 - Earth's Climate Past and Future

354 PART V • Historical and Future Climate Change

FIGURE 19-12 4 × CO world The
2
4 × CO world that may come into
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existence centuries from now would
be similar to the one that existed 50
or more million years ago, with little
or no sea ice or mountain glaciers
?
and forests growing at high Arctic
? ? latitudes.

?
?

No mountain glaciers left Conifer forest replacing tundra
Winter sea ice only ? Greener deserts
All ice sheets melting More evaporation

A likely consequence of such a circulation change the warming at middle latitudes (40°–60°N) where many
would be colder temperatures in northern Europe. humans in North America, Europe, and eastern Asia live.
Today the heat extracted from North Atlantic surface Temperature increases in these latitudes will be at least
waters during the formation of deep water is carried 3°–3.5°C for the annual mean and 4°–4.5°C or more
eastward and helps to keep Europe warmer in winter during the coldest winter months (Figure 19–13).
than North American or Siberian regions at the same These temperature changes can best be understood
latitude. Without this heat, Europe would become more by comparing them against modern seasonal changes.
like northern Canada, perhaps 5°C or more colder than Today, on landmasses at 40°–60°N latitude, mean daily
it is now. temperatures of ~0°C (32°F) in January rise to ~25°C
Recent reassessments suggest, however, that a com- (77°F) by July and fall back again the following winter.
plete cutoff of deep-water formation in the Atlantic is This 25°C shift over 6 months amounts to an average
unlikely. The planetary wind system will continue to change of 4.25°C (7.5°F) per month, approximately the
drive relatively warm and salty surface water northward same amount as the anticipated cold season temperature
from the tropical Atlantic Ocean, where relatively cold increase at high latitudes caused by a CO doubling. As
2
winter air masses will continue to extract heat. Partial a result, the temperature response of northern mid-
reductions (or relocations) of deep-water formation latitudes to a CO doubling should feel like a one-month
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could occur, but they are unlikely to offset the opposing shift of the seasons. Future Aprils will be like mod-
effect of the ongoing warming in Europe. ern Mays and future Novembers like modern Octobers
(Figure 19–14). Summer will last for an extra two
months, while winters will be shorter and less harsh.
19-7 How Will Greenhouse Warming Change
These changes will come on slowly enough and be
Human Life?
sufficiently masked by typical year-to-year variability
The projected climatic transformations in the 2 × CO that the trend should not be readily apparent to a per-
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and 4 × CO worlds are dramatic, but they omit an every- son whose life spans the rest of this century. If that same
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day perspective on how climate will change for people person fell into a Rip Van Winkle sleep of 90 years,
living at middle latitudes, including the children and however, the changes on awakening would be striking,
grandchildren of the students now reading this book. although no doubt overshadowed by far more bewilder-
Consider the 2 × CO world with a 2.5°C global ing societal and cultural transformations.
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mean temperature increase. Poleward amplification of Outside this somewhat narrow mid-latitude per-
this temperature change by albedo feedback will increase spective, Earth’s responses to climate changes will vary