Page 251 - Earth's Climate Past and Future

CHAPTER 12 • Last Glacial Maximum 227

The large-cooling view also has problems. The drier while the glacial methane concentration was about half
glacial climate in most of the tropics would have steep- of typical interglacial values of ~700 ppb.
ened the lapse rate from its present 6.5°C/km toward A 3°C drop in tropical temperatures is close to the
the 9.8°C/km rate typical of very dry air. A steeper lapse amount of cooling expected from general circulation
rate in the drier glacial tropics could account for part of model simulations of the tropical response to this low-
the discrepancy between the ocean and land evidence. In ering of greenhouse-gas concentrations. The evidence
addition, the evidence from mountain glaciers is poorly from the last glacial maximum indicates that the models
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dated. Only a handful of regions have C dates that capture the effect of greenhouse gases on climate rea-
closely constrain the glacial lowering of 600–1000 m to sonably well. This match gives climate scientists confi-
the exact glacial maximum time. Some glacial moraines dence that these same models are useful in forecasting
initially thought to date from the glacial maximum future climate changes caused by increases in CO and
2
turned out to have formed before 30,000 years ago other greenhouse gases.
during a time of cooler but also wetter climates. Lower
glacier limits at such times could have been caused at Key Terms
least in part by greater snowfall.
Other factors contribute to the apparent discrepancy CLIMAP (Climate Barents ice sheet
between land and ocean temperature changes. One Mapping and (p. 212)
factor is a result of sea level lowering. A sea level drop of Prediction) Project glacial outwash (p. 214)
110–125 m increases the “height” of the mountains (p. 210) COHMAP (Cooperative
(relative to the new sea level) by that amount. For a lapse Laurentide ice sheet Holocene Mapping
rate of 6.5°C/1000 m, this effect would make high (p. 212) Project) (p. 216)
mountain elevations cooler by ~0.75°C without any Cordilleran ice sheet biome models (p. 218)
actual change in climate. Another factor is the greater (p. 212)
responsiveness of the land than the ocean to climatic Scandinavian ice sheet permafrost (p. 223)
forcing. Climate model simulations of land-surface reac- (p. 212) alkenones (p. 226)
tions to seasonal changes tend to exceed those of the
ocean, which integrates year-round forcing. Still
another factor is the fact that the very low glacial CO Review Questions
2
values may fail to provide trees with enough CO for
2
photosynthesis and thereby cause the upper tree line to 1. What is the major uncertainty about the size of ice
drop even without changes in temperature. sheets at the glacial maximum?
2. Earth’s radius r is 6371 km, and its surface area
3
(4/3 π r ) is 70% water. If sea level was lower by
IN SUMMARY, the most likely resolution to the
controversy over the glacial tropical cooling is that 120 m during the most recent glacial maximum and 2
the cooling of some ocean regions (especially the if the surface of ice sheets was larger (45 million km
Pacific Ocean) was larger than the CLIMAP as against the modern surface area of 15 million
2
estimate, but the cooling in many tropical land areas km ), what was the average thickness of the ice
was not as large as the evidence from the mountains sheets?
suggests. The CLIMAP estimates are also likely to 3. In what ways did ice sheets make the glacial world
be in error along some ocean margins and in nearly a “dirty” place?
enclosed seas where CLIMAP had little or no core
coverage or where glacial maximum planktic 4. How does the composition of pollen in lake
assemblages had unusual combinations of species sediments tell us about climate?
that make it risky to apply techniques based largely 5. How and why did the glacial climate of the
on assemblages from the open ocean. southwestern United States differ from the climate
there today? Did the changes there generally agree
with those in other regions?
What is the implication for Earth’s sensitivity to
CO and other greenhouse gases? We can estimate that 6. How and why did glacial climates of Europe and
2
the actual tropical cooling was about 3°C, roughly northern Asia differ from the climate there today?
midway between the CLIMAP estimate (1.5°C) and
the initial land-based estimate (5°C). And we know 7. What caused the cooling of the tropics during the
from measurements of air bubbles in ice cores that the last glacial period?
glacial atmospheric CO concentration was 90 ppm 8. Explain why a large versus small tropical cooling is
2
lower than the typical interglacial value of ~280 ppm, important for understanding our future.