Page 246 - Earth's Climate Past and Future

222 PART IV • Deglacial Climate Change

Distribution of elm pollen forest in the north near the ice sheets and grassy steppe
vegetation prevailing farther to the south and east.
One reason for this harsh glacial climate was the clock-
wise outflow of cold winds from the Scandinavian ice
sheet (see Figure 12–11B). A second reason was the
large chilling of the North Atlantic Ocean, which
removed its moderating influence on winters in Europe
(see Figure 11–3).
One of the most striking features of the last glacial
maximum was the vast extent of steppe and tundra that
covered much of northern Asia (Figure 12–16). A region
covered today by forests of larch, birch, and alder trees
A Glacial observed B Glacial simulated was at that time a treeless expanse of grasses and herbs.
<1% 1–2% 2–5% >5%

FIGURE 12-14 Data-model mismatch in the southeastern
United States (A) Observed abundances of warm-adapted
deciduous pollen such as elm in the southeastern United
States during the glacial maximum are smaller than (B) the
amounts simulated by climate models. (Adapted from T. Webb
III et al., “Late Quaternary Climate Change in Eastern North
America: A Comparison of Pollen-Derived Estimates with Climate
Model Results,” Quaternary Science Reviews 17 [1998]: 587–606.)

This mismatch suggests that the model-simulated
cooling for the southeastern United States underrepre-
sents the cooling that actually occurred by permitting too
many warm-adapted trees. One cause of this mismatch
may have been an unusual geographic configuration in
which cold meltwater from the southern margin of the
great Laurentide ice sheet flowed down the Mississippi
River and emptied directly into the Gulf of Mexico at A Modern vegetation
subtropical and tropical latitudes. If the sea-surface Ice Boreal forest Mediterranean scrub
boundary conditions used in the model had incorporated Tundra and Deciduous Prairie-steppe
mountain and conifer forest
this cold inflow, the model might have simulated cooler
temperatures across a broad region of the southeastern
United States influenced by air masses from the nearby
Gulf. Disagreements like these in initial data-model
comparisons point the way toward future improvements
in model simulations and data interpretation.
Changes in Eurasia Europe was completely trans-
formed at the glacial maximum. The conifer and decid-
uous forests typical of today’s interglacial climate
(Figure 12–15A) were absent from most of Europe
south of the Scandinavian ice sheet. In their place,
grass-covered steppes and herb-covered tundra vegeta-
tion covered much of the continent, with bits of forest
scattered in the south (Figure 12–15B). The moderate B Glacial vegetation
maritime climate of today was preceded by a far harsher
FIGURE 12-15 Glacial north-central Europe was treeless
continental climate, more like that of modern northern (A) Vegetation in modern Europe is dominated by forest, with
Asia. These differences in vegetation agree with the dry, conifers in the north and deciduous trees to the south. (B) At
windy conditions indicated by the greater prevalence of the glacial maximum, Arctic tundra covered a large area south
windblown loess (see Figure 12–4). of the ice sheet, with grassy steppe farther south and east and
Biome models simulate glacial vegetation in Europe patchy forests near the Mediterranean coasts. (Adapted from
similar to that observed, with Arctic tundra instead of R. F. Flint, Glacial and Quaternary Geology [New York: Wiley, 1971].)