The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 285

during the Pleistocene epoch. By changing tempera-
tures, the Milankovitch cycles also caused the corre-
sponding cyclical changes in CO 2 (g) and CH 4 (g) in
Figure 12.20.
Forexample, higher temperatures decrease the solu-
bility of CO 2 (g) in seawater, increasing evaporation of
CO 2 (g) from seawater to the air. Changes in temperature
also change vertical mixing rates of ocean water, nutri-
ent uptake rates by phytoplankton, and rates of erosion
of continental shelves (which affect biomass loadings),
all of which feed back to change CO 2 (g) in the air (e.g.,
Crowley and North, 1991).
Similarly, higher temperatures increase the microbi-
ological production of methane by methanogenic bac-
teria (Section 2.3.3.2). Also, higher temperatures melt
ice faster in permafrost soil ,which is soil that remains
below the freezing point of water, 0 ◦ C, for 2 or more
years. The melting of permafrost soil releases methane
stored under it to the air. Similarly, warmer ocean tem-
peratures cause methane hydrates ,which are com-
pounds containing methane trapped in ice that form
under high pressure and low temperature deep in the
ocean, to melt and release their methane into ocean
water. Dissolved methane then bubbles to the surface
and escapes to the atmosphere.
In sum, the natural changes in carbon dioxide and
methane seen in Figure 12.20 were primarily a response
to changes in temperature rather than a cause of changes
in temperature. The situation today differs, where
increases in carbon dioxide, methane, and other chem-
icals emitted by humans are causing warming above
what nature would cause and at a rate much more rapid
than in the historical record, as discussed shortly.
Figure 12.20 shows that a temperature minimum two
ice ages ago occurred about 150,000 y.a. Near that time,
glaciers extended down to Wisconsin in the United
States, and possibly farther south in Europe (Kukla,
1977). About 130,000 y.a., temperatures increased
again, causing deglaciation. Over the Antarctic, temper-
atures rose 2 ◦ Cto3 ◦ C above what they are today (Figure
12.20). As the eccentricity of the Earth's orbit increased,
temperatures decreased again, causing renewed glacia-
tion. During this period ( the last glacial period ), two
major stages of glaciation occurred, the first starting
115,000 y.a. and the second 75,000 y.a. The second
stage continued until about 12,000 years ago.

5

Holocene maximum

Greenland

0

5 -5

Younger-Dryas

0 -10

Antarctic

-5

-10

20 15 10 5 0

Thousands of years before present

Figure 12.23. Temperature variation in the Northern
Hemisphere (top line) and in the Antarctic (bottom
line) during the past 20,000 years. The deviations for
the ice core data are relative to a modern surface air
temperature over the ice of

55 ◦ C. The Greenland
data are from North Greenland Ice Core Project
Members (2004). Antarctic data are from the Vostok
ice core (Jouzel et al., 1987, 1993, 1996; Petit et al.,
1999).

−

period is referred to as the Wisconsin , Weichselian , and
W urm ,respectively. During the last glacial maximum,
an ice sheet called the Laurentide ice sheet covered
North America, and another called the Fennoscandian
ice sheet covered much of northern Europe. These ice
sheets were about 3,500 to 4,000 m thick and drew
up enough ocean water to decrease the sea level by
about 120 m (CLIMAP Project Members, 1981; Fair-
banks, 1989). The decrease in sea level was sufficient to
expose land connecting Siberia to Alaska, creating the
Bering land bridge .Thisland bridge allowed humans
to migrate from Asia to North America and, ultimately,
to Central and South America. The Laurentide ice sheet
extended from the Rocky Mountains in the west to the
Atlantic Ocean in the east, but only as far south as the
Missouri and Ohio valleys. The melting of the Lauren-
tide ice sheet resulted in the flooding of significant land
off the East Coast of North America, creating shallow
ocean water out to a great distance offshore.
Temperatures during the last glacial maximum were
5 ◦ Cto8 ◦ Clower than they are today over the Northern
Hemisphere and 8 ◦ Clower than they are today over
the Antarctic (Figure 12.23). During the last glacial
maximum, Antarctic ice area expanded as did Arctic
sea ice area. In the tropics, precipitation decreased,
reducing inland lake and river levels. Globally, near-
surface winds may have been 20 to 50 percent faster
than those today. CO 2 (g) mixing ratios were about
200 ppmv (Figure 12.20), almost half their current

12.3.2.11. From 20,000 to 9,000 Years Ago
The last glacial maximum occurred from 22,000 to
12,000 y.a. (Figure 12.23). For glaciation over eastern
North America, western Europe, and the Alps, this