Page 204 - Earth's Climate Past and Future

180 PART III • Orbital-Scale Climate Change

FIGURE 10-7 Interglacial-to-
glacial changes in carbon reservoirs
Atmosphere During the glacial maximum 20,000
600 420 years ago, large reductions in carbon
(–30%)
biomass occurred in the atmosphere,
Vegetation and soil in vegetation and soils on land, and in
2160 1630 the surface ocean. The total amount
(–25%)
of carbon removed from these
reservoirs (more than 1000 billion
Ocean mixed layer: 1000 700 (–30%) tons) was added to the much larger
reservoir in the deep ocean.

Deep ocean
38,000 39,010
(+2.7%)

One place to look for this “missing” CO carbon is and 30%, but in any case the decrease was substantial.
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the vegetation-soil reservoir. Scientists have abundant Clearly, the carbon removed from the atmosphere did
information on the amount of carbon stored on land in not go into land vegetation, and now we face the added
vegetation and soils during the last glacial maximum, problem of explaining the carbon missing from not just
20,000 years ago. The evidence shows that the conti- the atmosphere but also the land.
nents had less net vegetation cover and held less carbon The only remaining place where the missing carbon
during glaciations than they did during warm inter- could have been stored is in the ocean. But which ocean
glacial intervals like today. reservoir took up the carbon, the small surface reservoir
The main reason for the decrease in glacial carbon or the much larger deep reservoir? The surface ocean is
was the expansion of ice sheets across large areas of not the answer. Ocean surface waters exchange all their
North America and Eurasia that during interglacial carbon with the atmosphere within just a few years.
times like today were covered by forests of conifers and Because of this rapid exchange of CO gas, most areas
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deciduous trees. In addition, other regions that were of the surface ocean today have CO values within
2
forested during warm interglacial times were covered 30 ppm of the value in the overlying atmosphere. Such
by steppe and grassland with lower amounts of carbon rapid exchanges mean that if CO values were 30%
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during glaciations. As a result, the forested regions that lower in the glacial atmosphere, they must have been
survived during full glaciations were smaller than those lower by nearly the same average amount in the glacial
today. surface ocean. This estimate adds another 300 billion
Continental lakes with sediments containing pollen tons to the growing list of carbon that was “missing”
can tell us about past changes of the nearby vegetation. during glacial times (Figure 10–7).
The picture that emerges from these lake core records The only available carbon reservoir left is the deep
is that most regions on Earth were drier and less vege- ocean. The missing glacial carbon removed from the
tated during maximum glaciations than they are today. atmosphere, vegetation, and surface waters must have
Even in the tropics, rain forests were less extensive. One been stored or sequestered there. The total amount of
region where a significant increase in glacial vegetation carbon missing from the other reservoirs adds up to
may have occurred was north of Australia. There, the about 1000 billion tons (see Figure 10–7). This amount
fall of sea level caused by storage of water in glacial ice must have ended up in the deep ocean during the last
sheets exposed large expanses of now-submerged conti- glaciation. But the deep ocean is such an enormous car-
nental shelf, and these regions were probably covered bon reservoir (~38,000 billion tons) that this additional
by tropical rain forests. But this regional increase was carbon would have increased the amount present dur-
not enough to offset losses elsewhere. ing interglacial times by only about 2.7%.
From this evidence, climate scientists estimate that
the total amount of vegetation on land was reduced by 10-5 δ C Evidence of Carbon Transfer
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roughly 25% (from 2160 to 1630 billion tons) during
the last glacial maximum 20,000 years ago (see Figure The amount of carbon transferred from the land to
10–7). These estimated reductions are uncertain, and the deep ocean can be estimated independently from
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the actual reduction could be anywhere between ~15% δ C measurements of foraminifera. Organic carbon in