Page 309 - Earth's Climate Past and Future

CHAPTER 15 • Humans and Preindustrial Climate 285

sea level and moved down the interior of North Amer- One criticism of the hunting hypothesis is that peo-
ica east of the Rockies. They passed through the ice- ple were too few in number to have caused so many
free corridor opened by early melting and separation of extinctions, but studies from population models refute
the Laurentide ice sheet to the east and the smaller this criticism. Because reproduction (gestation) times
Cordilleran ice sheet over the Canadian Rockies (see for large mammals are long, hunters only need to cull
Figure 13–2). an extra 1–2% of a species per year to drive them extinct
This view is now in dispute. Scattered but still dis- within a single millennium. In addition, these people
puted evidence hints at the arrival of humans 30,000 or worked in groups to drive animals to their death over
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40,000 years ago, although most undisputed C dates steep cliffs at the end of narrow bluffs. So many animals
still support a much later arrival. Another challenge is were killed that only a fraction were used for food and
that these people could have arrived by water, either clothing.
traveling along northeastern Pacific coastlines or cross- Another criticism of the hunting hypothesis is that
ing at lower latitudes from eastern Asia. some creatures that do not seem likely to have been
Whatever the date of the first humans in the Amer- hunted also went extinct, including large meat-eating
icas, a new hunting technology appeared at the same mammals that may have preyed on humans rather than
time that the extinctions occurred (12,500 years ago). becoming their prey. A plausible response is that carni-
Many archeological sites that date near 12,500 calendar vores that depended on the carcasses of large mammals
years ago contain spears fitted with a new and elegant for food may have gone extinct because much of their
kind of point fashioned by humans (Figure 15–13). This natural prey had gone extinct at the hands of humans.
new technological development could have helped peo- A still-unanswered criticism of the hunting hypothesis is
ple hunt large mammals more effectively. that it does not explain why some large mammals that
would seem to have been likely targets for hunters
(moose, musk ox, and one species of bison) survived.

IN SUMMARY, both explanations of the pulse of
extinctions have their critics, but the absence of any
extinction pulse during all the previous ice-age
cycles is a powerful argument against the hypothesis
that climate was responsible. The cause appears to
have been humans.

15-7 Did Early Farmers Alter Climate?
Introducing a controversial early anthropogenic
hypothesis, the marine geologist William Ruddiman
claimed that early agriculture had a substantial impact
on greenhouse gases and on global climate thousands
of years ago, much earlier than previously thought. He
based this claim on the fact that concentrations of
CO and methane had fallen during the initial stages
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of the last four interglaciations, but they instead rose
during the later part of the current interglaciation
(Figure 15–14). Because the rest of this book will take
an increasingly historical approach, the axes of all time
plots from this point on have been rotated to the typical
historical perspective: younger to the right (and warmer
upward).
FIGURE 15-13 Pulse of mammal extinctions Woolly
mammoths and other large mammals abruptly became extinct Deforestation is the proposed explanation of the
in North America near 12,500 years ago. Distinctive grooved anomalous rise in CO that began near 8000 years ago.
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spear points (“Folsom points,” named for the site in New Stone Age humans with flint axes began to cut the
Mexico where they were first found, shown here with bones) forests of Europe, China, and India to create clearings
suddenly appeared during this interval of widespread for growing crops. The first appearance of cereal grains
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extinction. (Denver Museum of Natural History.) and other crop remains in hundreds of C-dated lake