Page 311 - Earth's Climate Past and Future

CHAPTER 15 • Humans and Preindustrial Climate 287

wetlands because they were shrinking as the monsoon
weakened (see Figures 13–13 and 13–16). Other studies
indicate that the boreal wetlands could not have been
the source of the extra methane, in part because Arctic
summers were growing colder (see Figure 13–19), and in
part because the wetlands in that region were slowly
changing to a type that emitted less methane. Neither
tropical nor boreal wetlands make sense as the cause of
the anomalous methane rise.
Humans are the other possibility. Several human
activities generate methane, including biomass burning,
tending of livestock, and production of human waste,
but the abrupt reversal in the methane trend seems to
require a more dramatic explanation. Rice irrigation
began in southeast Asia between 7000 and 6000 years
ago and expanded rapidly by 5000 years ago, the same
time the methane concentration began to rise. Rice and
weedy plants grow in standing water and then die and
rot. As the carbon in the plant remains is oxidized, the
water loses its oxygen and begins to emit methane (with
reduced carbon) into the atmosphere.
Irrigation had spread across all of Southeast Asia
from China to India by 3000 years ago, as the CH trend
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continued to rise. By 2000 to 1000 years ago, the Asian
people were beginning to construct small rice paddies
even on steep hillsides (Figure 15–16). The fact that so
much effort was being put into adding so little cultivated
area suggests that many flat valley floors were already in
irrigation by this time. Rice is a highly nutritious food,
and the success of Asians in growing this crop must have
led to rapid increases in population. By 2000 years ago,
census data show that 50 million people already lived in FIGURE 15-16 Hillside rice paddies By 2000 years ago,
China. Emissions from livestock, biomass burning, and Asians were constructing rice paddies on steep hillsides like
human waste must have risen accordingly. these in Guizhou Province, China. (Georg Gerster/Photo
Critics of the early anthropogenic hypothesis ques- Researchers, Inc.)
tion whether human activities can explain the large
anomalies in CO (35–40 ppm) and methane (~225–250
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ppb) that had developed prior to the 1800s (see Figure called on to explain the rise in gas concentrations dur-
15–14). Underlying this criticism is the reaction that ing this interglaciation? If the natural trend is down,
too few humans were present on Earth to have taken then an upward trend cannot be natural.
control of atmospheric greenhouse-gas trends so many Another implication of the early anthropogenic
millennia ago. This criticism appears to have particular hypothesis is that the anomalous rises in CO and CH
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merit in the case of the large CO anomaly, which can- kept the atmosphere warmer than it would have been if
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not be accounted for even if all the forests of southern nature had remained in control. Throughout the last
Eurasia had been cut by a few centuries ago. And yet, several millennia, the decrease in summer insolation
the anomalies remain, so they still require an explana- across the northern hemisphere cooled the high north-
tion. One possibility now being explored is that positive ern latitudes (see Figure 13–19). If the greenhouse-gas
feedbacks in the climate system have amplified the CO concentrations had fallen as they had done during the
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emissions by humans during the last several thousand previous interglaciations, climate would have cooled
years. even more, perhaps to the point of allowing new ice
Attempts at natural explanations face a seemingly caps to form.
more daunting problem. If natural processes in the cli- Sensitivity tests with general circulation models
mate system caused gas concentrations to fall early in support this idea. If greenhouse-gas concentrations are
the last four interglaciations (all of which occurred prior reduced to the “natural” level predicted by the hypoth-
to agriculture), how can the same natural processes be esis (~240 ppm for CO and ~450 ppb for CH ), some
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