Look back at the CO2 trend during the last 10,000 years in figure 11.1B. One odd thing is that the rise in CO2 values slows in the last 2,000 years compared to the preceding millennia. This seems strange, given that human populations had increased and technological improvements had continued over this interval. Why wouldn't CO2 concentrations have risen even faster? Even odder was the fact that the CO2 values began oscillating up and down, sometimes dropping by as much 10 parts per million from the general trend. Because these wiggles are much larger than any possible errors in measuring the CO2 concentrations, they must be real.
One seemingly likely explanation was that these wiggles were the result of natural changes in climate occurring over decades to centuries. Short-term oscillations are well known from climatic records during times when Northern Hemisphere ice sheets were large, but also when ice sheets were absent from North America and Europe. The cause of these fluctuations is not completely understood, but two plausible factors are sporadic volcanic explosions and small changes in the brightness of the Sun. Other evidence, however, convinced me that these natural variations in climate could not explain the large dips in CO2 during the last 2,000 years (chapter 12). It then occurred to me that the explanation might lie in some process tied to human activities. After all, the oscillations were superimposed on a rising CO2 trend that I had concluded was caused by humans over thousands of years. Could the answer lie in a process that reversed that trend, perhaps one that had killed enough humans to reverse the process of gradual deforestation for several decades or centuries? This idea sent me into the history books, where I stumbled on the forbidding image of the four horsemen of the apocalypse. One of these (pestilence, or disease) proved to be the most plausible culprit (chapter 13).
Out of this exploration emerged one more challenge to current thinking: several times in the last 2,000 years, major pandemics had literally "put a plague on" (killed) tens of millions of people, reversing the gradual clearance of forests for agriculture, and contributing to short-term climatic cooling, including the Little Ice Age interval between 1300 and 1900. Disease had become a factor in climate change.
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