Most scientists accept the view that human effects on global climate began during the 1800s and have grown steadily since that time. The evidence supporting this view looks quite solid: two greenhouse gases (carbon dioxide, or CO2, and methane, or CH4) that are produced both in nature and by humans began unusual rises like the pattern shown in figure 1.1A. Both the rate of change and the high levels attained in the last 100 to 200 years exceed anything observed in the earlier record of changes from ancient air bubbles preserved in ice cores. Because greenhouse gases cause Earth's climate to warm, these abrupt increases must have produced a warming.
But one aspect of the evidence shown in figure 1.1A is deceptive. Magicians use a form of misdirection in which flashy movements with one hand are used to divert attention from the other hand, the one slowly performing the magic trick. In a sense, the dramatic change since 1850 is exactly this kind of misdirection. It distracts attention from an important rise in gas concentrations that was occurring during the centuries before the 1800s. This more subtle change, happening at a much slower rate but extending very far back in time, turns out to be comparably important in the story of humanity's effects on climate.
I propose that the real story is more like the one shown in figure 1.1B. Slower but steadily accumulating changes had been underway for thousands of years, and the total effect of these earlier changes nearly matched the explosive industrial-era increases of the last century or two. Think of this as like the fable of the tortoise and hare: the hare ran very fast but started so late that it had trouble catching the tortoise. The tortoise moved at a slow crawl but had started early enough to cover a lot of ground.
The tortoise in this analogy is agriculture. Carbon dioxide concentrations began their slow rise 8,000 years ago when humans began to cut and burn forests in China, India, and Europe to make clearings for croplands and pastures. Methane concentrations began a similar rise 5,000 years ago when humans began to irrigate for rice farming and tend livestock in unprecedented numbers. Both of these changes started at negligible levels, but their impact grew steadily, and they had a significant and growing impact on Earth's climate throughout the long interval within which civilizations arose and spread across the globe.
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