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(3) A wide array of archeological, cultural, historical and geologic evidence points to viable explanations tied to anthropogenic changes resulting from early agriculture in Eurasia, including the start of forest clearance by 8000 years ago and of rice irrigation by 5000 years ago. (1) Cyclic variations in CO 2 andCH 4 driven by Earth-orbital changes during the last 350,000 years predict decreases throughout the Holocene, but the CO 2 trend began ananomalous increase 8000 years ago, and the CH 4 trend did so 5000 years ago.(2) Published explanations for these mid- to late-Holocene gas increases basedon natural forcing can be rejected based on paleoclimatic evidence. This hypothesis is based on three arguments. A different hypothesis is posed here: anthropogenic emissions of these gases first altered atmospheric concentrations thousands of years ago. The anthropogenic era is generally thought to have begun 150 to 200 years ago, when the industrial revolution began producing CO 2 andCH 4 at rates sufficient to alter their compositions in the atmosphere.
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