The Anthropogenic Greenhouse Era Began Thousands of Years Ago

Abstract

The anthropogenic era is generally thought to have begun 150 to 200 years ago, when the industrial revolution began producing CO₂ andCH₄ at rates sufficient to alter their compositions in the atmosphere. A different hypothesis is posed here: anthropogenic emissions of these gases first altered atmospheric concentrations thousands of years ago. This hypothesis is based on three arguments. (1) Cyclic variations in CO₂ andCH₄ driven by Earth-orbital changes during the last 350,000 years predict decreases throughout the Holocene, but the CO₂ trend began ananomalous increase 8000 years ago, and the CH₄ 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. (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. In recent millennia, the estimated warming caused by these early gas emissions reached a global-mean value of ∼ 0.8 °C and roughly 2 °C at high latitudes, large enough to have stopped a glaciation of northeastern Canada predicted by two kinds of climatic models. CO₂ oscillations of ∼ 10 ppm in the last 1000 years are toolarge to be explained by external (solar-volcanic) forcing, but they can be explained by outbreaks of bubonic plague that caused historically documented farm abandonment in western Eurasia. Forest regrowth on abandoned farms sequestered enough carbon to account for the observed CO₂decreases. Plague-driven CO₂ changes were also a significant causal factor in temperature changes during the Little Ice Age (1300–1900 AD).