The Anthropogenic Greenhouse Era Began Thousands of Years Ago
The anthropogenic era is generally thought to have begun 150 to 200 years ago, when the industrial revolution began producing CO2 and CH4 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 and CH4 driven by Earth-orbital changes during the last 350,000 years predict decreases throughout the Holocene, but the CO2 trend began an anomalous increase 8000 years ago, and the CH4 trend did so 5000 years ago. (2) Published explanations for these mid- to late-Holocene gas increases based on 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 ~0.8oC and roughly 2oC at high latitudes, large enough to have stopped the initial stages of a glaciation of northeastern Canada predicted by two kinds of climatic models.
CO2 oscillations of 5-10 ppm during the last 2000 years are too large to be explained by external (solar-volcanic) forcing, but they can be explained by pandemics that afflicted western Eurasia and the Americas. The pandemics led to forest regrowth on abandoned farms, and they reduced carbon emissions from ongoing deforestation. The disease-driven CO2 changes were a significant factor contributing to temperature changes during the Little Ice Age (1300-1900 AD).
About the Lecturer
William Ruddiman is a marine geologist. He received his PhD from Columbia University, and is currently Professor Emeritus of Environmental Sciences at the University of Virginia. He was the Chair of this department from 1993 -1996. Prior to this, he was a senior research scientist at the Lamont-Doherty Observatory in New York, a program associate with the National Science Foundation, and a Senior Scientist/Oceanographer with the U.S. Naval Oceanographic Office in Maryland.