Special Oceans Seminar: Humans have influenced global climate for thousands of years

Speaker: Dr. William Ruddiman, professor emeritus of Environmental Sciences at the University of Virginia

Greenhouse-gas concentrations diverged from their natural (orbital-scale) trendsmillenniaago. CO2 levels began an

anomalous rise 8,000 years ago just as deforestation began in Europe, and methane levels began an anomalous rise 5,000 years ago when irrigation for wet rice farming began in Southeast Asia. Long before the industrial era, much of the southern tier of Eurasia was deforested and many of the river valleys and even terraced hillsides were used to grow rice. The anthropogenic anomalies -- the difference between the observed gas concentrations and those expected

from the orbital trends -- were 35-40 ppm for CO2 and ~200-250 ppb for methane. The warmth of the current interglacial is not natural -- had nature remained in control, Earth would have cooled significantly by now, but the gases generated by agriculture kept climate in a warm interglacial state.