UPDATE ON THE HYPOTHESIS OF EARLY ANTHROPOGENIC EFFECTS ON CLIMATE

UPDATE ON THE HYPOTHESIS OF EARLY ANTHROPOGENIC EFFECTS ON CLIMATE

Bill Ruddiman
Environmental Sciences, University of Virginia
2004/10/20

ABSTRACT
I recently proposed that farming has altered greenhouse-gas levels in the atmosphere for thousands of years. Clearance of forests for early agriculture raised atmospheric CO2 levels, and irrigation for rice farming increased methane levels. I also proposed that these early gas emissions were large enough to cancel most of a natural cooling that would otherwise have occurred and to stop the early stages of a glaciation. The early part of marine isotopic stage 11 near 400,000 years ago provides the closest analog to recent solar radiation levels of any interglaciation in the last million years. At the time that was most similar to the last few millennia, the CH4 concentration measured in Vostok ice fell to ~450 ppb, and CO2 values to ~250 ppm. These natural decreases contrast with the large increases (methane to 700 ppb and CO2 to 280 ppm) during pre-industrial time, thus providing strong support for the early anthropogenic hypothesis. In addition, deuterium (hydrogen-isotope) values in Vostok ice fell from typical interglacial to nearly glacial values, indicating that a major cooling occurred in Antarctica, as postulated in the hypothesis. Other evidence from both ice-core and marine-sediment sources suggests that new ice was accumulating in the northern hemisphere during the closest insolation analog to the present day, supporting the hypothesis that a glaciation is now overdue.