Humans have been tinkering with greenhouse gas levels in Earth's atmosphere for at least 2,000 years and probably longer, according to a surprising new study of methane trapped in Antarctic ice cores conducted by an international research team.
The study showed wild gyrations of methane from biomass burning from about 1 A.D. to present, said Dominic Ferretti, lead study author and a University of Colorado at Boulder researcher with a joint appointment at the National Institute of Water and Atmospheric Research, or NIWA in Wellington, New Zealand. Scientists had expected to see slowly increasing concentrations of methane, a major greenhouse gas produced primarily by burning and anaerobic activity from agriculture, livestock and natural sources, up until the onset of the Industrial Revolution in the late 1700s, he said.
For the first time, researchers were able to separate "pyrogenic" and anaerobic methane sources using a stable-isotope analysis of the ice cores, said James White of CU-Boulder's Institute of Arctic and Alpine Research and study co-author. They found methane emissions from burning dropped about 40 percent from 1000 to 1700, likely due in large part to decreased landscape burning by indigenous populations in the Americas devastated by diseases brought to the New World by European explorers.
Undertaken by a team from CU-Boulder, NIWA, Australia's Commonwealth Scientific and Industrial Research Organization, or CSIRO, Australia's Department of the Environment and Heritage and the National Oceanic and Atmospheric Administration, the study was published in the Sept. 9 issue of Science. "The results frankly were a shock," said White. "We can see human fingerprints all over atmospheric methane emissions for at least the last 2,000 years. Humans have been an integral part of Earth's carbon cycle for much longer than we thought."
The researchers recorded a huge drop in methane levels from biomass burning from 1500 to 1600, when anthropologists say indigenous humans in South and Central America -- who had been expanding in population and range -- declined by 90 percent. Since most forests in Europe and China had been mostly cleared for agricultural or habitable lands by 1 A.D., "the seemingly small indigenous populations of the Americas would have had a disproportionate influence on anthropogenic methane emissions from fires," the researchers wrote in Science.
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http://www.colorado.edu/news/releases/2005/342.html
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