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.

The study is important because methane increases have had the second highest impact on climate change over the past 250 years behind carbon dioxide, accounting for about 20 percent of the warming from all greenhouse gas increases, White said. Methane is more powerful than carbon dioxide in slowing the release of radiated heat away from Earth, he said.

About 60 percent of atmospheric methane is generated from human-related activities, according to the International Panel on Climate Change. Methane increases in the past 200 years are due to increased burning of grasslands, forests and wood fuels, more intense livestock activity and rice cultivation and gas leaked from fossil fuel production and waste management. In addition, natural sources of methane include wetlands, termites and wildfires.

Overall methane levels in the atmosphere increased about 2 percent from about 1 A.D. to 1000 and decreased by 2 percent from 1000 to 1700, according to the study. Since the 1700s, the levels have increased by nearly 300 percent, said White.

Surprisingly, the study showed the amount of methane produced from burning was about the same 1,000 years ago as it is today, said White. "There has been a naÔve idea out there that humans were just passive, pastoral passengers on the planet up until just a few hundred years ago," he said. "We have shown that is not the case."

The study also suggests that natural climate change has played a role in changing methane levels in the atmosphere, at least on a regional level, White said. During the Medieval Warm Period from about 1000 to 1270, there appears to have been a slight increase in biomass burning in Europe. In cooler periods like the Little Ice Age from roughly 1300 to 1850, biomass burning in the Northern Hemisphere appears to have decreased somewhat while anaerobic activity by bacteria in bogs and swamps probably increased, he said.

Involving the United States, New Zealand and Australia, the international project focused on ice cores from Antarctica's Law Dome, White said. "We could not have undertaken this study without all three countries," he said. "These types of projects are not cheap, and each group brought a unique line of expertise."

White said the team hopes to look at methane levels going back prior to 2,000 years ago. "The larger question is when humans began influencing the climate and nutrient system," he said. "We are in an unusually long interglacial period right now, and another interesting but unresolved question is whether humans, without forethought, have inadvertently kept Earth out of the next ice age by altering its energy budget."

The National Science Foundation, NIWA, CSIRO and Australia's Antarctic Climate and Ecosystems program were the primary funding agencies for the study.

Other co-authors on the study included CU-Boulder's Mark Dreier, NIWA's Keith Lassey and Dave Lowe, CSIRO's David Ethridge, Cecelia MacFarling-Meure, Cathy Trudinger and Ray Langenfelds, Tas Van Omen of Australia's DEH and NOAA's John Miller, also a research associate at the CU-Boulder-based Cooperative Institute for Research in Environmental Sciences.