Methane Gyrations Last 2,000 Years Show Human Influence On Atmosphere

The study showed wild gyrations of methane from biomass burningfrom about 1 A.D. to present, said Dominic Ferretti, lead study authorand a University of Colorado at Boulder researcher with a jointappointment at the National Institute of Water and AtmosphericResearch, or NIWA in Wellington, New Zealand. Scientists had expectedto see slowly increasing concentrations of methane, a major greenhousegas produced primarily by burning and anaerobic activity fromagriculture, livestock and natural sources, up until the onset of theIndustrial Revolution in the late 1700s, he said.

For the first time, researchers were able to separate"pyrogenic" and anaerobic methane sources using a stable-isotopeanalysis of the ice cores, said James White of CU-Boulder's Instituteof Arctic and Alpine Research and study co-author. They found methaneemissions from burning dropped about 40 percent from 1000 to 1700,likely due in large part to decreased landscape burning by indigenouspopulations in the Americas devastated by diseases brought to the NewWorld by European explorers.

Undertaken by a team from CU-Boulder, NIWA, Australia'sCommonwealth Scientific and Industrial Research Organization, or CSIRO,Australia's Department of the Environment and Heritage and the NationalOceanic and Atmospheric Administration, the study was published in theSept. 9 issue of Science.

"The results frankly were a shock," said White. "We can seehuman fingerprints all over atmospheric methane emissions for at leastthe last 2,000 years. Humans have been an integral part of Earth'scarbon cycle for much longer than we thought."

The researchers recorded a huge drop in methane levels frombiomass burning from 1500 to 1600, when anthropologists say indigenoushumans in South and Central America -- who had been expanding inpopulation and range -- declined by 90 percent. Since most forests inEurope and China had been mostly cleared for agricultural or habitablelands by 1 A.D., "the seemingly small indigenous populations of theAmericas would have had a disproportionate influence on anthropogenicmethane emissions from fires," the researchers wrote in Science.

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

About 60 percent of atmospheric methane is generated fromhuman-related activities, according to the International Panel onClimate Change. Methane increases in the past 200 years are due toincreased burning of grasslands, forests and wood fuels, more intenselivestock activity and rice cultivation and gas leaked from fossil fuelproduction and waste management. In addition, natural sources ofmethane include wetlands, termites and wildfires.

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

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

The study also suggests that natural climate change has playeda role in changing methane levels in the atmosphere, at least on aregional level, White said. During the Medieval Warm Period from about1000 to 1270, there appears to have been a slight increase in biomassburning in Europe. In cooler periods like the Little Ice Age fromroughly 1300 to 1850, biomass burning in the Northern Hemisphereappears to have decreased somewhat while anaerobic activity by bacteriain bogs and swamps probably increased, he said.

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

White said the team hopes to look at methane levels going backprior to 2,000 years ago. "The larger question is when humans beganinfluencing the climate and nutrient system," he said. "We are in anunusually long interglacial period right now, and another interestingbut unresolved question is whether humans, without forethought, haveinadvertently kept Earth out of the next ice age by altering its energybudget."

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

Other co-authors on the study included CU-Boulder's MarkDreier, NIWA's Keith Lassey and Dave Lowe, CSIRO's David Ethridge,Cecelia MacFarling-Meure, Cathy Trudinger and Ray Langenfelds, Tas VanOmen of Australia's DEH and NOAA's John Miller, also a researchassociate at the CU-Boulder-based Cooperative Institute for Research inEnvironmental Sciences.