methane_gyrations_in_past_2000_years_show_human_influence_on_atmosphere

News Sep 09, 2005

Methane gyrations in past 2,000 years show human influence on atmosphere

Field tents at Law Dome, Antarctica. View Large

Dominic Ferretti, Jim White (ENVS, Geological Sciences, and INSTAAR), and colleagues from the US, New Zealand, and Australia used pioneering stable isotopic techniques on air samples extracted from the tiny bubbles trapped in Antarctic ice cores to show that methane, a potent greenhouse gas in Earth's atmosphere, has been altered by humans over the past 2,000 years. Atmospheric methane (CH4) varied as expected over the past few centuries when methane concentrations in the atmosphere rose by nearly 300 percent and other greenhouse gas levels are known to have increased sharply due to human influences. But the results further back in time came as a shock. Measurements of the stable carbon isotopes in methane (d13C of CH4) fluctuated much more than expected before the industrial revolution. The gyrating ratio combined with other geochemical measurements are evidence for massive fires set by humans clearing land for agriculture and hunting for at least 2,000 years. A prominent feature is a huge drop in the the d13C ratio from ca. 1500 to 1600 A.D., and this was attributed to decreased grassland and forest burning by indigenous peoples in the South and Central America Americas, whose population was devastated by diseases brought to the New World by European explorers. The study is particularly 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. Methane is more powerful than carbon dioxide on a per molecule basis in slowing the release of radiated heat away from Earth. Previous work by other groups indicates that methane emissions from wildfires are likely to be higher during warm and dry periods, such as El Niño events, and may therefore increase with future climate change.

Ferretti has a joint appointment with INSTAAR and the National Institute of Water and Atmospheric Research (NIWA) in Wellington, New Zealand. Other collaborating institutions include Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO), Australia's Department of the Environment and Heritage and the National Oceanic and Atmospheric Administration (NOAA).

The paper was published in the 9 September 2005 issue of Science.

News Source: CU-Boulder News Center

Websites: CU-Boulder News Release | Science Magazine

ENVS Faculty: James White

ENVS News Category: Publication

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