Gases Trapped in Ice Cores Show Probable Influence of Ancient Human

October 20, 2005

A NOAA Joint Institute scientist working in the Earth System Research Laboratory (ESRL) Global Monitoring Division (GMD) was a co-author of a recent report published in Science (Vol. 309, 5741, 1714-1717, 9 September, 2005) describing changes in the budget of atmospheric methane (CH4) over the past 2000 years. The report has been receiving considerable international attention and was featured on the BBC World and on the BBC science website.

In the study, a 2000 year history of methane and its stable carbon isotope ratio was reconstructed from gases trapped in the Law Dome Antarctic ice cores. High methane carbon 13 isotope ratios showed that global levels of biomass burning were significantly larger than previously thought around 1000 AD, but also that global burning levels decreased dramatically, especially between 1500 - 1700 AD. The timing of this change corresponds closely with the large drop in human population in the Americas (from 60 million people down to 6 million) following the introduction of European diseases and exploitation in the 16th and 17th centuries. The depopulation could have produced the observed biomass burning decreases due to reduced agricultural and hunting activities by the 90% smaller North and South American native populations.

Background: Methane is the second most important greenhouse gas in the atmosphere. Understanding the evolution over time of climatically important trace gases like methane and carbon dioxide (CO2) is important for understanding current trends in these key greenhouse gases.

Significance: This research shows the important role of fires in the global carbon cycle of not just CH4 but also CO2. Combined with other recent studies, the results point to fire (combustion) as a potentially major driver of both short and long-term trends of climatically important gases.

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