Ancient Methane Mirrors Climate RecordUniversity Park, Pa. -- Analysis of the methane concentration of fossil air trapped in Greenland ice cores indicates that the methane levels closely follow other measures of ancient climate change on millennial time scales, according to researchers at Penn State and the University of Rhode Island, Narragansett.
The researchers reported in today's (Aug. 23) issue of the journal Science, that their data verify general patterns of methane concentration changes seen in Antarctic and other Greenland ice cores and provide a detailed picture of methane variations with respect to longer, orbital climate cycles and short term warming events.
Methane concentrations in the atmosphere are primarily driven by changes in temperatures and precipitation.
"Before 1800 A.D., the dominant methane source was decomposition of organic material in natural wetlands," says Dr. Todd Sowers, assistant professor of geosciences at Penn State. "Other sources including termites, wild animals, wildfires, methane hydrate release and the oceans may have accounted for as much as 40 percent of the total methane sources during the pre-industrial Holocene and the last glacial maximum."
Because dispersion of methane through the atmosphere takes a relatively short time, methane that becomes trapped in the layers of glacial snow reflects global changes in the methane balance and therefore global changes in temperature and precipitation. Just like tree rings, yearly deposits of snow on glaciers can be counted and analyzed to determine the year the layer was created and something about the global climate at that time.
Previous examination of both Antarctic and Greenland ice cores indicate that there are a variety of factors which may have influenced the paleoatmospheric methane record retrieved from ice cores, including seasonal solar variations and rapid warming events. The methane concentration changes indicate that elevated methane levels occurred during warm episodes, but the absolute methane level appears to be modulated on a longer time scale by orbital forcing.
Sowers, Dr. Edward J. Brook, post-doctoral researcher and Joe Orchardo of the graduate school of oceanography, University of Rhode Island, Narragansett, worked with the Greenland Ice Sheet Project 2 (GISP2) ice core to develop a methane record for the past 110,000 years.
In today's wetlands, wetter conditions and warmer temperatures increase methane production. It is likely that the same conditions held in the past and that increased methane in the ice cores reflect wetter and warmer climates in methane producing areas, many of which are in the tropics.
The methane record confirms high methane during the early Holocene and the dramatic changes that occurred during the Younger Dryas some 12,000 years ago. The researchers note that within the uncertainty associated with dating gases relative to the ice surrounding the gas, the rapid concentration increases of the methane and the rapid warming events -- derived from variations in the heavy isotope enrichment of water -- are in phase
While the methane concentration variations and temperatures tend to agree in nearly all cases, there appear to be times when methane concentrations and climactic temperatures were decoupled. In general, however, the researchers believe that there is a tight link between the beginnings of warmer periods and the methane budget.
"The rapidity and magnitude of methane shifts during warmer periods argues for large scale regional to global changes in terrestrial climate associated with these periods," says Brook.
According to the researchers, besides causing increased methane production in the tropics, rapid warming events could have activated large areas of temperate wetlands which had previously been inactive methane producing regions. These temperate wetlands, in areas like Siberia and Alaska, today contribute significant methane to the atmosphere and these high latitude wetlands may have been significant factors in creating elevated methane levels in the past.
EDITORS: Dr. Sowers may be reached at (814) 863-8093. Dr. Brook may be reached at (401)874-6535 until Aug 29 and then at (360) 546-9762. Mr. Orchardo may be reached at (401) 874-6220.
A'ndrea Elyse Messer
University of Rhode Island