By Matt Owens December 6, 2013
Above: Researcher Logan Mitchell standing on the vast expanse of the Antarctic ice cap with the clean Antarctic air above - and the ice that records a history of that air beneath. Photo credit: Susanne Lilja Buchardt.
How much of preindustrial methane can we attribute to human activity? A sizable fraction according to the much-debated “early anthropogenic hypothesis,” first proposed by William Ruddiman, a climate scientist with the University of Virginia. According to the theory, the prehistoric human greenhouse footprint, although exponentially smaller than it is today, may have been enough to stave off the onset of a new ice age thousands of years ago. Researcher Logan Mitchell has now added a new dimension to the issue with work just printed in Science.¹
Mitchell is with the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University, and has previously worked on the issue of methane variability over decadal time scales as a function of temperature and precipitation trends.² He has also been researching the chemistry and physics of ice core air bubble formation. It is those air bubbles that are in fact the proxy record for the past 800,000 years of methane.
I spoke with Mitchell earlier this week about his latest research findings. He explained that this work indicates human sources, especially rice farming in Asia, contributed to a rise in atmospheric methane levels starting at least several hundred years ago. He found this by comparing high-resolution ice core data from both the North and South Poles, essentially triangulating the geographic sources of methane within certain latitudes, distinguishing between Northern and Southern Hemisphere sources, and thereby also establishing limits on contributions from tropical latitudes.