A network of interdisciplinary scholars studying past climate change
A study out in Science this week by Mitchell et al. (see also accompanying news article) measures the differences between methane levels in Arctic and Antarctic ice cores over the late Holocene, and builds a model to test whether those differences are likely natural or anthropogenic. They conclude that higher concentrations of methane in the Northern Hemisphere over the last few millennia must come in large part from human activities, particularly paddy rice agriculture. Although the authors don’t discuss it, the data in Mitchell et al. also point to a substantial drop in the difference between Northern and Southern Hemisphere methane concentrations around 1400AD, pointing to the impact of the Black Death and possibly Little Ice Age on rice farming.
The study lends some support to William Ruddiman’s controversial thesis that humans have substantially influenced global climate for perhaps 7000 years. In essence, Ruddiman’s 2005 book Plows Plagues and Petroleum (which I reviewed a few years ago) argued that based on past glacial cycles, atmospheric methane and carbon dioxide should have started falling millennia ago. The reason they did not is human farming and forest clearance, which released those greenhouse gasses. In the wake of the Black Death and die-off of most Amerindians from Eurasian diseases, farming and burning retreated, contributing to a drop in CH4 and CO2 and exacerbating the early modern Little Ice Age. In recent years, the thesis has gathered some modest empirical support (particularly regarding the impacts of the Columbian Exchange on land use — see Dull et al. 2010; Nevle et al. 2011) as well as criticism. The new study adds weight to his argument that humans influenced greenhouse gas concentrations even before the Industrial Revolution. The question remains how large that influence was and how substantially it might have altered global climate in the context of other natural drivers.