Early Agriculture & Anthropogenic Climate

A quick note on the publication of the updated rice archaeology database and a model effort based on it examining the spread of rice with an attempt to test its hypothesized contribution to rising global methane levels between 3000 BC and 1000 AD. This was a team effort involving students and post-docs with Early Rice Project (Kingwell-Banham; Castillo; Weisskop; Qin Ling) collaborators from abroad (Sato (Kyoto); Hijmans (David)) and some nice GIS modelling work by Jacob van Etten (Madrid). Some maps wet rice distirbution in selected  time-slices are shown left; but for more details read the paper.  This article "The contribution of rice agriculture and livestock pastoralism to prehistoric methane levels" is available ahead-of-print on-line from The Holocene.  Our key conclusion is that by 1000 AD perhaps 80% of 'anomalous' methane could be attributed to rice cultivation but also at 2000 BC rice is still not that significant and other sources should be sought such as the rapid spread of pastoralism around this period especially through the savannas of Africa and South Asia.

I have previously blogged the rice-part of the Ruddiman (Early Anthropogenic) hypothesis i.e. that early rice framing (and its spread) contributed enough extra methane to the atmosphere to make a global impact from sometime just after 3000 BC. I did raise some questions about the quality of the data: how many early rice finds actually represent cultivated rice (not wild) and how many represent flooded or paddyfield rice rather than upland rainfed rice? Also what role did the spread of cattle pastoralism over the Old World play in contributing to Mid-Holocene methane levels? So this new paper is an attempt to address some of these questions. We make a first stab at mapping the areas over which pastoralism spread in addition to modelling the spread of rice and the land area under wet rice cultivation. For rice at least we are able to estimate methane output but further work is needed for an equivalent calculation from cattle. I think we also would all admit that our estimate from rice remains imperfect and there is a lot of additional work to do! Collecting better quality archaeobotany (and more of); more zooarchaeology; and more sophisticated modelling...

A recent summary of the Early Anthropogenic Greenhouse Gas hypothesis has been published on-line by Bill Ruddiman at realclimate.org. It has attracted a lot of discussion. Essentially this provides a preview to some of the results due out in the August issue of the journal The Holocene (although many of the paper are already available on-line). This issue has also received attention in Nature in their News section (in March) and was one of the issue debated at the AGU Chapman conference in Santa Fe in March.