August 21, 2009

Burning the biosphere before you were born

800px-JhumMillennia before humans discovered coal, indeed, millennia before there was civilization, Homo sapiens had discovered fire and was making extensive use of it. In a study just published by Bill Ruddiman and myself (Ruddiman and Ellis, 2009), we show that early farmers using fire likely cleared vast areas of forest thousands of years ago, even when human populations were small, thus supporting the Early Anthropocene Hypothesis– that humans began altering climate more than 7,000 years ago.

By burning away forests and other woody vegetation, early hunter gatherers enhanced the productivity of landscapes for game and the first farmers cleared lands for planting without tillage and other labor intensive practices. To maintain productivity with a minimum of labor, these early farmers burned one area after another using “shifting cultivation” methods (also known as “slash and burn” and “swidden”) and thereby used an order of magnitude more land per person than do farmers today, even using traditional farming practices. By using far more land per capita, even small bands of prehistoric humans regularly burned huge areas of forests, keeping regrowth at bay such that forests regained only a fraction of their original carbon density – causing carbon to build up in the atmosphere.

This paper was set in motion after I met Bill Ruddiman last year at the annual meetings of the American Geophysical Union (AGU) in San Francisco. Over lunch with Bill and Kees Klein Goldewijk, we discussed land use patterns of the first farmers, with Kees supporting the ruddiman_Boserup_factorconventional wisdom of “why would farmers use more land than they need” (i.e. the same amount they use today), and me supporting the Boserup hypothesis (Ester Boserup, 1965) that farmers minimized their use of labor wherever possible, only adopting more labor-intensive practices when population densities have increased to the point where land becomes scarce. Thus, the first farmers, having no shortage of forest to burn, used shifting methods that used lots of land but little labor, and only later began to use land intensively and efficiently. The first farmers easily cleared huge areas of forests, far more than would be needed by later farmers.

The funny thing about this paper is that it was inspired by facts well known to agricultural economists for decades, and even to myself, but it was Bill Ruddiman who put this together as an explanation for anthropogenic forest clearing trends that help to explain atmospheric carbon dioxide trends over the past 7,000 years. We had been discussing this in general, and then Bill had an epiphany when he tested out the numbers for some simple scenarios relating to this hypothesis- generating the hand-drawn curves at left and an email about these results with the subject line “holy moly!”. A great example of the magic of interdisciplinary collaboration!

The paper:

Ruddiman, W. F. and E. C. Ellis. in press. Effect of Per-Capita Land use Changes on Holocene Forest Clearance and CO2 Emissions. Quaternary Science Reviews. < http://dx.doi.org/10.1016/j.quascirev.2009.05.022> < download>

The Media:

Further Reading:

Ruddiman, W. E. 2005. Plows, Plagues, and Petroleum: How Humans Took Control of Climate. Princeton University Press. < my review>

Ruddiman, W. F. 2003. The anthropogenic greenhouse era began thousands of years ago. Climatic Change 61:261-293. < http://www.springerlink.com/openurl.asp?id=doi:10.1023/B:CLIM.0000004577.17928.fa>

Ruddiman, W. F. 2007. The early anthropogenic hypothesis: Challenges and responses. Reviews of Geophysics 45:RG4001. < http://www.agu.org/pubs/crossref/2007/2006RG000207.shtml>

Boserup, E. 1965. The Conditions of Agricultural Growth: The Economics of Agrarian Change under Population Pressure. Allen & Unwin, London.

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