An Emerging View on Early Land Use

Guest article by William Ruddiman

More than 20 years ago, analyses of greenhouse gas concentrations in ice cores showed that downward trends in CO2 and CH4 that had begun near 10,000 years ago subsequently reversed direction and rose steadily during the last several thousand years. Competing explanations for these increases have invoked either natural changes or anthropogenic emissions. Reasonably convincing evidence for and against both causes has been put forward, and the debate has continued for almost a decade. Figure 1 summarizes these different views.

An August 2011 special issue of the journal The Holocene will help to move this discussion forward. All scientists who have been part of this debate during the last decade were invited to contribute to the volume. The list of those invited was well balanced between the two views, both of which are well represented in the issue. The papers have recently begun to come online, but unfortunately behind a paywall.

Arguably, the most significant new insight emerging from this issue comes from several papers that converge on a view of pre-industrial land use that is very different from the one that has prevailed until recently. Most previous modeling simulations relied on the simplifying assumption that per-capita clearance and cultivation remained small and nearly constant during the late Holocene, but historical and archeological data now reveal much larger earlier per-capita land use than used in these models. The emergence of this view was reported in several presentations at a March 2011 Chapman Conference, and it has attracted recent attention both in Nature and Science News. The following article summarizes this new evidence.

Historical data on land use extending back some 2000 years exists for two regions — Europe and China. In a 2009 paper, Jed Kaplan and colleagues reported evidence showing nearly complete deforestation in Europe at mid-range population densities, but very little additional clearance at higher densities. Embedded in this historical relationship was a trend from much greater per-capita clearance 2000 years ago to much smaller values in recent centuries. Similarly, a Holocene special-issue paper by Ruddiman and colleagues pointed to a pioneering study of early agriculture in China published in 1937 by J. L. Buck. Paired with reasonably well-constrained population estimates that extend back to the Han dynasty 2000 years ago, these data show a 4-fold decrease in per-capita land area cultivated in China from that time until the 1800’s.

These two re-evaluations of per capita land use have important implications for global pre-industrial carbon emissions. A special issue paper by Kaplan and colleagues used the historical relationships from Europe to estimate worldwide clearance, with smaller per-capita land needs in tropical regions due to the longer growing season that allows multiple crops per year. Their model simulated major forest clearance thousands of years ago not just in Europe and China, but also in India, the Fertile Crescent, Sahelian Africa, Mexico and Peru. The pattern of clearance is nicely shown in a time-lapse sequence available in the Science News article cited above. Kaplan and colleagues estimated cumulative carbon emissions of ~340 GtC (1 Gt = billion metric tons) before the industrial-era CO2 rise began in 1850. This estimate is 5 to 7 times larger than those based on the assumption that early farmers cleared forests and cultivated land in the small per-capita amounts typical of recent centuries.

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