An Emerging View on Early Land Use

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.

Over millennial time scales, roughly 85% of CO2 emissions to the atmosphere end up in the deep ocean. As a result, Kaplan’s estimated 340 Gt of early anthropogenic carbon emissions to the atmosphere would have resulted in a total pre-industrial CO2 increase of ~24 ppm (340 Gt divided by 14.2 Gt per ppm). A mismatch in timing remains, however, between the early rise of the ice-core CO2 trend and the later rise of Kaplan’s carbon-emissions estimate. One possibility currently being investigated by Kaplan and colleagues is larger early per-capita burning by farmers (and those cultures still in the hunter-gatherer stage).

A similar story of decreasing per-capita land use holds for farming practices that generate methane. The paper by Ruddiman and colleagues cites a 1997 study by Ellis and Wang in Agriculture, Ecosystems, and Environment (61: 177-193) reporting a 4-fold decrease from 1000 to 1800 AD in the per-capita size of rice paddy fields in the lower Yangtze River valley. Because of ongoing population growth and the lack of additional arable land, farmers were forced to produce rice on ever-smaller land holdings, resulting in the typical ‘garden-style’ Chinese agriculture.

For longer time scales, an in-press paper by Fuller and colleagues on ‘The contribution of rice agriculture and livestock pastoralism to prehistoric methane levels: an archeological assessment’ assembled archeological evidence from hundreds of well-dated sites showing the spread of irrigated rice across southern Asia between 5000 and 1000 years ago. Based on modern regional relationships, they assumed that rice farming in each region subsequently filled in with the log of population density. Combining the first arrival of rice and the subsequent infilling, Fuller and colleagues projected the progressive increase in the total area of southern Asia devoted to irrigated rice.

Their estimate showed a rising exponential trend in total area that reached more than 35% of the modern value by 1000 years ago, even though the population in the rice-growing areas of Asia at that time was only 5-6% of modern levels. This mismatch again indicates much greater per-capita land use early in the historical era than in later pre-industrial time. According to this analysis, the increase in CH4 emissions from rice irrigation can account for most of the CH4 rise measured in ice cores between 5000 and 1000 years ago. Fuller and colleagues also mapped the first arrival of domesticated livestock across Asia and Africa and found that a major expansion of pastoralism into wet areas with high carrying capacities began after 5000 years ago. They noted that this spread of livestock would also have made a major contribution to anthropogenic methane emissions and atmospheric concentrations but did not attempt to estimate the amount.

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