An Emerging View on Early Land Use

This emerging view bears on a current discussion over whether or not to designate an ‘Anthropocene’ interval (a time of major human influence on Earth’s system) and, if so, when to place its beginning. Although most opinion seems to favor using the industrial era (the last two centuries or less) as the start, this new evidence offers a different perspective. Forest clearance for cultivation and pastureland is the largest transformation of Earth’s surface that has occurred to date. If well over half of this key transformation occurred prior to the industrial era, then an argument can be made for placing the start of the anthropocene at an earlier time. One possible solution would be to designate two stages: an ‘early anthropocene’ (a time of slow but growing and cumulatively large transformations that began ~7000 years ago for CO2 and ~5000 years ago for CH4) and a ‘late anthropocene’ to mark the many accelerating changes of the industrial era.

Other special-issue papers also point to a revised interpretation of a related kind of evidence that bears on early deforestation— the meticulous analyses of the carbon-isotopic composition of CO2 in ice-core air bubbles by the Bern group. Elsig et al. concluded in 2009 in an article in Nature that the small (~0.05o/oo) amplitude of the δ13CO2 decrease during the last 7000 years constrains net emissions of terrestrial carbon to ~50 GtC (one Gt is a billion tons), if fully equilibrated with the deep ocean. As part of their proposed balancing of various carbon sources and sinks, they estimated an anthropogenic contribution of ~50 GtC to the δ13CO2 trend, equivalent to a CO2 increase of 3.5 ppm.

But the mass balance calculation in Elsig et al. entailed the questionable assumption that only 40 Gt of carbon has been buried in boreal peats during the last 7000 years, yet this value lies well below a long-respected estimate of 300 GtC by Eville Gorham (e.g. Ecological Applications 1: 182-195, 1991; Gajewski et al., Global biogeochemical Cycles 15: 297-310; 2001). A new analysis by Zicheng Yu in the special issue takes into account both the initial burial of peat carbon and, for the first time in any study, the subsequent decomposition and release of peat carbon after burial. Yu arrives at an estimate of ~300 Gt of carbon burial in peat during the last 7000 years.

This much higher value (~300 GtC versus 40 GtC) requires much larger offsetting emissions of terrestrial carbon to satisfy the overall δ13CO2 constraint, but the additional carbon is unlikely to have come from natural sources. Model studies have, on average, placed the net carbon balance caused by natural changes in monsoon vegetation and carbon fertilization close to the 30 Gt size estimated by Elsig and colleagues. These changes cannot account for the emissions needed to offset the much larger amount of carbon buried in peat.

The only remaining source left is anthropogenic emissions. The resulting estimate of >300 GtC of preindustrial anthropogenic emissions is in the same ballpark as the land-use simulation estimate from Kaplan and colleagues. If the earlier (Gorham) and more recent (Yu) estimates of large carbon burial in boreal peat are correct, the small negative δ13CO2 trend during the last 7000 years is not an argument against the early anthropogenic hypothesis, but rather an argument in its favor.

The two estimates of a pre-industrial anthropogenic CO2 increase of as much as 24 ppm are much larger than previous estimates of 3-5 ppm, but still short of the 40 ppm proposed in the early anthropogenic hypothesis. However, another factor that would have contributed to the pre-industrial anthropogenic total was CO2 feedback from an ocean kept warmer by agricultural emissions of CO2 and CH4 to the atmosphere. A special-issue paper by Kutzbach and colleagues estimates a contribution of as much as 9 ppm from the reduced solubility of CO2 in an ocean warmed by the early anthropogenic CO2 and CH4 emissions to the atmosphere. This, and other possible feedbacks from the ocean, put the total pre-industrial CO2 effect at >30 ppm, closer to the 40 ppm in the original hypothesis.

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