Early Anthropogenic Overprints on Holocene Climate

PAGES research on Holocene climate rests in part on the implicit assumption that humans had negligible effects on large-scale climate until the industrial era, followed by large and accelerating impacts during the last 200 years. With this two-part division, scientists use pre-industrial climatic proxies to defi ne the natural state of the climate system, and thereby to isolate and quantify the anthropogenic overprint during the last 200 years. A paper recently published in the journal Climatic Change (Ruddiman, 2003) suggests that this basic premise is fl awed. The interval between 8000 years ago and the industrial era was a time of signifi cant and slowly increasing human impact on greenhouse-gas concentrations and global climate, and the cumulative impact of these changes by 200 years ago was equivalent to the subsequent impacts during the industrial era. Long-term orbital-scale cycles predict ongoing decreases in CO2 and methane through the entire Holocene, but ice-core trends show a ~100-ppb rise in atmospheric methane during the last 5000 years and a 20-25 ppm rise in CO2 during the last 8000 years. Natural orbitalscale variations cannot account for these increases. I attribute these anomalous increases to early anthropogenic activity. The CO2 rise occurred during a time of large-scale deforestation in southern Eurasia, as agriculture advanced from the primitive practices of the late Stone Age to the much more sophisticated package of skills in the early Iron Age. The methane increase correlates with an interval in which wet-rice irrigation began in the lowlands of Southeast Asia and later spread to hillside rice paddies. The observed increases are only part of the story, because the full anthropogenic signal must also include the natural decreases that should have happened but did not. I estimate the total anthropogenic anomalies by the start of the industrial era at 40 ppm for CO2 and 250 ppb for methane (Fig. 1). For the 2.5oC IPCC (2001) estimate of global climate sensitivity to CO2 doubling, the pre-industrial global-mean warming effect from anthropogenic sources would have been ~0.8oC, about the same size as estimates of the greenhouse-gas contribution to the measured industrialera warming. The pre-industrial warming at high latitudes would have been larger (~2oC) because of amplifi cation by snow and sea ice feedback. This large signal has escaped notice until now because of an even larger natural cooling caused by decreasing insolation. In summary, humans were altering global climate well before we built cities, discovered writing, or founded religions. In addition, I investigated relatively rapid CO2 oscillations of 5-10 ppm found in high-resolution icecore records of the last 2000 years from Antarctica. Natural variations (solar-volcanic forcing) do not appear to be capable of explaining such large CO2 changes. Simulations with the Bern carbon-cycle model (Gerber et al., 2003) indicate that each 1-ppm change in CO2 in response to solar-volcanic forcing