CO and CH changes and the Anthropocene hypothesis

In a series of stimulating publications, Ruddiman interprets the increase in atmospheric CO2 and methane at 8000 and 5000 years ago in ice-core records, respectively, as largely the result of human activity (Ruddiman and Thomson 2001; Ruddiman 2003, 2005a,b) Ruddiman's hypothesis of early anthropogenic influences on Holocene climate consists of three parts (Oldfield 2005). First, humans reversed a natural decrease in atmospheric CO2 concentrations 8000 years ago through the early development of agriculture and associated forest clearance and carbon release to the atmosphere. Second, humans reversed a natural methane decrease about 5000 years ago by the development of intensive rice cultivation and flood-irrigated regions in Asia. Third, these human activities and associated changes in CO2 and methane atmospheric concentrations caused an anthropogenic warming sufficient to counter a natural cooling and avoided the onset of a new glaciation in the past several thousand years (Ruddiman 2005a). Ruddiman (2003, 2005b) presents many arguments to support this hypothesis. He emphasizes that the orbital configurations to which CO2 and methane concentrations have been linked in the past would have led to declines in both CO2 and methane during the past 5000 years of the Holocene. He also emphasizes that none of the previous interglacials in the Antarctic ice-cores show increases in CO2 or methane similar to the Holocene. He argues, using a wide range of paleoenvironmental records, that fluctuations in CO2 in the past 2000 years are correlated with human demographic changes including plagues that result in forest regrowth, increased carbon sequestration, and reduced atmospheric CO2 concentrations (Oldfield 2005). Oldfield (this volume) presents an up-to-date review of the evidence being presented from several sources for and against Ruddiman's hypothesis.

A global synthesis of fire-history records for the Holocene for different geographic regions (Carcaillet et al. 2002) shows that global fire indices parallel the increase in atmospheric CO2 concentrations, suggesting that biomass burning may have been a major cause for the increase in CO2 concentrations from 8000 years ago. Studies on the loss of storage of organic carbon in Chinese soils as a result of cultivation (Wu et al. 2003) similarly support Ruddiman's main hypothesis.

Joos et al. (2004) tested Ruddiman's hypothesis concerning CO2 changes by driving the carbon component of a carbon-cycle climate model with simulated climate from two global climate models for the past 21 000 years. They conclude that the Holocene ice-core record of CO2 is well reproduced (within a few ppm) by the model results and by the processes incorporated in the models without invoking any anthropogenic influences suggested by Ruddiman. Joos et al. (2004) argue that the early Holocene CO2 rise was a result of several processes, including terrestrial carbon uptake and release, coral-reef build-up, and sea-surface temperature changes. They further claim that the level of terrestrial carbon release suggested by Ruddiman's hypothesis is not compatible with the ice-core record of 813C change in the pre-industrial times of the Holocene.

Ruddiman's hypotheses about CO2 changes and associated climate responses have stimulated much interest and further work (e.g. Claussen et al. 2005; Crucifix et al. 2005; Ruddiman et al. 2005) and are already making a major impact on research and thinking (Mason 2004; Oldfield 2005, this volume). The hypotheses challenge current ideas on the spatial extent of deforestation and land abandonment in the Holocene as deduced from pollen-analytic data and on estimating carbon sequestration and release as a result of human activity. Attempts at quantitative modeling of the extent of forested land-area required to be cleared to be registered in pollen-stratigraphic data (Sugita et al. 1997, 1999) suggest that traditional interpretations by pollen analysts of the areas involved may be a gross underestimate.

As Oldfield (this volume) notes, Ruddiman's interpretation of the rise in atmospheric methane concentrations about 5000 years ago (see also Ruddiman and Thompson 2001) has not received as much criticism or attention as Ruddiman's hypothesis about changes in atmospheric CO2 concentrations. A recent attempt at identifying possible causes of changes in atmospheric methane concentrations in the mid-Holocene by MacDonald et al. (2006) has involved a synthesis of over 1500 radiocarbon-dates from circum-arctic peatlands. This synthesis shows that the mid-Holocene rise in atmospheric methane concentrations cannot be explained by any major expansion of northern peatlands in the mid-Holocene. It adds further credibility to Ruddiman's hypothesis of an anthropogenic explanation for the mid-Holocene rise in methane concentrations.

Ruddiman's hypotheses are a major potential paradigm shift in Holocene climate research. As Oldfield (2005) concludes "Ruddiman has opened up a range of possibilities that will take a long time to evaluate fully."

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