The global array of ice and lake cores in the EARLYhumanIMPACT project allow for direct investigations into discerning regional or global increases in fire associated with the timing of the advent of agriculture. A recent provocative hypothesis by Ruddiman suggests that humans may have had a significant impact on the Earth’s climate thousands of years ago through carbon and methane emissions originating from biomass burning and deforestation associated with early agriculture. This hypothesis is centred on the observation that atmospheric carbon dioxide and methane levels were at their minima around 7,000 to 5,000 years before present, respectively, and then slowly increased until the rapid rise in GHGs caused by the industrial revolution. The increase in methane is attributed to biomass burning and rice cultivation in the tropics. The carbon dioxide increase is more difficult to ascribe to human activity, but Ruddiman argues that deforestation and biomass burning may be a primary factor.
Ruddiman contends that without human interference, greenhouse gas concentrations would have continued to decrease, leading to the return of glacial conditions. Figure 5 demonstrates the GHG concentrations across the last seven interglacial periods as measured in polar ice cores. Most interglacial periods peak at a GHG atmospheric concentration between ~260 to 300 ppm and then sharply decrease. The modern interglacial (Stage 1, as shown by the red dots in Figure 4) appears to initially follow this trend, peaking at ~10,000 years BP, but then reverses between ~7000 to 5000 years BP and increases until the present day. As the orbital configurations are different for each interglacial period, the GHG concentrations and behaviour of the climate system vary (Figure 4). The fact that Stage 1 does not have a past analogue in the other known interglacials does not mean that it cannot have natural causes.
Figure 4: Carbon dioxide flux during the past seven interglacial (warm) periods as measured in the EPICA Dome C ice core2
Isotopic (δ13C) data suggests that the increase in carbon dioxide over the past 7000 years may be caused by natural changes in the carbon cycle. Changes in the land biosphere may be insufficient to cause the Holocene carbon dioxide rise but may also necessitate changes in the marine-carbonate cycle. It is difficult to ascribe late-Holocene methane concentrations to purely natural causes as Antarctic and Greenland ice core data show that the methane rise came from the tropics where human activity may have had a pivotal role. The hypothesis of a detectable early human impact on climate directly relies on biomass burning as the cause of an increase in GHG concentrations. However, it is unknown if fire activity increased ~7000 and ~5000 years ago.
The quantification of biomass burning at ~7000 and ~5000 years ago can test the hypothesis of an early detectable human impact on the climate system. The negation or confirmation of this hypothesis will have an important interdisciplinary impact on both climate science and anthropology.