6.5.1.2 Why Did Holocene Atmospheric Greenhouse Gas Concentrations Vary Before the Industrial Period?

Recent transient carbon cycle-climate model simulations with a predictive global vegetation model have attributed the early Holocene CO₂ decrease to forest regrowth in areas of the waning Laurentide Ice Sheet, partly counteracted by ocean sediment carbonate compensation (Joos et al., 2004). Carbonate compensation of terrestrial carbon uptake during the glacial-interglacial transition and the early Holocene, as well as coral reef buildup during the Holocene, likely contributed to the subsequent CO₂ rise (Broecker and Clark, 2003; Ridgwell et al., 2003; Joos et al., 2004), whereas recent carbon isotope data (Eyer, 2004) and model results (Brovkin et al., 2002; Kaplan et al., 2002; Joos et al., 2004) suggest that the global terrestrial carbon inventory has been rather stable over the 7 kyr preceding industrialisation. Variations in carbon storage in northern peatlands may have contributed to the observed atmospheric CO₂ changes. Such natural mechanisms cannot account for the much more significant industrial trace gas increases; atmospheric CO₂ would be expected to remain well below 290 ppm in the absence of anthropogenic emissions (Gerber et al., 2003).

It has been hypothesised, based on Vostok ice core CO₂ data (Petit et al., 1999), that atmospheric CO₂ would have dropped naturally by 20 ppm during the past 8 kyr (in contrast with the observed 20 ppm increase) if prehistoric agriculture had not caused a release of terrestrial carbon and CH₄ during the Holocene (Ruddiman, 2003; Ruddiman et al., 2005). This hypothesis also suggests that incipient late-Holocene high-latitude glaciation was prevented by these pre-industrial greenhouse gas emissions. However, this hypothesis conflicts with several, independent lines of evidence, including the lack of orbital similarity of the three previous interglacials with the Holocene and the recent finding that CO₂ concentrations were high during the entire Stage 11 (Siegenthaler et al., 2005a; Figure 6.3), a long (~28 kyr) interglacial (see Section 6.4.1.5). This hypothesis also requires much larger changes in the Holocene atmospheric stable carbon isotope ratio (¹³C/¹²C) than found in ice cores (Eyer, 2004), as well as a carbon release by anthropogenic land use that is larger than estimated by comparing carbon storage for natural vegetation and present day land cover (Joos et al., 2004).