Holocene vegetation changes and atmospheric methane and carbon dioxide concentrations

To what extent have changes in land-use and land-cover modified the atmospheric concentrations of both methane and carbon dioxide during the pre-industrial parts of the Holocene? Forest clearance, followed by cultivation or pastoralism, reduces carbon sequestration in the terrestrial biosphere by generating an initial release to the atmosphere usually through burning, by diminishing the store of carbon in standing crops and soils, and by increasing the rate of carbon turnover. Moreover, Lal (2002) shows that much of the carbon transported from sites as a result of land-use-driven soil erosion is also emitted to the atmosphere. The initial changes occur rapidly and recovery from them in the event of reforestation is a much slower process. Rice paddy cultivation, especially under conditions where drainage is poorly controlled, leads to methane release.

Several types of conversion are important in changing the carbon budgets of terrestrial ecosystems, for example, from most types of "natural" plant cover to permanent cropland, slash and burn agriculture or pasture; or from undisturbed forest to commercial plantations or managed forest (see Lawrence and Chase 2006). Wu et al. (2003) have attempted to quantify the loss of carbon from soils as a result of anthropogenic land-cover changes. Despite indications that paddy and irrigated soils have experienced an increase in soil organic carbon, they estimate a loss of some 7.1 pg of soil organic carbon over the country as a whole as a result of human activities, land-cover conversion, and associated soil degradation in many regions, especially in north-east China. Other activities, such as drainage of swamps and peatlands, can also lead to reduced carbon sequestration in the terrestrial biosphere as well as to increased vulnerability to wildfires.

Ruddiman's proposal (2003) that human activities have had significant effects on atmospheric carbon dioxide and methane from 8000 and 5000 years BP, respectively, have attracted considerable interest and controversy (EPICA Community members 2004; Claussen et al. 2005; Crucifix et al. 2005; Ruddiman 2005a,b; Ruddimann et al. 2005; Schmidt and Shindell 2005; Broecker 2006; Birks, this volume). The significance of this debate goes beyond improving reconstructions of past changes in the carbon cycle. Ruddiman's hypothesis has wider implications in relation to past climate change and future climate sensitivity to greenhouse gas forcing.

Part of Ruddiman's claim was based on the fact that increases in atmospheric CO2 and methane concentrations from 8000 and 5000 years BP onwards, respectively, appeared to be unique to the Holocene (Figure 3.7). At the time of proposing his hypothesis, data for atmospheric greenhouse concentrations were only available from Marine Isotope Stage (MIS) 9 and subsequent interglacials . The subsequent availability of data for MIS 11, the interglacial associated with orbital forcing most comparable to that prevailing during the Holocene, has provided a more appropriate basis for comparing the behavior of each greenhouse gas with and without the possibility of human intervention. For some (EPICA Community Members 2004; Broecker 2006), the comparison disposes of Ruddiman's claim that the Holocene CO2 increase is unique. For Ruddiman (2005a) their argument is not conclusive, for he adopts a different part of the MIS 11 CO2 curve for comparison and advances credible reasons for his choice (Figure 3.8). At the same time, and in parallel with Ruddiman, Carcaillet et al. (2002) provide a wealth of evidence in support of the view that fire alone may have made a significant contribution to the Holocene increase in atmospheric CO2 (Figure 3.9).

Another dimension to the controversy over Ruddiman's hypothesis arises from the mismatch between inferences based on empirical evidence for past land-use/cover change and model-based reconstructions incorporating stable

Figure 3.7 Actual and inferred "natural" trends of atmospheric concentrations of CO2 and CH4 during the Holocene (Ruddiman 2003). The histograms ascribing methane to different source regions during the Holocene (Fluckiger et al. 2004) are shown at the top ofthe graph.

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Holocene Methane Concentration Graph
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Years ago isotope evidence (Joos et al. 2004). Figure 3.10, which shows an estimate of the change in total biospheric carbon during the Holocene, indicates the extent to which any attempt to calculate past carbon budgets depends on the approach used. Broecker (2006) casts doubt on Ruddiman's hypothesis by claiming that for it to work "the forest biomass of 8000 years ago must have been more than double that in the year 1800 A.D.". This requirement he finds quite unlikely to have been met, but it may seem less incredible to many Old World paleoecologists. More difficult to counter are the arguments based on the latest measurements of the 13C to 12C ratio in Antarctic ice, quoted by Broecker (2006). These are interpreted as favoring the view that the main changes in atmospheric CO2 were the result of changes in the world's oceans. In response to the stable isotope data, Ruddiman has revised his latest estimate of anthropogenic carbon release downwards to 14 ppm CO2

Figure 3.8 Ruddiman's interpretation of the comparison between greenhouse gas concentration trends during the Holocene (red line - from the Vostok core measurements) and those during Marine Isotope Stage 11 (blue line - from Dome C). (From Ruddiman et al. (2005a) with kind permission from Springer Science and Business Media.)

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rather than the original 40. This would imply a much lower biomass reduction of 17-20 percent.

Ruddiman's ascription of the post-5000 year BP rise in atmospheric methane concentrations (see also Ruddiman and Thompson 2001) has provoked rather less controversy and the ascription of the main part of the increase to tropical sources (Fluckiger et al. 2001; Figure 3.7) lends credibility to Ruddiman's hypothesis. Schmidt and Shindell (2005), however, consider that "in the absence of further studies ruling out boreal wetlands, tropical river deltas and peatlands as sources of the late Holocene increase in methane emissions, a definitive attribution of the trend to anthropogenic sources is premature".

Up to now, much of the debate over Ruddiman's hypothesis has been in terms of acceptance or rejection as it stands. It seems quite likely that any resolution will require more empirical evidence to constrain the range of possibilities and to address the issue in less extreme terms. The extent to which human activities such as deforestation and paddy cultivation contributed to the observed increases remains an open question. Even if the greater percentage of the Holocene increases proves to be unrelated to anthropogenic activities, there still remains the intriguing possibility that on shorter, more recent time-scales the fortunes of human populations may have led to sufficiently large oscillations in forest biomass to have produced significant fluctuations in atmospheric CO2.

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