In China, as in other nations that produce carbon dioxide from fossil fuels on a large scale, the terrestrial biosphere mops up a proportion of the emissions. Estimates of the amounts involved are now available.
According to recent estimates1,2, in late 2006 China overtook the United States as the world's number one emitter of carbon dioxide, the primary greenhouse gas. This dubious honour highlighted the swift growth of CO2 emissions in China, much of that growth being due to rapid industrialization fuelled by coal-powered energy and cement manufacturing (during which especially large amounts of CO2 are produced).
As reported on page 1009 by Piao and colleagues3, and as in other large CO2 emitters of the Northern Hemisphere, China's trees, shrubs and soils are acting to partly offset the CO2 emissions resulting from fossil-fuel combustion. Piao et al. estimate this biospheric offset by examining changes in soils and vegetation across China and comparing these changes with fluxes calculated both from models of the land biosphere and from measurements in the atmosphere. They conclude that, over the 1980s and 1990s, the biosphere in China removed an average of 0.19–0.26 PgC yr−1(petagrams of carbon per year, where peta is 1015). That figure roughly corresponds to 28–37% of the accumulated CO2 emissions from fossil fuels during that time.
The ability of the global terrestrial biosphere to remove atmospheric CO2 produced by fossil-fuel combustion and tropical deforestation is an essential element in understanding the global carbon cycle4. Although the exact mix of mechanisms remains unclear, the Intergovernmental Panel on Climate Change estimates5 that 2.6 PgC yr−1 (range 0.9–4.3 PgC yr−1) was removed by the global terrestrial biosphere during the 1990s. When compared with the total anthropogenic CO2 emissions due to fossil-fuel combustion, cement manufacture and deforestation (8.0 PgC yr−1; range 6.9–9.1 PgC yr−1)5, the relevance of the biospheric sink becomes clear. The point is all the more significant given the hypothesis that this uptake will slow as climate changes or, worse, turn into a source of CO2 to the atmosphere6. Hence, reliable projections of climate change are inextricably linked to our understanding of carbon uptake in the terrestrial biosphere.
One difficulty in building this understanding is the often contradictory results researchers arrive at when they compare estimates of biospheric uptake from measurements in forests and grasslands at the regional scale with estimates inferred from atmospheric CO2 (known as the 'inverse' approach)7. To get these different approaches to converge, care must be taken to account for all components of carbon exchange between the land and the atmosphere, and the movement of carbon in and out of the region in question8.
To arrive at their estimate for China, Piao et al.3 exhaustively catalogue data from all of the major vegetation and soil categories in China through the use of inventories, field measurements, surveys and remote sensing. In addition to this 'bottom-up' estimate, the researchers used five different process-based ecosystem models and an ensemble of atmospheric inversions to arrive at a convergent estimate.
Of the categories of uptake included, three are especially prominent as contributors to the biological uptake. First, increased summer precipitation and reforestation–afforestation programmes have resulted in an increase in vegetation across China, particularly in the southern regions. Second, reduction in fuel-wood collection has led to an accelerated recovery of shrubland. Third, the expansion of crop production, combined with higher levels of crop residues returned to the soil, has produced increases in agricultural soil carbon.
Inverse studies have previously estimated net biological uptake in the temperate Asia region as a whole9. The convergence of the three methods used by Piao et al. strengthens that conclusion considerably, and offers insight into what components of China's complex landscape are responsible for carbon uptake. As might be expected, the uncertainties on the uptake figures are large for each of the approaches, and some of the terms in the bottom-up estimates rely on limited data and must, by necessity, incorporate assumptions. It could also be argued that there is some overlap in the methods, in that data sets used to drive the ecosystem modelling are also used in the inventory estimation. However, the agreement is striking even in light of these difficulties.
A broader context to the estimates of Piao et al. is provided by Table 1 and Figure 1. These give a comparison of the fossil-fuel CO2 emissions for China and the United States, normalized by the main drivers of emissions such as population, energy consumed and gross domestic product (GDP)10. The table and figure highlight the fact that, although China is the smaller emitter on a per-capita basis, it is the larger emitter when normalized by GDP (carbon intensity of GDP), a reflection of the relatively carbon-intensive power production and inefficiencies in the use of energy for making goods and services. However, the carbon intensity of GDP has been trending down, relative to the United States, as is evidenced by the halving of this measure between 1990 and 2006. This was driven primarily by more efficient use of energy as opposed to a less fossil-fuel-based energy-production system. The trend reversed in 2001, driven by an increasing reliance on carbon-intensive primary energy consumption.
In this broader context, a relevant question is what will happen to China's net carbon balance in the future. Although the land biosphere has partly offset Chinese fossil-fuel emissions of CO2 in the past, this offset is very likely to diminish in percentage terms as China continues to emit more fossil-fuel CO2 each year. For example, on the basis of an estimate of emissions for the year 2007, derived from data provided by G. Marland (Oak Ridge National Laboratory), the mean uptake estimated by Piao et al. would have reduced the fossil-fuel CO2 emissions by 10–15%. By contrast, projections for the year 2030, produced by the International Energy Agency11, place Chinese fossil-fuel CO2 emissions at 3.1 PgC yr−1, of which only 6–8% will be offset by biospheric uptake should China's biosphere continue to remove carbon at its current rate (a questionable prospect).
The next round of climate-change treaty negotiations will commence in December 2009, in Copenhagen, where increasing pressure will be placed on all countries of the world to join the international Kyoto Protocol, and mitigate their emissions. China is clearly an important player in this process. Likewise, the status of China's land sink will also figure prominently because intentional (as opposed to passive) sink enhancement has a role in the international treaty. Because only a portion of the biospheric uptake estimated by Piao et al.3 can be construed as intentional, the allowed biotic offset is likely to be small. But unless China curbs its fossil-fuel CO2 emissions, even this intentional land sink will be of diminishing help.
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Netherlands Environmental Assessment Agency. 'China now no. 1 in CO2 emissions; USA in second position' (press release, Bilthoven, Netherlands, 19 June 2007).
Piao, S. et al. Nature 458, 1009–1013 (2009).
Sarmiento, J. L. & Gruber, N. Phys. Today 55, 30–36 (2002).
Denman, K. et al. in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (eds Solomon, S. D. et al.) 499–587 (Cambridge Univ. Press, 2007).
Friedlingstein, P. et al. J. Clim. 19, 3337–3353 (2006).
House, J. I., Prentice, I. C., Ramankutty, N., Houghton, R. A. & Heimann, M. Tellus B 55, 345–363 (2003).
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