Global change

Indirect feedbacks to rising CO2

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There have been many studies on the effects of enriched levels of atmospheric carbon dioxide on soils. A meta-analysis shows that emissions of other greenhouse gases increase under high-CO2 conditions. See Letter p.214

Human activities have caused atmospheric concentrations of carbon dioxide, a major greenhouse gas, to increase at an accelerating pace. Starting at around 280 parts per million (p.p.m.) in pre-industrial times, they have now exceeded 390 p.p.m., and are expected to reach 600–800 p.p.m. by the end of the century1. On page 214 of this issue2, van Groenigen and colleagues add to our awareness of the complex consequences of this trend, in terms of the effect that it will have on emissions of other greenhouse gases from various ecosystems.

In producing global warming, CO2 is responsible for the largest part of the anthropogenic impact on Earth's energy balance. It is, of course, also an essential nutrient for plant metabolism. Numerous CO2-enrichment experiments over the past two decades have demonstrated the positive effect of elevated CO2 on plant growth — increased biomass and increased carbon storage in soils3. The vegetation response to elevated CO2 might be constrained by various interactions with water and nutrients such as nitrogen4,5. However, experiments and model projections suggest that accelerated plant growth due to CO2 fertilization could draw down some of this gas from the atmosphere, and hence could weaken future rates of CO2 increase and lessen the severity of climate change6.

Van Groenigen et al.2 present evidence that rising levels of CO2 are not only resulting in an increased carbon sink in terrestrial ecosystems, but could also cause increased emissions of other, much more potent, greenhouse gases such as methane (CH4) and nitrous oxide (N2O) from soils. Methane is produced by anaerobic methanogenic microorganisms that thrive in wetlands, including rice paddies, where labile (biologically accessible) carbon is available and diffusion of oxygen into the soil is severely restricted. Nitrous oxide is mainly produced in soils by aerobic nitrifying and anaerobic denitrifying bacteria. The interaction between nitrogen availability and soil water content controls the rate of N2O production. The respective global-warming potentials of CH4 and N2O are 25 and 298 times greater than that of CO2, and thus they influence Earth's energy balance even though they occur in much smaller concentrations.

Van Groenigen and colleagues collected information from 49 published studies that reported the effect of atmospheric CO2 enrichment on CH4 and N2O fluxes from soils. Using a meta-analysis, they show that elevated CO2 stimulated N2O emissions by 18.8%, and that CH4 emissions from wetlands increased by 13.2% and from rice paddies by as much as 43.4%. Notably, they also suggest the mechanisms that are probably responsible for these observed increases in greenhouse-gas emissions (Fig. 1).

Figure 1: Proposed mechanisms of increased N2O and CH4 emissions from soils.
figure1

From their meta-analysis, van Groenigen et al.2 estimate that rising levels of atmospheric CO2 will result in more output of N2O from upland soil (at a rate equivalent to 0.57 Pg CO2 yr−1) and of CH4 from rice paddies and wetlands (equivalent to 0.56 Pg CO2 yr−1). They suggest that these increases are caused by reduced plant transpiration under conditions of elevated CO2, resulting in increased soil moisture. Together with increased root biomass, this leads both to greater denitrification (and hence increased N2O emission) and to more methanogenic activity (and hence increased CH4 emission). The increase in these greenhouse gases will thus partially offset the predicted enhanced uptake of carbon by terrestrial ecosystems in a high-CO2 world.

Their suggestion goes as follows. Elevated CO2 led to reduced plant transpiration (the evaporation of water from plant surfaces, leaves in particular), which increased soil water content and promoted the existence of anaerobic microsites in soils. This, together with increasing biological activity, probably stimulated denitrification and consequently N2O production. Also, the CO2-induced increase in root biomass may have contributed by increasing the availability of labile carbon, a crucial energy source for denitrification. The CO2-induced stimulation of CH4 emissions from wetlands and rice paddies was probably the result of higher net plant production, leading to increasing carbon availability for substrate-limited methanogenic microorganisms. Extrapolating their results to the global scale, van Groenigen et al.2 estimate that the combined effect of stimulated N2O and CH4 emissions could be equivalent to at least 1.12 Pg CO2 yr−1 (Pg = petagrams = 1015 grams). This is around 17% of the expected increase of the terrestrial CO2 sink as a result of higher CO2 concentrations.

Earlier studies have shown that long-term carbon sequestration in a CO2-enriched atmosphere can be constrained by nitrogen availability5,6. Critics may wonder how these studies and van Groenigen and colleagues' analysis fit together, as it seems unlikely that denitrification would be stimulated by elevated CO2 in nitrogen-limited ecosystems. This apparent discrepancy may be explained by the geographical bias in the present paper. The large majority of the 49 studies included in the meta-analysis were located in temperate regions, in areas — the United States, Europe, China and Japan — that are nowadays subject to considerable deposition of atmospheric nitrogen7. Some ecosystems included in the meta-analysis, such as agricultural areas receiving little or no fertilizer, and regions of natural vegetation, may thus have been subject to the input of considerable anthropogenic nitrogen through the atmosphere. Because nitrogen deposition is predicted to increase in the coming decades, the studies may therefore be more representative of future conditions, when nitrogen deposition will have become a global feature.

Another striking point is the almost complete lack of studies in the tropics and subtropics, where the strongest increases in nitrogen deposition are expected to occur7. Some tropical ecosystems may react differently from temperate ecosystems to elevated CO2 concentrations. Many intact tropical forests tend to cycle large quantities of nitrogen8, and an increase in soil-moisture content may have strong effects on N2O emissions even without nitrogen deposition. Tropical grasslands are dominated by grasses using the C4 photosynthetic pathway, which may improve their water-use efficiency to different extents from that of plants using the C3 pathway. There is a clear need for field studies in these ecosystems, in order to improve our ability to evaluate the overall effect of elevated CO2 on the budgets of greenhouse-gas emissions.

Obviously, the report by van Groenigen et al.2 is not the end of the story, and future research may provide evidence of other feedbacks that have not yet been quantified or even hypothesized. Nevertheless, this study provides the first comprehensive analysis of available data that shows the importance of indirect feedbacks of elevated CO2 on CH4 and N2O emissions on a global scale. It is now up to the scientific community to include these feedbacks in global climate models and to fill in the large gaps in information that still exist.

References

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Correspondence to Alexander Knohl or Edzo Veldkamp.

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Knohl, A., Veldkamp, E. Indirect feedbacks to rising CO2. Nature 475, 177–178 (2011) doi:10.1038/475177a

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