Carbon cycle: A dent in carbon's gold standard Matthias Cuntz Nature 477, 547–548 (29 September 2011) doi:10.1038/477547a Published online 28 September 2011 The global uptake of carbon by land plants may be greater than previously thought, according to observations based on the enigmatic Keeling curve of rising atmospheric carbon dioxide. See Letter p.579
Estimates of how much carbon is taken up each year by the world's land plants are derived mainly from models of the carbon cycle. Worldwide measurements of terrestrial carbon exchange have yielded an estimate1 of this global carbon uptake as 123 ± 8 petagrams carbon per year (Pg C yr−1; 1 Pg is 1015 g). This is so close to earlier estimates derived from models and biomass production that 120 Pg C yr−1 can be taken as carbon's 'gold standard'. But Welp and colleagues2 remind us, on page 579 of this issue, that we should not be complacent — land ecosystems might be taking in considerably more carbon than we thought. Our atmosphere is a perfect blender. Changes in its levels of trace gases — such as carbon dioxide — reveal variations in the total influx and uptake of its constituents. So if you measure the carbon exchange of a forest ecosystem, for example, you get the net exchange of all the carbon taken up by the trees for photosynthesis and all the carbon released by the trees and soils through respiration. These gross-exchange fluxes — photosynthesis and respiration — are much larger than the net ecosystem exchange that is actually measured. On the global scale, the net flux is only a few per cent of the gross fluxes. Because small changes in photosynthesis and respiration can have big consequences for the net carbon uptake of terrestrial ecosystems, the interplay between photosynthesis and respiration must be well described in carbon-cycle models if they are to reliably project into the future. It is, however, almost impossible to measure individual components on scales larger than the size of a leaf, let alone on a regional or continental scale. This is where Welp et al.2 take advantage of the composition of oxygen isotopes in CO2 — the chemical signature of which changes if one 16O oxygen atom in CO2 is replaced by a heavier 18O atom. Carbon dioxide dissolves in water and exchanges its oxygen with water's oxygen to equilibrium, so CO2 is tagged by the water it comes into contact with. Different waters have distinct isotopic compositions owing to evaporation processes in soils and leaves — the lighter molecules evaporate faster, and the heavier ones fall behind. As a result, the oxygen isotopic composition in CO2 is very sensitive to photosynthesis and respiration: more photosynthesis means more 18O, and hence higher oxygen-isotope ratios in the atmosphere. Using an impressive 30-year record of the isotopic composition of atmospheric CO2, Welp et al.2 assess the mean atmospheric residence time for oxygen atoms in CO2. Their 11 time series were started in the 1970s by the late Charles Keeling, alongside the famous record of total atmospheric CO2 at Mauna Loa in Hawaii (Fig. 1). Welp et al. identified a strong correlation between the observed interannual variability of the oxygen isotopes and the El Niño–Southern Oscillation (ENSO). Such a correlation has previously been established for the isotopic composition of water3 and, consequently, is now found in the oxygen isotopes of CO2 as well. From the mean residence time of the oxygen atoms in CO2, Welp et al. arrive at a best guess of global productivity of 150–175 Pg C yr−1 — some 25–45% more than the gold standard. Figure 1: Atmospheric CO2 concentrations and isotope composition measured at Mauna Loa, Hawaii. a, Keeling curve of CO2 concentrations since the 1980s (ref. 7). Dots are single measurements or daily averages; line indicates the long-term trend. b, Carbon-isotope composition of the CO2 at Mauna Loa. Here, δ13C is the deviation of the 13C/12C ratio from a standard value. Because the carbon cycle is the major influence on both CO2 concentrations and 13C/12C ratios, the curves in a and b correlate well with each other (that is, the downward trend in b mirrors the upward trend in a and so do the seasonal variations). c, The oxygen-isotope composition of the CO2 is influenced not only by the carbon cycle, but also by the water cycle, and so does not correlate simply with CO2 concentration; δ18O is the deviation of the 18O/16O ratio from a standard value. Welp and colleagues2 find that the interannual variations in δ18O correlate with the El Niño–Southern Oscillation (arrows indicate El Niño events). Their analysis of the oxygen-isotope data also provides a new estimate of global carbon uptake on land. p.p.m., parts per million. (Data are publicly available on the Scripps Institution of Oceanography website8.) Full size image (155 KB) This inference hinges on a set of assumptions and estimates. It depends, for example, on how many CO2 molecules actually enter a plant before one molecule is fixed by photosynthesis. The authors think that plants eventually fix some 43% of all CO2 molecules entering a leaf; however, if this were only 34%, the isotope-based estimate would fall to about 120 Pg C yr−1, the current gold standard. The global value of 43% is derived from carbon-cycle models and remains uncertain, because it depends on the details of the models' formulation. It also depends on the distribution of different plant types. For example, some savannah grasses and maize (corn) fix carbon more efficiently through the C4 metabolic pathway, rather than by the usual C3 route, thereby enabling them to fix about 60% of the CO2 molecules that enter the plant. Hence, the global abundance and distribution of C4 plants are important in estimates of global productivity, whether these are derived from modelling, actual measurements or isotope-composition data. One carbon-cycle model, for example, increased global productivity by more than 20% simply by substituting a new map of C4-plant distribution4. So it looks as though we are stuck with model-based estimates that are hard to validate globally. But other isotopes might yet come to the rescue: the isotopic composition of the carbon atoms in CO2 provides a measure of the percentage of carbon that is fixed5. This could constrain estimates such as that offered by Welp et al., but it could also constrain C4-plant distribution and therefore help non-isotopic estimates of global production as well. And the carbon-isotope estimate of the percentage of carbon that is fixed might be further refined with the help of carbonyl sulphide, a new tracer of leaves' ability to take up CO2 (ref. 6). Gold does not tarnish easily. With their approach, and by making their long-term records publicly available, Welp and colleagues2 are preparing the ground to combine these pieces of information and polish up carbon's gold standard of the future. References Author information References Author information Comments Affiliations Matthias Cuntz is at the UFZ – Helmholtz Centre for Environmental Research, Permoserstrasse 15, 04318 Leipzig, Germany. Corresponding author Correspondence to: Matthias Cuntz -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to geoengineering@googlegroups.com. To unsubscribe from this group, send email to geoengineering+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en.
