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Freshwater Methane Emissions Offset the Continental Carbon Sink

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Science  07 Jan 2011:
Vol. 331, Issue 6013, pp. 50
DOI: 10.1126/science.1196808

Abstract

Inland waters (lakes, reservoirs, streams, and rivers) are often substantial methane (CH4) sources in the terrestrial landscape. They are, however, not yet well integrated in global greenhouse gas (GHG) budgets. Data from 474 freshwater ecosystems and the most recent global water area estimates indicate that freshwaters emit at least 103 teragrams of CH4 year−1, corresponding to 0.65 petagrams of C as carbon dioxide (CO2) equivalents year−1, offsetting 25% of the estimated land carbon sink. Thus, the continental GHG sink may be considerably overestimated, and freshwaters need to be recognized as important in the global carbon cycle.

A cornerstone of our understanding of the contemporary global carbon cycle is that the terrestrial land surface is an important greenhouse gas (GHG) sink (1, 2). The global land sink is estimated to be 2.6 ± 1.7 Pg of C year−1 (variability ± range, excluding C emissions because of deforestation) (1). Lakes, impoundments, and rivers are parts of the terrestrial landscape, but they have not yet been included in the terrestrial GHG balance (3, 4). Available data suggest, however, that freshwaters can be substantial sources of CO2 (3, 5) and CH4 (6). Over time, soil carbon reaches freshwaters by lateral hydrological transport, where it can meet several fates, including burial in sediments, further transport to the sea, or evasion to the atmosphere as CO2 or CH4 (7). CH4 emissions may be small in terms of carbon, but CH4 is a more potent GHG than CO2 over century time scales. This study indicates that global CH4 emissions expressed as CO2 equivalents correspond to at least 25% of the estimated terrestrial GHG sink.

CH4 can be emitted from freshwaters through several different pathways, including ebullition (bubble flux from sediments), diffusive flux, and plant-mediated transport through emergent aquatic plants (6). Additional pathways may be important for hydroelectric reservoirs, such as emissions upon passage through turbines and downstream of reservoirs (8, 9). We compiled CH4 emission estimates from 474 freshwater ecosystems for which the emission pathways were clearly defined (Table 1) (10).

Table 1

Freshwater CH4 emissions (Emiss., in Tg CH4 year−1) estimated from average areal estimates (flux m−2 year−1) times the areal estimates for different latitudes (10). Total open water is the sum of open water fluxes, that is, ebullition, diffusive flux, and flux when CH4 stored in the water column is emitted upon lake overturn (Stored). n and CV (%) denote the sample size (number of systems) and the coefficient of variation. Note the small sample size for many large emission values. The total sums of the yearly fluxes are expressed in Tg CH4. Lake and river areas are from (11); reservoir areas, from (12). Plant fluxes (plant-mediated emission) are according to (10).

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By using recent data on the area and distribution of inland waters (11, 12), we estimate the total CH4 emission from freshwaters to be 103 Tg of CH4 year−1 (Table 1). Expressed as CO2 equivalents (eq), this corresponds to 0.65 Pg of C (CO2 eq) year−1 or 25% of the estimated land GHG sink, assuming that 1 kg of CH4 corresponds to 25 kg of CO2 over a 100-year period (13). Ebullition and plant flux, which are both poorly represented in the data set, dominate the other flux pathways that have been studied more frequently (Table 1). Ebullition is most likely to be underestimated because it is episodic and not representatively captured by the usual short-term measurements (6). Accordingly, our global estimate of freshwater CH4 emissions is probably conservative. For further discussion of the results, see supporting online material (SOM) text.

This study indicates that CH4 emissions from freshwaters can substantially affect the global land GHG sink estimate. Moreover, proper consideration of ebullition and plant-mediated emission will most likely result in increased future estimates of CH4 emission. Combining the present CH4 emission estimate of 0.65 Pg of C (CO2 eq) year−1 with the most recent estimate of freshwater CO2 emissions, 1.4 Pg of C (CO2 eq) year−1 (5)—together corresponding to 79% of the estimated land GHG sink—it becomes clear that freshwaters are an important component of the continental GHG balance. Accordingly, the terrestrial GHG sink may be smaller than currently believed, and data on GHG release from inland waters are needed in future revision of net continental GHG fluxes.

Supporting Online Material

www.sciencemag.org/cgi/content/full/331/6013/50/DC1

Materials and Methods

SOM Text

References

References and Notes

  1. Materials and methods are available as supporting material on Science Online.
  2. We thank J. Cole, N. Than Duc, and H. Marotta for valuable input. This study was supported by the Swedish Research Council (VR) and the Swedish Research Council for Environment, Agricultural Sciences, and Spatial Planning (Formas). Analyses of global surface water area come from the ITAC (Integrating the Terrestrial and Aquatic Carbon Cycle) Working Group supported by the National Center for Ecological Analysis and Synthesis, a center funded by NSF (grant DEB-94-21535), the University of California at Santa Barbara, and the state of California.
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