The boundless carbon cycle
Tom J. Battin, Sebastiaan Luyssaert, Louis A. Kaplan, Anthony K. Aufdenkampe, Andreas Richter and Lars J. Tranvik The terrestrial biosphere is assumed to take up most of the carbon on land. However, it is becoming clear that inland waters process large amounts of organic carbon and must be considered in strategies to mitigate climate change.
tmospheric carbon dioxide concentrations increased from ~280 ppm before the industrial revolution to over 384 ppm in 2008 (ref. 1). This increase reflects only about half of the CO2 emissions from human activities; the other half has been sequestered in the oceans and on land2,3 (Box 1). Although the location and magnitude of continental carbon sinks remain uncertain4, they are assumed to lie within the terrestrial biosphere. We argue that inland waters have a significant role in the sequestration, transport and mineralization of organic Box 1 | Balancing the carbon cycle
carbon. Integration of these fluxes into the traditional carbon cycle is needed for appropriate CO2 management and climate change mitigation. Inland waters — such as ponds, lakes, wetlands, streams, rivers and reservoirs — permeate terrestrial ecosystems and often shape the Earth’s landscapes. Although only about 1% of the Earth’s surface is assumed to be covered by inland waters, their collective contribution to global carbon fluxes is substantial compared with terrestrial and marine ecosystems5–10. Specifically, current estimates
carbon dioxide sinks Since 1750, continuously increasing anthropogenic CO2 emissions and land-use change have perturbed the natural carbon cycle. Of the 9.1 Pg C yr–1 (1 Pg C = 1 petagram or 109 metric tons of carbon) emitted in this way between 2000 and 2006, 4.1 Pg C yr–1 have accumulated in the atmosphere, 2.2 Pg C yr–1 have been assigned to marine sequestration and the residual 2.8 Pg C yr–1 have been assigned to sequestration within the terrestrial biosphere3. At regional and continental scales the terrestrial carbon sink has been evaluated by top-down and bottom-up carbon balances20,21. estimating from the top down In the top-down approach, the carbon balance from an atmospheric perspective is compiled by running an atmospheric transport model (the so-called inverse model) back in time. The distribution of sources and sinks at land and ocean surfaces is then optimized for observed atmospheric CO2 concentrations. This approach has confirmed the location of the residual carbon sink over continents. However, state-of-the-art inverse models have a spatial resolution too coarse to account for most inland waters. Therefore, CO2 outgassing from inland waters is assigned to terrestrial ecosystem respiration, blending the carbon sink in inland waters with the terrestrial carbon sink. scaling from the bottom up The bottom-up approach compiles the carbon balance by scaling up site-level observations of sinks and sources of croplands, grasslands and forests as the main land-use types. Inland waters are usually not considered among the main land-use types, with the exception of reservoirs for the carbon sink of the coterminous US4. Furthermore, study sites are typically located in uplands to catch a terrestrial signal with little interference from aquatic ecosystems. Consequently, carbon export from terrestrial ecosystems to inland waters is not typically accounted for in regional estimates that scale-up from the bottom-up approach. This in turn contributes to the discrepancy between estimates based on the bottom-up and top-down approaches. 598
suggest that inland waters transport, mineralize and bury ~2.7 Pg C yr−1 (ref. 5; Fig. 1). This is similar to the size of the terrestrial carbon sink for anthropogenic emissions of 2.8 Pg C yr−1 (ref. 3). So far, carbon fluxes into and out of inland waters have received little attention in global-scale analyses. However, their quantification could prove critical for constraining estimates of...