Freshwater Methane Emissions Offset the Continental Carbon Sink

摘要

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) (7). 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 CO_2 (3, 5) and CH_4, (6). Over time, soil carbon reaches freshwaters by lateral hydro-logical transport, where it can meet several fates, including burial in sediments, further transport to the sea, or evasion to the atmosphere as CO_2 or CH_4 (7). CH_4 emissions may be small in terms of carbon, but CH_4 is a more potent GHG than CO_2 over century time scales. This study indicates that global CH_4 emissions expressed as CO_2 equivalents correspond to at least 25% of the estimated terrestrial GHG sink.