Abstract
Accounting for the residual land sink (or missing carbon sink) has become a major budget focus for global carbon cycle modelers. If we are not able to account for the past and current sources and sinks, we cannot make accurate predictions about future storage of fossil fuel combustion emissions of carbon in the terrestrial biosphere. Here, we show that the autochthonous production (AP) in inland waters appears to have been strengthening in response to changes in climate and land use, as evidenced by decreasing CO2 emissions from and increasing dissolved organic carbon storage and/or organic carbon burial in inland waters during recent decades. The increasing AP may be due chiefly to increasing aquatic photosynthesis caused by global warming and intensifying human activities. We estimate that the missing carbon sink associated with the strengthening AP in inland waters may range from 0.38 to 1.8 Gt C yr-1 with large uncertainties. Our study stresses the potential role that AP may play in the further evolution of the global carbon cycle. Quantitative estimates of future freshwater AP effects on the carbon cycle may also help to guide the action needed to reduce carbon emissions, and increase carbon sinks in terrestrial aquatic ecosystems.
Highlights
Following a thorough review of previous publications, we have found that the autochthonous production (AP) in inland waters (Liu et al, 2010; Liu et al, 2018) appears to have been strengthening in response to the changes of climate and land-use (Figures 1, 2), as evidenced by decreasing CO2 emissions from and increasing dissolved organic carbon (DOC) storage and organic carbon (OC) burial in inland waters during the past 60–150 years (e.g., Figures 3–5)
If we take three-fold (Anderson et al, 2013) as the conservative estimate for the increase in OC burial flux in inland waters in the recent past, with 0.6 Gt C yr−1 (Tranvik et al, 2009) and 0.2 Gt C yr−1 (Cole et al, 2007) as maximum and minimum global OC burial rates in inland waters respectively, a maximum of 1.8 Gt C yr−1 and a minimum of 0.6 Gt C yr−1 are obtained for the missing carbon sink, which are the same order of magnitude as those obtained above for the estimates of decreasing CO2 emissions
We have shown that the AP in inland waters appears to have been strengthening in response to global changes of climate and land use, as evidenced by decreasing CO2 emissions from, and increasing DOC storage and OC burial in, inland waters during recent decades
Summary
For the past three centuries CO2 emissions from human activities, including the burning of fossil fuels and extensive land-use changes, have unbalanced the natural cyclical growth and rise of CO2 in the atmosphere and the accumulation of CO2 in the oceans (Broecker et al, 1979; Houghton et al, 1990; Tans et al, 1990; Houghton, 2003; Melnikov and O’Neill, 2006; Houghton, 2007; Ballantyne et al, 2012; Ciais et al, 2013; Houghton et al, 2018; Kirschbaum et al, 2019). These studies suggest that anthropogenic change leads to higher OC burial rates by increasing autochthonous and allochthonous carbon fluxes, and that global carbon burial in lakes will become more significant with continuing eutrophication caused by human activities.
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