Attribution of increasing global lake methane emissions to climate and eutrophication using the FLaMe model

Methane (CH₄) emissions from lakes were considered entirely natural by the Intergovernmental Panel on Climate Change (IPCC) and the Global Methane Budget (Saunois et al., 2020). However, eutrophication, via enhanced inputs of nutrients (mostly total dissolved phosphorus, TDP) from the surrounding catchments has been shown to be a substantial control factor of both diffusive and ebullitive lake CH₄ fluxes, suggesting that a fraction of these emissions are in fact attributable to human factors. Here, we adopted a newly developed physically-resolved process-based model of the coupled carbon-oxygen-methane cycles, FLaMe (Fluxes of Lake Methane), to simulate decadal trends (1901-2070) in CH4 emissions and decompose them into natural and anthropogenic components. By configurating global lakes from the HydroLAKES database (with an area of 2.47 million km²), we estimated that global lake CH₄ emissions already increased by about 20 % over the last century (from 28±1 to 34±1 Tg CH₄ yr^-1). Furthermore, we adopted a factorial experiment approach to conduct the attribution analysis, and found that over this time period, eutrophication and climate contributed to 70% and 30% of the cumulative growth in global lake CH₄ emissions, respectively. Moreover, we identified a progressive shift from eutrophication to climate control on global lake CH₄ emissions from the early part till the end of the last century. In the future, we project that global lake CH₄ emissions will further increase to reach 39±2, 44±4 and 45±5 Tg CH₄ yr^-1 by year 2070 under climate scenarios SSP1-2.6, SSP3-7.0 and SSP5-8.5, respectively. Our analysis implies that the future risks stemming from lake CH₄ emissions could be reduced by efficient nutrient removal from urban and agricultural sources.