Eutrophication will increase methane emissions from lakes and impoundments during the 21st century

Lakes and impoundments are an important source of methane (CH4), a potent greenhouse gas, to the atmosphere. A recent analysis shows aquatic productivity (i.e., eutrophication) is an important driver of CH4 emissions from lentic waters. Considering that aquatic productivity will increase over the next century due to climate change and a growing human population, a concomitant increase in aquatic CH4 emissions may occur. We simulate the eutrophication of lentic waters under scenarios of future nutrient loading to inland waters and show that enhanced eutrophication of lakes and impoundments will substantially increase CH4 emissions from these systems (+30–90%) over the next century. This increased CH4 emission has an atmospheric impact of 1.7–2.6 Pg C-CO2-eq y−1, which is equivalent to 18–33% of annual CO2 emissions from burning fossil fuels. Thus, it is not only important to limit eutrophication to preserve fragile water supplies, but also to avoid acceleration of climate change.

and used to parameterize the future scenarios simulated in this study. The principal mechanism of increasing eutrophication will be increases in P release from populationdriven agriculture and sewage release. These increases will be further augmented by the increasing spatial extent of lakes and impoundments, increased nutrients due to intense storm-and flood-driven runoff, and the warming of water bodies due to climate change. Export or loading of N and P are assumed to be near-linearly related to concentrations in inland waters because, unless future water loads drastically increase in regions producing agricultural crops 2 , loading of N and P are known to be tightly coupled with concentrations in lakes 3 . Further, concentrations of N and P have been shown to be quite tightly correlated with each other 4 . This analysis does not account for increased nutrient concentrations in warm regions due to concentration from enhanced evaporative losses 2, 5 so overall factors are likely underestimates. If a range of values was reported by the publication, the median or average is reported here and in Table 1.

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Change 1.23× --1.41x --Predictions of increased N export to oceans derived from a nitrate export model for river basins. Concentration increases in inland waters will be higher than this because these estimates are for effluent from inland waters after nutrients have moved downstream from lakes to river tributaries to the sea. Export is considered proportional to concentration because the predictions are based entirely on human population increases driving releases of nutrients from sewage and agriculture.
Bouwman et al (2013) 1.19x 1.50x ----Increased run-off P from global food animal production, regardless of animal production scenarios. Manure and urine from animal production is highly mobile, containing high concentrations of N and P. Animal production is projected to be strongly correlated with export, loading, and concentrations in receiving waters.
Prediction of N and P-driven eutrophication increase due to expanded agriculture and population growth. These are direct estimates of increased nutrient concentrations in freshwaters based on conversion of natural lands to agriculture, losses of nutrients from fertilized crop, and untreated manure production.
Cordell et al (2009) --1.97× --3.19× Based on the "peak P" concept concerning depleting reserves and the idea that 55% of all mined P is released to water. Both estimates are derived from cumulative P mined since 1950 and assume no major improvement in conservation of P and no increase in identified P rock reserves. Because most changes in nutrient concentrations in freshwaters are linked to increased loading, except in regions where water discharge is likely to increased faster than nutrient export, nutrient concentrations in global waters will climb proportionately with increased P depletion.
Samir & Lutz (2017) 1.37× --1.50× --Projected population increase without utopian assumptions, assuming a wealthy and fragmented world (SSP3). Assumes proportionality of water quality impact and population growth and no change in per capita food demands and sewage production.
Concentrations are projected to rise because exports will increase. This will be true unless water discharges increase markedly in population centers and agricultural areas supporting them. nutrient loading to inland waters by 1.10×. These projections are based on the fact that intense storms liberate more nutrients than the amount mobilized by the same precipitation delivered gradually.
Downing (2014) --1.30× Increased global primary production of lakes and impoundments due to increased temperature alone. Reported is the median rate of increase for the latitudes with most ample agriculture and aquatic systems. Global water temperature is projected from known models of the relationship between ambient and surface water temperature and known relationships between aquatic primary production (directly related to P) and water temperature.