Year‐2020 Global Distribution and Pathways of Reservoir Methane and Carbon Dioxide Emissions According to the Greenhouse Gas From Reservoirs (G‐res) Model

Abstract

Collectively, reservoirs constitute a significant global source of C-based greenhouse gases (GHGs). Yet, global estimates of reservoir carbon dioxide (CO2) and methane (CH4) emissions remain uncertain, varying more than four-fold in recent analyses. Here we present results from a global application of the Greenhouse Gas from Reservoirs (G-res) model wherein we estimate perarea and per-reservoir CO2 and CH4 fluxes, by specific flux pathway and in a spatially and temporally explicit manner, as a function of reservoir characteristics. We show: 1) CH4 fluxes via degassing and ebullition are much larger than previously recognized and diffusive CH4 fluxes are lower than previously estimated, while CO2 emissions are similar to those reported in past work; 2) per-area reservoir GHG fluxes are >29% higher than suggested by previous studies, due in large part to our novel inclusion of the degassing flux in our global estimate; 3) CO2 flux is the dominant emissions pathway in boreal regions and CH4 degassing and ebullition are dominant in tropical and subtropical regions, with the highest overall reservoir GHG fluxes in the tropics and subtropics; and 4) reservoir GHG fluxes are quite sensitive to input parameters that are both poorly constrained and likely to be strongly influenced by climate change in coming decades (parameters such as temperature and littoral area, where the latter may be expanded by deepening thermoclines expected to accompany warming surface waters). Together these results highlight a critical need to both better understand climate-related drivers of GHG emission and to better quantify GHG emissions via CH4 ebullition and degassing.

Spatiotemporal Methane Emission From Global Reservoirs

“…The 9.9 Tg CH 4 yr −1 from diffusion + ebullition emission pathways is almost identical to our results although the spatial distributions may differ. Degassing alone accounts for ∼55% of the global total from Harrison et al (2021) but this study indicates that this pathway is very uncertain, flux observations are limited, and methods to characterize reservoirs and management should be improved.…”

“…10.1029/2021JG006305 10 of 19 Lehner et al (2011), HydroLAKES (Messager et al, 2016), and GRanD are the same data. Harrison et al [2021]). It should be noted that both studies used air temperature as a proxy for the temperature at the air-water and sediment-water interface (where a majority of reservoir CH 4 is produced).…”

“…Emission season lengths for high latitude lakes derived with satellite data are also noticeably shorter (by 9%-32%) than those assumed in other lake studies . Harrison et al (2021) recently reported total emissions of 22 Tg CH 4 yr −1 from 350 × 10 3 km 2 of global reservoir areas (comprising 1.6 Tg CH 4 yr −1 via diffusion and 8.3 Tg CH 4 yr −1 via ebullition). The 9.9 Tg CH 4 yr −1 from diffusion + ebullition emission pathways is almost identical to our results although the spatial distributions may differ.…”

“…vidual reservoir systems whereas Rosentreter et al (2021) and Harrison et al (2021) defined emission periods using air temperature for reservoirs in locations >0°C (>4°C for diffusion in Harrison et al (2021)) and ice-cover periods for those locations <0°C (Table 2). Specific season lengths are not reported for either of these studies.…”

“…Most importantly, none of the existing global estimates provide CH 4 emissions which are both spatially and temporally explicit. While the recent study by Harrison et al (2021) incorporated temporal variability into their estimates of annual-scale emission, gridded estimates of sub-annual CH 4 emission are not provided. This study provides gridded estimates of daily CH 4 emission from reservoirs which is a critical requirement for use in bottom-up biogeochemical and top-down atmospheric CH 4 emission modeling.…”