Long-term changes in inland water CH4 emissions across the United States in response to climate change and anthropogenic activities

Abstract:

Emissions of CH₄ from inland waters, including rivers, lakes, and reservoirs, have been characterized as essential parts of the regional Greenhouse Gas budget. However, accurate estimation of long-term changes in inland-water CH₄ emissions in response to climate change and human activities at a continental scale is still poorly constrained. That is because no modeling tool can simulate the CH₄ emissions along the linked river-lake-reservoir corridor simultaneously, and fully couple with a terrestrial ecosystem model. Inspired by this knowledge gap, we developed an aquatic CH₄ module within an improved large-scale channel routing scheme. We coupled this aquatic model with the DLEM (Dynamic Land Ecosystem Model) to simulate the riverine carbon fluxes and the associated inland-water CH₄ emissions across over the Conterminous United States during the period from 1860 to 2019. The simulated water quality variables and CH₄ fluxes were well validated against observations. Additionally, we conducted attribution analysis to quantify the impacts of climate change, land conversion, N deposition, land N applications, and atmospheric CO₂ concentration on the terrestrial ecosystems and the resultant inland-water CH₄ emissions. Our results suggested that the CH₄ emission from rivers and natural lakes decreased significantly from the 1860s to the 1900s primarily due to land-use change. Climate variability dominate the changes in CH₄ emissions from rivers and lakes, and substantially increased the level of inland water CH₄ emissions since the 1900s. The CH₄ emissions from reservoirs increased continuously and became the largest source of inland water CH₄ emissions since the 1960s primarily due to the increasing dam buildings. Through incorporating the sub-grid routing scheme, we also found that the headwater streams (1^st –3^rd order streams), which is extremely sensitive to climate variations, account for most (53%) of the diffusive CH₄ emissions followed by high-order streams (27%), reservoirs (12%), and lakes (8%) during the 2010s.