Intense methane ebullition from urban inland waters and its significant contribution to greenhouse gas emissions

Abstract

The evasions of methane (CH₄) and carbon dioxide (CO₂) from inland waters represent substantial fluxes of greenhouse gases into the atmosphere, offsetting a large part of the continental carbon sink. However, the CH₄ and CO₂ emissions from urban inland waters are less constrained. In particular, ebullitive CH₄ emissions from these waters are poorly understood. Here, we measured the concentrations and fluxes of CH₄ and CO₂ in rivers and lakes in the megacity of Beijing, China, between 2018 and 2019. The CH₄ concentration ranged from 0.08 to 70.2 µmol L^-1 with an average of 2.5 ± 5.9 µmol L^-1. The average CH₄ ebullition was 11.3 ± 30.4 mmol m^-2 d^-1 and was approximately 6 times higher than the global average. The average total CH₄ flux (14.2 ± 35.1 mmol m^-2 d^-1) was 3 times higher than the global average, with ebullition accounting for 80% of the flux. The high surface water CH₄ concentrations and ebullitive fluxes were caused by high sediment organic carbon/dissolved organic carbon contents, high aquatic primary productivity and shallow water depths in the urban inland waters. The CH₄ emissions accounted for 20% of CO₂ emissions in terms of the carbon release and were 1.7 times higher in terms of CO₂ equivalent emissions from Beijing inland waters. Furthermore, the CH₄ ebullition and its contribution to the total carbon gas emissions increased exponentially with the water temperature, suggesting a positive feedback probably occurs between the greenhouse gas emissions from urban inland waters and climate warming. This study confirms the major role of CH₄ ebullition from urban inland waters in the global carbon budget under the rapid progress of global urbanization.

Keywords: Carbon dioxide (CO(2)); Climate warming; Ebullition; Methane (CH(4)); Urban heat island; Urban inland waters.