Global methane and nitrous oxide emissions from inland waters and estuaries

Abstract

Inland waters (rivers, reservoirs, lakes, ponds, streams) and estuaries are significant emitters of methane (CH₄ ) and nitrous oxide (N₂ O) to the atmosphere, while global estimates of these emissions have been hampered due to the lack of a worldwide comprehensive data set of CH₄ and N₂ O flux components. Here, we synthesize 2997 in-situ flux or concentration measurements of CH₄ and N₂ O from 277 peer-reviewed publications to estimate global CH₄ and N₂ O emissions from inland waters and estuaries. Inland waters including rivers, reservoirs, lakes, and streams together release 95.18 Tg CH₄ year^-1 (ebullition plus diffusion) and 1.48 Tg N₂ O year^-1 (diffusion) to the atmosphere, yielding an overall CO₂ -equivalent emission total of 3.06 Pg CO₂ year^-1 . The estimate of CH₄ and N₂ O emissions represents roughly 60% of CO₂ emissions (5.13 Pg CO₂ year^-1 ) from these four inland aquatic systems, among which lakes act as the largest emitter for both CH₄ and N₂ O. Ebullition showed as a dominant flux component of CH₄ , contributing up to 62%-84% of total CH₄ fluxes across all inland waters. Chamber-derived CH₄ emission rates are significantly greater than those determined by diffusion model-based methods for commonly capturing of both diffusive and ebullitive fluxes. Water dissolved oxygen (DO) showed as a dominant factor among all variables to influence both CH₄ (diffusive and ebullitive) and N₂ O fluxes from inland waters. Our study reveals a major oversight in regional and global CH₄ budgets from inland waters, caused by neglecting the dominant role of ebullition pathways in those emissions. The estimated indirect N₂ O EF₅ values suggest that a downward refinement is required in current IPCC default EF₅ values for inland waters and estuaries. Our findings further indicate that a comprehensive understanding of the magnitude and patterns of CH₄ and N₂ O emissions from inland waters and estuaries is essential in defining the way of how these aquatic systems will shape our climate.

Keywords: estimate; estuaries; indirect emission factor; inland waters; methane; nitrous oxide.