Global methane and nitrous oxide emissions from inland waters and estuaries

Inland waters (rivers, reservoirs, lakes, ponds, streams) and estuaries
are globally significant emitters of methane (CH4) and nitrous oxide (N2O)
to the atmosphere, while global estimates of these emissions have been
hampered due to the lack of a worldwide comprehensive dataset with the
collection of complete CH4and N2O flux components. Here, we synthesize
2,997 in-situ flux or concentration measurements of CH4 and N2O from 277
peer-reviewed publications to explore the role of inland waters and
estuaries in shaping climate change. We estimate that inland waters
including rivers, reservoirs, lakes, and streams together release 95.18 Tg
CH4 yr-1 (ebullition plus diffusion) and 1.48 Tg N2O
yr-1 (diffusion) to the atmosphere, yielding an overall
CO2-equivalent emission total of 3.06 Pg CO2 yr-1, representing roughly
60% of CO2 emissions (5.13 Pg CO2 yr-1) from these four
inland aquatic systems, among which lakes act as the largest emitter for
both CH4and N2O. Ebullition is noticed as a dominant flux component of
CH4, contributing up to 62–84% of total CH4fluxes across all inland
waters. Chamber-derived CH4 emission rates are significantly greater than
those determined by diffusion model-based methods for commonly capturing
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 N2O fluxes from inland waters. Our study reveals a
major oversight in regional and global CH4budgets from inland waters,
caused by neglect of the dominant role of ebullition pathways in those
emissions. The indirect N2O EF5 values established in this study generally
suggest a downward revision 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 N2O emissions from inland waters and estuaries is essential
in defining how these aquatic systems will shape our climate.