Estuaries exchange greenhouse gases with the atmosphere and can either potentially warm or cool the climate. Most studies focus on CO2, but other gases such as CH4 and N2O can also be important. In several tropical deltas, mixing between river water and sea water can lower CO2 levels, leading these systems to act as CO2 sinks even without biological uptake. Here, we measured air‐water CH4 and N2O fluxes in a tropical estuarine delta in Brazil and combined these data with CO2 flux measurements collected at the same time and locations. We found that CH4 was consistently released to the atmosphere, especially during periods of high river discharge, while N2O showed smaller and more variable exchanges. When all gases are expressed in a common climate metric, CH4 and N2O substantially altered the overall greenhouse gas balance. They reduced the apparent cooling effect of CO2 uptake during dry conditions and increased warming effect of CO2 release during wet conditions. Their influence was strongest where CO2 exchange was weak or near equilibrium. Because many tropical deltas show similar CO2 behavior driven by mixing, focusing only on CO2 can lead to misleading conclusions. Considering multiple greenhouse gases is essential to accurately assess their climate impact.
Tropical estuaries and mangroves in the global carbon cycle