The Global Importance of CO2 and CH4 Emissions from Ponds: A Large-Scale Data Perspective

Inland waters, including small lakes and ponds, play a major role in the global carbon cycle. While they typically act as organic carbon sinks, they also emit the greenhouse gases (GHGs) carbon dioxide (CO₂) and methane (CH₄) to the atmosphere. Nonetheless, small inland waters, defined as those with a surface area (SA) of less than 5 hectares (further referred as “ponds”), are often excluded from large-scale CO₂ and CH₄ budgets. To help overcome this gap, we reviewed available global datasets on inland waters and selected G1WBM, GLCF GIW, GSW, and OSM, because these datasets provide sufficient information on ponds to assess their global distribution. We further compiled a dataset of CO₂ and CH₄ emissions plus water chemistry data from 950 ponds worldwide from existing literature and databases. Next, we applied a Monte Carlo analysis to the estimated surface areas and CO₂ and CH₄ emission ranges of ponds. The results suggest that ponds with SA < 1 ha emit 0.25–0.42 Pg C yr^-1, and those of 1–5 ha emit 0.18–0.45 Pg C yr^-1, accounting for up to 14 and 17%, respectively, of the total carbon gas emissions from all-sized lakes and ponds worldwide. Our estimates thus further highlight the potentially disproportionate, yet poorly constrained, importance of ponds in global GHG budgets. In addition to water chemistry data, we also extracted global gridded hydrometeorological and socio-economic data matched to each HydroBASIN-delineated basin of the 950 lakes. Using Random Forest regression (RFR) models, we found that pond water pH and watershed urbanization were the most important predictors of the CO₂ emissions, while electrical conductivity (EC), SA, and pond depth were the most important variables for the CH₄ emissions. The RFR modeling revealed that ponds in urban areas typically exhibit elevated pH levels, probably due to the ubiquitous use of cement-based construction materials. High pH levels, in turn, suppress CO₂ emissions by retaining dissolved inorganic carbon under the form of aqueous bicarbonate (HCO₃^-). The role of non-sulfate-derived EC in modulating the CH₄ emissions is attributed to the effect of salinity on the mixing intensity and the associated impact on water column oxygenation. Overall, our findings confirm the significant role of ponds in global carbon cycling. At the same time, they suggest that site-specific characteristics, including land use and water chemistry, can induce considerable variability in GHG emissions from ponds.