Cross-continental importance of CH₄ emissions from dry inland-waters

Despite substantial advances in quantifying greenhouse gas (GHG) emissions from dry inland waters, existing estimates mainly consist of carbon dioxide (CO₂) emissions. However, methane (CH₄) may also be relevant due to its higher Global Warming Potential (GWP). We report CH₄ emissions from dry inland water sediments to i) provide a cross-continental estimate of such emissions for different types of aquatic systems (i.e., lakes, ponds, reservoirs, and streams) and climate zones (i.e., tropical, continental, and temperate); and ii) determine the environmental factors that control these emissions. CH₄ emissions from dry inland waters were consistently higher than emissions observed in adjacent uphill soils, across climate zones and in all aquatic systems except for streams. However, the CH₄ contribution (normalized to CO₂ equivalents; CO₂-eq) to the total GHG emissions of dry inland waters was similar for all types of aquatic systems and varied from 10 to 21%. Although we discuss multiple controlling factors, dry inland water CH₄ emissions were most strongly related to sediment organic matter content and moisture. Summing CO₂ and CH₄ emissions revealed a cross-continental average emission of 9.6 ± 17.4 g CO₂-eq m^-2 d^-1 from dry inland waters. We argue that increasing droughts likely expand the worldwide surface area of atmosphere-exposed aquatic sediments, thereby increasing global dry inland water CH₄ emissions. Hence, CH₄ cannot be ignored if we want to fully understand the carbon (C) cycle of dry sediments.