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 (CO) emissions. However, methane (CH) may also be relevant due to its higher Global Warming Potential (GWP). We report CH 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. CH 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 CH contribution (normalized to CO equivalents; CO-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 CH emissions were most strongly related to sediment organic matter content and moisture. Summing CO and CH emissions revealed a cross-continental average emission of 9.6 ± 17.4 g CO-eq m d 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 CH emissions. Hence, CH cannot be ignored if we want to fully understand the carbon (C) cycle of dry sediments.