Carbon burial outweighs the climate impact of methane emissions across global aquatic ecosystems

Different aquatic ecosystems may have opposing effects on global climate acting as sources and/or sinks of greenhouse gases. The conversion of sediment organic carbon to the powerful greenhouse gas methane is a key indicator of the potential climate impact of a given ecosystem. Here, we assess the lifetime climatic role of aquatic ecosystems by contrasting methane (CH ₄ ) emissions with long-term organic carbon burial. Global compilations revealed that the ecosystem-specific ratio of organic carbon burial (kg CO ₂ ) to CH ₄ emissions (kg CH ₄ ) ranged from 20:1 in lakes to 2500:1 in continental shelves. Using 100-year sustained-flux global warming potential and a radiative balance model, we show that most ecosystems currently exhibit a net negative radiative balance (i.e., net cooling or a CO ₂ -equivalent sink). The lifetime cooling effect per area was strongest in the coastal ocean and fjords. Freshwater ponds, lakes, and inland wetlands had lowest organic carbon burial to CH ₄ emission ratios, requiring ~ 0-700 years after their establishment to shift from a positive to net negative radiative balance. All aquatic ecosystems gradually increase their net climate benefit via long-term enhancement of the ratio of carbon sequestration to CH ₄ emissions. Most aquatic ecosystems will thus achieve negative radiative balance over their lifetime.