Methane emissions partially offset “blue carbon” burial in mangroves

Organic matter burial in mangrove forests results in the removal and long-term storage of atmospheric CO₂, so-called "blue carbon." However, some of this organic matter is metabolized and returned to the atmosphere as CH₄. Because CH₄ has a higher global warming potential than the CO₂ fixed in the organic matter, it can offset the CO₂ removed via carbon burial. We provide the first estimate of the global magnitude of this offset. Our results show that high CH₄ evasion rates have the potential to partially offset blue carbon burial rates in mangrove sediments on average by 20% (sensitivity analysis offset range, 18 to 22%) using the 20-year global warming potential. Hence, mangrove sediment and water CH₄ emissions should be accounted for in future blue carbon assessments.