Coastal wetlands undermine climate benefits by releasing powerful methane gas
Coastal ecosystems promoted as natural carbon sinks are emitting significant methane that cancels out roughly one-third of their climate benefits, new research shows. The finding threatens to reshape carbon credit valuations and climate policy that relies on these habitats for emissions reductions.
Originaltitel: Methane emissions offset atmospheric carbon dioxide uptake in coastal macroalgae, mixed vegetation and sediment ecosystems
<p>Coastal ecosystems can efficiently remove carbon dioxide (CO<sub>2</sub>) from the atmosphere and are thus promoted for nature-based climate change mitigation. Natural methane (CH<sub>4</sub>) emissions from these ecosystems may counterbalance atmospheric CO<sub>2</sub> uptake. Still, knowledge of mechanisms sustaining such CH<sub>4</sub> emissions and their contribution to net radiative forcing remains scarce for globally prevalent macroalgae, mixed vegetation, and surrounding depositional sediment habitats. Here we show that these habitats emit CH<sub>4</sub> in the range of 0.1 – 2.9 mg CH<sub>4</sub> m<sup>−2</sup> d<sup>−1</sup> to the atmosphere, revealing in situ CH<sub>4</sub> emissions from macroalgae that were sustained by divergent methanogenic archaea in anoxic microsites. Over an annual cycle, CO<sub>2</sub>-equivalent CH<sub>4</sub> emissions offset 28 and 35% of the carbon sink capacity attributed to atmospheric CO<sub>2</sub> uptake in the macroalgae and mixed vegetation habitats, respectively, and augment net CO<sub>2</sub> release of unvegetated sediments by 57%. Accounting for CH<sub>4</sub> alongside CO<sub>2</sub> sea-air fluxes and identifying the mechanisms controlling these emissions is crucial to constrain the potential of coastal ecosystems as net atmospheric carbon sinks and develop informed climate mitigation strategies.</p>