Long-term warming raises risks of seasonal seafloor methane release in the coastal Baltic Sea

Climate change driven ocean warming is a worldwide environmental issue that can impact cycling of greenhouse gases. However, how methane production in marine sediments as a potential contributor to atmospheric greenhouse gases versus its consumption at the sulfate-methane transition zone will be affected by climate change related warming is still not well constrained. In this study, sediments from two Baltic Sea bays with long-term temperature differences were collected during summer and winter. The primary difference between the two bays was that one had been heated by a nearby power plant for 50 years, resulting in a 5.1 °C increase in annual average temperature compared to an unheated control bay. The results showed that near-seafloor sediment methane concentrations were 50 times higher compared to present-day conditions. Furthermore, the sediment fluxes along with microbial community composition changes suggested that long-term warming may thin the sulfate reduction zone, such that methanotrophic archaea and sulfate reducing bacteria peaked at shallower sediment depths in the heated bay. Overall, the results from long-term warming in natural sediment environment indicated that future climate change warming may increase the risk of methane release to the water and eventually the atmosphere.