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Compound weather events found to have greater effect on wetland methane emissions than discrete weather extremes do

Human-caused climate change is driving an increase in extreme weather. Heat waves, droughts, and extreme precipitation are occurring more frequently, growing more intense, and directly affecting ecosystem function. For instance, the 2003 European heat wave—the continent's hottest summer in centuries—caused a substantial die-off of Sphagnum moss in alpine bogs, and the bogs took at least four years to recover.

As ecosystems falter under extreme conditions, the exchange of gases between land, water, and atmosphere changes too. For example, under extreme heat, plants prevent water loss by closing off the stoma they use to absorb carbon, leaving more carbon in the atmosphere and creating a feedback loop that can enhance warming.

Wetlands, in which organic material is broken down by microbes underwater to produce methane, are the largest natural source of global atmospheric methane. Using these soggy environs as a natural laboratory, T. J. R. Lippmann and colleagues evaluated how extreme climate events have affected wetland methane emissions at 45 flux tower sites around the world. Their findings are in the journal Global Biogeochemical Cycles.

The authors used climate data stretching from January 1982 to December 2020 to identify extreme events such as heat waves and droughts. They found both discrete (e.g., only hot) and compound (e.g., both hot and dry) events.

The findings showed that methane emissions were more affected by compound extreme events than by discrete events. Hot-and-dry events led to the largest increases in methane emissions, more so than either hot or dry events. Because droughts can be very long in duration, dry-only extremes led to the largest total decrease in methane emissions.

Heavy precipitation alone did not significantly alter emissions, which the authors say was unexpected because soil saturation is a critical requirement of microbial methane production. Emissions' responses to climate extremes differed by season and by wetland type; marsh and upland sites appeared to be the most sensitive.

Notably, the authors found that the effects of extreme climate events can persist in an ecosystem for at least a year following the end of an event. The findings suggest that as the climate changes, resulting in more hot extremes and fewer cold extremes, wetland methane emissions may increase as well.