Massachusetts’ 2022 drought pushed seawater into a coastal marsh, triggering an 83% crash in methane emissions; the wetland still hadn’t recovered a year later

A severe drought in Massachusetts in 2022 allowed seawater to enter a coastal freshwater marsh, causing methane emissions to plunge by nearly 83% almost overnight, a new study has found. However, the marsh also failed to return to its previous state more than a year after normal rainfall resumed, showing how a single extreme weather event can trigger lasting changes in wetland ecosystems.

Researchers studied the Plum Island Estuary in northeastern Massachusetts, where waterlogged soils normally create oxygen-poor conditions that support methane-producing microbes. During the 2022 drought, reduced river flows allowed unusually salty tides to move farther inland than usual.

The sulfate-rich seawater changed the chemistry of the marsh and suppressed the microorganisms responsible for producing methane. Measurements showed that methane emissions fell by nearly 83% after an unusually high tidal event, highlighting how quickly wetland greenhouse gas emissions can respond to changes in salinity.


The effect did not disappear when wetter conditions returned. More than a year later, methane emissions remained well below their pre-drought levels, suggesting that prolonged saltwater exposure may have altered the microbial communities involved in methane production.

Seawater introduces sulfate, allowing sulfate-reducing microbes to compete with methane-producing microorganisms for organic matter. As a result, methane production can remain suppressed even after salinity levels begin returning to normal.

'Saltwater can alter soil chemistry'

While a reduction in methane emissions might appear beneficial because methane is a potent greenhouse gas, researchers caution against viewing saltwater intrusion as an environmental benefit. The intrusion can stress freshwater vegetation, alter soil chemistry and disrupt biological communities that are important to wetland health.

Coastal marshes provide several critical ecosystem services. They store carbon, filter pollutants, support wildlife and help protect coastlines from storms. Persistent saltwater intrusion could weaken these functions even if methane emissions decline temporarily.

With climate models projecting more frequent droughts, rising sea levels and increasing saltwater intrusion in many coastal areas, the findings suggest that wetlands may undergo abrupt ecological shifts rather than respond gradually to climate change.

Researchers say understanding how extreme events affect wetland microbes, vegetation and carbon emissions will be important for improving climate models and protecting coastal marshes as climate conditions become increasingly unpredictable.