'Exceptional' surge in methane emissions from wetlands worries scientists

Methane emissions from wetlands have risen faster this century than in even the most pessimistic climate scenarios, new research finds.

From the Arctic to the tropics, wetlands encompass around 6% of the planet’s surface. These waterlogged soils are the planet’s largest natural source of methane – a potent greenhouse gas that plays a key role in global temperature rise.

As climate change raises global temperatures and disrupts rainfall patterns, wetlands are releasing methane into the atmosphere more rapidly – a phenomenon known as the “wetland methane feedback”.

New research, published as a “brief communication” in Nature Climate Change, finds that 2020-21 marked “exceptional” growth in wetland methane emissions.

The paper adds that tropical wetlands in particular are “hotspots” of methane wetland emissions, with South America the largest contributor to increased tropical wetland emissions over the 21st century.

Meanwhile, a separate study – also published in Nature Climate Change – finds that global warming is also affecting wetland emissions of carbon dioxide and nitrous oxide.

The paper concludes that “warming undermines the mitigation potential of pristine wetlands even for a limited temperature increase of 1.5-2C”.

Methane is a potent greenhouse gas that has caused around 30% of all human-caused global warming since the Industrial Revolution. Most methane emissions come from human activity – including from the fossil fuel industry, landfill sites and agriculture.

In 2021, the US, EU, Indonesia, Canada, Brazil, UK and many others signed the “Global Methane Pledge” promising to cut their methane emissions by 30% over 2020-30.

Meanwhile, a report published last year by the International Energy Agency’s global methane tracker concluded that “the most cost-effective opportunities for methane abatement are in the energy sector, especially in oil and gas operations”.

However, 40% of methane emissions are from natural sources. Waterlogged soils called wetlands, which are inundated with water for at least part of the year, are the world’s largest natural source of methane emissions.

Wetlands take many different forms, ranging from Arctic permafrost peatlands to tropical mangrove plantations to salt marshes. Around 40% of all species live or breed in wetlands. They also provide key ecosystem services, such as water filtration and are important carbon sinks. As such, wetland restoration is often discussed as an important climate mitigation option.

However, wetlands also release greenhouse gases into the atmosphere. The new study explores how climate change is affecting methane emissions in two key types of wetlands – permafrost wetlands and tropical wetlands.

Found in cold temperatures at high latitudes, permafrost wetlands consist of partially frozen and waterlogged soils. As the climate warms and the permafrost thaws, long-dormant microbes are starting to “wake up” and release methane into the atmosphere.

Meanwhile, tropical wetlands which are typically found in hot and humid climates. As the changing climate causes shifts in rainfall patterns, new soils are becoming waterlogged and these wetlands are expanding, the paper says.

Overall, this means that global warming is driving greater wetland methane emissions. This process is called the “wetland methane feedback”.

The paper assesses the wetland methane feedback using two different types of data – samples collected over many decades of fieldwork and “reanalysis” data that combines observations from multiple sources with model simulations.

The authors use the two data sources to run simulations of a wetland methane model, which they use to project future methane emissions from both tropical and permafrost wetlands under a range of warming scenarios.

The plot below shows wetland methane emissions over 2000-22, relative to 2000-06 levels, as estimated by fieldwork data (dotted black line) and reanalysis data (solid black line). It also shows projected emissions taken from the fifth Coupled Model Intercomparison Project (CMIP5) up to the year 2100. CMIP is a framework for climate model experiments, which allows scientists to study and compare the outputs of different climate models.

The dark blue, light blue, yellow and red lines show, respectively, the low (RCP2.6), moderate (RCP4.5), high (RCP 6.0) and extremely high (RCP8.5) emissions scenarios.