'Extraordinary' increase in methane emissions from wetlands worries scientists

From the Arctic to the tropics, wetlands occupy approximately 6 % of the planet's surface. These waterlogged soils are the largest natural source of methane, a powerful greenhouse gas that plays a key role in increasing global temperatures. As climate change increases global temperatures and disrupts rainfall patterns, wetlands release methane into the atmosphere at a faster rate – a phenomenon known as the "wetland methane feedback". The new research, published as a "brief notice" in the journal Nature Climate Change, says there was an "exceptional" increase in methane emissions from wetlands in 2020-21. The document adds that tropical wetlands in particular are "hotspots" for wetland methane emissions, with South America accounting for the largest increase in emissions from tropical wetlands in the 21st century. A separate study, also published in the journal Nature Climate Change, said global warming is also affecting carbon dioxide and nitrous oxide emissions from wetlands. The paper concluded that "warming undermines the potential of intact wetlands to mitigate climate change even with a limited temperature increase of 1.5-2°C".

Methane feedback in wetlands
Methane is a powerful greenhouse gas that has caused about 30 % of all human-caused global warming since the Industrial Revolution. Most methane emissions come from human activity – including the fossil fuel industry, landfills and agriculture. In 2021, the US, EU, Indonesia, Canada, Brazil, the UK and many other countries signed the "Global Methane Commitment", pledging to reduce their methane emissions by 30 % over the period 2020-30. Meanwhile, a report released last year by the International Energy Agency as part of its Global Methane Monitor concluded that "the most cost-effective options for reducing methane emissions are in the energy sector, particularly oil and gas operations." However, 40 % of methane emissions come from natural sources. The largest natural source of methane emissions in the world are waterlogged soils called wetlands, which are flooded with water for at least part of the year.

Wetlands take many different forms, from arctic permafrost bogs to tropical mangrove plantations to salt marshes. About 40 % of all species live or breed in wetlands. They also provide key ecosystem services, such as water filtration, and are important carbon sinks. Therefore, wetland restoration is often discussed as an important option for mitigating climate change. However, wetlands also release greenhouse gases into the atmosphere. A new study examines how climate change affects methane emissions in two key types of wetlands – permafrost and tropical wetlands. Permafrost wetlands, found at low temperatures in high latitudes, consist of partially frozen and waterlogged soil. As the climate warms and permafrost thaws, long-dormant microbes begin to "wake up" and release methane into the atmosphere. Meanwhile, tropical wetlands, which are usually found in hot and humid climates. As a changing climate causes changes in rainfall patterns, new soils are becoming waterlogged and these wetlands are expanding, the paper says. Overall, this means that global warming is causing more methane emissions from wetlands. This process is called "wetland methane feedback".

Underestimation of emissions
The paper assesses wetland methane feedback using two different types of data—samples collected over many decades of fieldwork and data from a "reanalysis" that combines observations from multiple sources with model simulations. The authors use these two data sources to run wetland methane model simulations, which they use to forecast future methane emissions from both tropical and permafrost wetlands under various warming scenarios. The following graph shows 2000-22 wetland methane emissions compared to 2000-2006 levels as estimated from field data (dashed black line) and reanalysis data (solid black line). It also shows projected emissions taken from the fifth Coupled Model Intercomparison Project (CMIP5) up to 2100. CMIP is a framework for climate model experiments that allows scientists to study and compare the output of different climate models.

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

Ayesha Tandon, Carbon Brief