New research shows that methane emissions from wetlands are set to increase faster this century than under even the most pessimistic climate scenarios.
From the Arctic to the tropics, wetlands cover about 6% of the planet’s surface. These wetlands are the planet’s largest natural source of methane – a potent greenhouse gas that plays a major role in rising global temperatures.
As climate change raises global temperatures and disrupts rainfall, wetlands are releasing methane into the atmosphere more rapidly – a phenomenon known as the “wetland methane feedback.”
The new research, published as a “brief” in the journal Nature Native local weather Change, found that 2020-21 marked “exceptional” growth in methane emissions from wetlands.
The paper adds that tropical wetlands in particular are “hotspots” for wetland methane emissions, with South America being the largest contributor to the increase in tropical wetland emissions in the 21st century.
Meanwhile, a separate study – also published in the journal Nature Native local weather Change – found that global warming is also affecting carbon dioxide and nitrous oxide emissions in wetlands.
The paper concludes that “warming weakens the mitigation potential of pristine wetlands even at the 1.5-2C temperature rise limit”.
Methane feedback in wetlands
Methane is a potent greenhouse gas, responsible for about 30% of human-caused global warming since the Industrial Revolution. Most methane emissions come from human activities – including the fossil fuel industry, landfills and agriculture.
In 2021, the United States, EU, Indonesia, Canada, Brazil, the United Kingdom and many other countries signed the “Global Methane Pledge” promising to cut methane emissions by 30% between 2020 and 2030.
Meanwhile, a report published last year by the International Energy Agency’s global methane tracker concluded that “the most cost-effective opportunities to reduce methane are in the energy sector, particularly in oil and gas operations.”
However, 40% of methane emissions come from natural sources. Wetlands, known as wetlands, which are flooded for at least part of the year, are the world’s largest natural source of methane emissions.
Wetlands come in many forms, from Arctic permafrost to tropical mangrove plantations to salt marshes. About 40% of all species live or breed in wetlands. They also provide important ecosystem services, such as water filtration, and are important carbon sinks. As such, wetland restoration is often discussed as an important option for mitigating climate impacts.
However, wetlands also release greenhouse gases into the atmosphere. New research explores how climate change is affecting methane emissions in two main types of wetlands – permafrost wetlands and tropical wetlands.
Found in cold temperatures at high latitudes, permafrost wetlands consist of soil that is partially frozen and flooded with water. As the climate warms and the permafrost thaws, long-dormant microbes begin to “wake up” and release methane into the atmosphere.
Meanwhile, tropical wetlands are typically found in hot, humid climates. As climate change causes changes in rainfall patterns, new areas are becoming waterlogged and these wetlands are expanding, the article said.
Overall, this means that global warming is driving greater methane emissions from wetlands. This process is known as the “wetland methane feedback.”
Underestimation of emissions
The paper assesses methane feedbacks in wetlands using two different types of data – samples collected over decades of field research and “reanalysis” data that combines observations from multiple sources with model simulations.
The authors used two data sources to run wetland methane model simulations that they used to predict future methane emissions from both tropical wetlands and permafrost under a range of warming scenarios.
The chart below shows wetland methane emissions from 2000-2022, compared to 2000-06 levels, as estimated from field research data (black dotted line) and reanalysis data (black solid line). It also shows projected emissions 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 outputs of different climate models.
The dark blue, light blue, yellow and red lines show the low (RCP2.6), medium (RCP4.5), high (RCP 6.0) and extremely high (RCP8.5) emission scenarios, respectively.
The authors found that, over the past 20 years, wetland methane emissions have increased by 1.2–1.4 million tons per year. This is faster than the median projection under the most pessimistic emissions scenario (RCP8.5), which shows an increase of 0.9 million tons of methane per year.
The authors also found “exceptional” increases in methane emissions in 2020-21.
Global average annual emissions increased by 8-10 million tonnes per year between 2007 and 2021 due to climate change, compared to the 2000-06 baseline. However, emissions increased by 14-26 million tonnes in 2020 and 13-23 million tonnes in 2021.
Dr Benjamin Poulter is a research scientist at NASA’s Goddard Space Flight Center and an author of the study. He told Carbon Transient that it is difficult to attribute methane emissions to climate change because the climate is highly variable from year to year, but said that “we are now becoming more confident that we are seeing the impact of climate change on methane emissions.”
Professor Grant Allen from the University of Manchester, who was not involved in the research, told Carbon Transient that the paper “adds weight to the conclusion made in many other recent studies that wetland-related methane emissions are increasing rapidly in the face of positive climate feedbacks.”
Tropical Hotspot
To investigate the global increase in wetland methane emissions in more detail, the authors examined emissions by continent.
The graph below shows the percentage increase in wetland methane emissions, as measured by field research data (orange) and reanalysis data (green), from 2000-2021, compared to a 2000-06 baseline. The “exceptional” methane emission years of 2020 and 2021 are shown by red and blue dots.
Results are displayed for the globe (GL), northern hemisphere (NH), southern hemisphere (SH), North America (NAm), South America (SAm), Africa (Afr), North Asia (NAs), South Asia (SAs) and Southeast Asia (SEA).
Ground-based data show that South America is the largest contributor to global wetland methane emissions, while satellite data indicate that South Asia and Southeast Asia also contribute.
The study concluded that the largest increase in global wetland methane emissions came from tropical wetlands, while high-latitude permafrost peatlands contributed less.
The models in the study “consistent with satellite remote sensing observations that some tropical wetlands are expanding as the weather gets warmer and warmer,” said Professor Euan Nisbet – emeritus professor at Royal Holloway University of London – in an accompanying commentary. He continued:
“Satellite observations indicate that much of the recent methane increase is due to increased emissions, particularly from flooding of wetlands in Africa. These findings are supported by large emissions recently measured during low-flying aircraft campaigns over wetlands in the Bolivian and Brazilian Amazon, as well as over the Upper Congo and Zambezi basins in Zambia. For example, methane emissions over large wetlands in Zambia are much larger than those simulated in land surface models.”
Although wetland methane feedbacks are well established in the scientific literature, scientists often “struggle” to represent wetland methane emissions in models, Poulter told Carbon Transient.
The study’s authors caution that most Earth system models and integrated assessment models – used to inform influential reports, such as the latest Intergovernmental Panel on Climate Change (IPCC) assessment report – do not directly incorporate this process into their climate models.
Dr Gabrielle Dreyfus is chief scientist at the Institute for Governance and Sustainable Development – a US-based non-profit organisation – and was also not involved in the study. She told Carbon Transient that current IPCC emissions pathways require “rapid and deep reductions in methane emissions associated with human activities” to limit warming to 1.5C. However, these “assume very limited increases in methane emissions from natural sources”.
In light of new work on methane feedbacks in wetlands, she concludes:
“The gaps highlighted in this study point to a future where mitigation of human-caused methane emissions may need to be supplemented with methane removal methods.”
Dr Mark Lunt – a postdoctoral research associate at the University of Edinburgh’s school of geosciences who was not involved in the study – warned Carbon Transient that “committed emissions from wetland methane feedbacks are likely to offset any reductions made through agreements such as this Global Methane Commitment”.
Allen said further measurements and modeling are “necessary” to “fully characterize this worrying trend.”
Source or sink?
Methane is not the only greenhouse gas emitted from wetlands. They are also a source of carbon dioxide and nitrous oxide.
A separate study in Nature Climate Change used observations from 167 sites to assess how global warming would impact wetland emissions of all three gases.
The authors found that although wetlands are currently a sink for greenhouse gases, the “100-year global warming potential of wetlands” could increase by 57% with a 1.5-2C global temperature rise.
The authors found that greenhouse gas emissions from wetlands depend in part on the types of plants found in the wetlands. They focused on vascular plants – a group of plants that includes conifers and flowering plants that have tubes called “xylem” and “phloem” to transport food and water inside – and cryptophytes that reproduce by spores, such as algae.
Dr. Xiyan Xu – a researcher at the Chinese Institute of Atmospheric Physics and author of the study – explains the key findings of the study with Carbon Transient:
“While warming increased CO2 uptake in vascular plant-dominated wetlands, it significantly enhanced CO2 sources in cryptogam-dominated wetlands. As a source of methane and nitrous oxide, wetlands showed a positive response to warming, especially in vascular plant-dominated permafrost wetlands.”
The study warns that “warming undermines the mitigation potential of pristine wetlands”, adding that there is “large uncertainty” about whether wetlands will remain carbon sinks as the planet warms.
Dreyfus told Carbon Transient that the study is a “great complement” to the first, adding that it “cleverly synthesizes data from higher latitude wetland sites in the northern hemisphere to show a more complex picture.”
Poulter added that the paper “provides support” that observed warming is “the culprit behind the rapid increase in atmospheric methane over the past decade.”