Global warming increases methane emissions even in untouched mangrove soils
Experiment simulated warming conditions and recorded changes in soil functioning that altered organic matter decomposition in mangroves
By: Amanda Yoshizaki*
Art by: Livia Bortoletto**
Mangroves are coastal transitional ecosystems between terrestrial and marine environments, located in estuarine areas –
Photo: Heris Luiz Cordeiro Rocha/Wikimedia Commons
The study, published in the journal Atmospheric Environment, found that CH4 emissions increased by about 6.4 times relative to the initial condition. Carbon dioxide (CO2) release, meanwhile, fell by about 73% after six months of experimental warming in mangroves located in the Cananéia-Iguape coastal system on the southern coast of the state of São Paulo, one of Brazil’s best-preserved estuarine complexes.
The change, according to the study, is linked to the soil’s own conditions. Mangroves are periodically flooded coastal ecosystems, which reduces the amount of oxygen (O2) available in the soil. According to Manoella Molitor, a doctoral student in USP’s Graduate Program in Physical Geography and first author of the article, in these environments organic matter, produced by vegetation or deposited there, is decomposed mainly through anaerobic processes, which do not depend on O2 and occur slowly. Because organic matter enters the system faster than the soil can decompose it, the result is substantial carbon accumulation over time.
To simulate a rise in temperature, the team installed open-top chambers over sections of soil. They allow solar radiation to enter while reducing heat loss, causing temperatures inside to remain 7 to 12 degrees Celsius (°C) above ambient conditions.
During the experiment, researchers monitored changes in the soil’s physicochemical characteristics and measured CO2 and CH4 fluxes. The goal was to observe how the environment would respond to higher temperatures. “We built experimental units in mangroves to simulate climate change scenarios”, said Hermano Queiroz, a professor in USP’s Department of Geography and the research supervisor.
The experiment took place on Pai Matos Island, within the Cananéia estuarine-lagoon system – Photo: Reproduced from the article
Changes in decomposition pathways
According to Manoella Molitor, rising temperatures change the conditions under which decomposition occurs because they intensify metabolic activity and accelerate the consumption of available oxygen. “The environment becomes more reducing, meaning that it begins to offer more favorable conditions for anaerobic processes”, said the researcher.
As the oxygen available in the soil decreases, microorganisms begin to use other substances, such as iron and sulfate, to decompose organic matter. Under even more oxygen-depleted conditions, this process may favor methane production. With warming, microorganisms become more active and consume oxygen more quickly, making the soil increasingly oxygen-poor and favoring the production and release of CH4 into the atmosphere.
During the experiment, CO2 emissions fell by about 73% in the first warming period, reflecting a rapid shift in soil carbon mineralization pathways. By contrast, CH4 emissions increased progressively throughout the experiment, reaching levels approximately 6.4 times higher under the warming scenario.
The reduction in CO2 emissions indicates a change in how carbon is being mineralized. “What concerns us is the progressive and consistent increase in CH4 emissions, which suggests a trend toward changes in the emissions regime of these soils under warming conditions”, Queiroz told Jornal da USP.
“There is an understanding that mangroves are extremely resilient ecosystems, but our data are showing that, in fact, they do have vulnerabilities”
Hermano Queiroz
According to the professor, CH4 has a global warming potential about 30 times greater than CO2. Therefore, the change observed in the experiment may compromise some of the climate benefit expected from soil that, under natural conditions, serves as an important carbon reservoir.
“Mangrove soils, which have been considered major allies in this climate change scenario, may become vulnerable because of this change in functioning and begin emitting large quantities of CH4”, emphasized the professor.
The researchers stress that the experiment does not allow them to conclude that this change will be permanent or will occur in the same way in all mangroves.
The chambers simulate a controlled scenario, and the results correspond to the soil’s initial response to warming. The team intends to monitor the phenomenon over longer periods to determine whether the increase in methane persists and to understand how this change might be prevented or reversed.
Mangrove soils are understudied
The study on the impacts of climate warming on mangroves was motivated by an earlier study by the team, published in the journal Forest, which analyzed how soils in this ecosystem are represented in the scientific literature. “We identified a knowledge gap: few scientists worldwide work on the soils of this ecosystem from the perspective of understanding how they function in terms of their potential to combat climate change”, said Queiroz.
The bibliometric analysis used the Scopus database and examined papers published between 1950 and 2025. The team began with broad searches on mangroves and progressively narrowed the survey to include climate change, mitigation and soil-related processes.
Of the 30,084 articles on mangroves found in the initial search, only 25 explicitly linked mangrove soils to climate change mitigation. This represents 0.08% of the total. According to Manoella Molitor, when studies examine carbon storage in mangroves, they tend to focus on vegetation and biomass. “The soil, despite holding a large share of the carbon, receives less attention”, said the author.
The study found that the increase in publication volume after 2009 coincided with the emergence of the concept of blue carbon, which recognizes the capacity of coastal and marine ecosystems to sequester carbon.
Beyond the temporal trend, the study also examined which countries produce the most articles on mangroves. “Publication output on mangroves from a broader perspective is dominated by countries that do not have large mangrove areas, such as the United States and China, for example”, explained Queiroz. He said this is due to the greater resources and infrastructure available in those countries.
More specific research on mangroves is led by countries with greater proximity to the ecosystem, such as Brazil, Australia and Malaysia. “What the results show is that, despite all the infrastructure available in countries such as the United States and China, geographic proximity to the ecosystem is a determining factor in the production of studies focused on mangrove soils and climate change, and also reflects greater participation by soil scientists”, said Queiroz.
The research project was funded with resources from the BlueShore Project, financed by Petronas Petróleo Brasil Ltda. through the Research, Development and Innovation (RD&I) investment clause regulated by the National Agency of Petroleum, Natural Gas and Biofuels (ANP). BlueShore is developed within USP’s Research Center for Greenhouse Gas Innovation (RCGI).
The articles Experimental warming alters iron–carbon interactions and CO2 and CH4 emissions in pristine mangrove soils and The Overlooked Carbon Reservoir: Marginalization of Mangrove Soils in Climate Change Mitigation Research are available online.
More information: Manoella Molitor Martins at manumolitor@usp.br and Hermano Melo Queiroz at hermanomelo@usp.br
*Intern under the supervision of Tabita Said
**Intern under the supervision of Simone Gomes
English version: Nexus Traduções, edited by Denis Pacheco