If your field work often takes you to the peat swamp forests of Borneo and the Brazilian Amazon as Sunitha Rao Pangala does, then hair-rising experiences are part of your job. Sunitha is a Royal Society Research Fellow and lecturer at Lancaster Environment Centre, Lancaster, soon to start as senior lecturer at Imperial College London, UK. She studies interactions between soils and plants, and how greenhouse gases move between soils, plants and atmosphere, especially how trees ferry methane from the soil to the atmosphere.
Her studies provide insights into feedback loops occurring in soils, plants and atmosphere, and contribute to understanding climate change specifically in the plant world. The team’s field work involves measuring emissions from trees. They strap chambers onto tree trunks to measure methane emissions.
To measure methane emissions, they strap a gas-tight chamber around the trunk, ensuring it is sealed so that no external air can enter or escape. This chamber is left in place for a set period, during which it accumulates methane emitted by the tree.
They then connect a laser-based methane analyser to the chamber to monitor the change in methane concentration over time. If the analyser shows methane levels are increasing within the chamber, it indicates that the tree stem is emitting methane. Conversely, if the methane concentration decreases, it suggests that the tree stem is absorbing methane. This method provides valuable insights into the tree's role in the methane cycle.
During a field trip, they once camped in a remote forest in Cuniã Ecological Station, Brazil, for seven days. They found jaguar footprints near their campsite, which was both thrilling and terrifying.
“The realisation that a jaguar had been so close added an intense layer of awareness to our work in the wilderness,” Sunitha says re-living the experience.
Another close encounter with jaguars happened four years later in Panama. They once again saw jaguar prints near the area where they were making measurements during the day. The local support team, who were helping them, warned them that jaguars were likely roaming nearby and advised them to vacate the area immediately and return to their camp base for safety.
On several occasions, snakes have unexpectedly leapt from the trees and swum away in the water below. During the wet season, when the forests are flooded, snakes seek refuge in the trees, making their work even more risky. One night, after a gruelling day of fieldwork, they were hit by an intense rainstorm while sleeping at one of their campsites. The rain was so heavy that their tent collapsed, and water gushed in, soaking everything.
Sunitha waxes eloquent about the field trips and how they knit people together, with their shared hardship and triumphs. Despite the challenges, she says, they work in some of the most beautiful ecosystems on earth. The boat rides they take to access field sites are stunning, teeming with life at every turn. The flow of water and the soupy air; the time squished in the ancient mud; the forever churn. The caimans, pink dolphins, capuchin monkeys and howler monkeys. The birds of the forests and of your imagination.
“The friendship we build during these gruelling times, both in the field and back at camp, creates lasting bonds with our team members. We share such intense and often difficult experiences that we become great friends, united by the ordeals we have overcome together,” she says.
“I genuinely love what I do.”
Their efforts tie into making measurements of exchange of gases between soils, trees and atmosphere, and most importantly the movements of methane—a gas responsible for global warming.
This is their team’s latest discovery: trees absorb methane. The paper titled Global atmospheric methane uptake by upland tree woody surfaces—Professor Vincent Gauci of the Birmingham University is the lead author and Sunitha, one of the co-authors—was published in July 24, 2024 in the journal Nature.
The concentration of methane in the atmosphere has more than doubled over the past 200 years. Scientists estimate that this increase is responsible for 20 to 30 per cent of climate warming since the Industrial Revolution which began in 1750, according to information available at NASA.
The thing about methane is that it traps more heat than a corresponding amount of carbon dioxide. It lingers in the atmosphere for about 10 years while carbon dioxide persists for hundreds of years.
The relatively short lifespan of methane makes it the go-to element to track the sources and processes of its movement and build-up in the atmosphere. By addressing those issues, people believe that methane draw-down is a quick way to address escalating warming.
The present study from the researchers helps show how trees and forests provide far more climate benefits than previously known by soaking methane from the atmosphere.
Sunitha Rao Pangala (right) conducting experiments to measure methane emissions by trees in the Amazon rianforests of Brazil.
Photo: Special arrangement.
The air we breathe consists of 21 per cent oxygen and 78 per cent nitrogen. Then the rest of the volume is filled by assorted gases like argon, xenon, helium, hydrogen and water vapour. In the mix is carbon dioxide, which is just 0.04 per cent, but the most powerful greenhouse gas that acts as a lid on the atmosphere, heating it up. Then there is its volatile cousin, methane. Its molecular weight of 16.04g/mol belies its menace. After carbon dioxide, methane is the second-largest contributor to global warming.
Methane comes from both natural sources and human activities. Agriculture, fossil fuel burning and decomposition of waste generate large amounts of methane, while in nature wetlands release copious amounts. Human activities account for an estimated 60 per cent of methane emissions while natural sources account for 40 per cent. According to the US Environment Protection Agency (EPA), China, the United States, Russia, India, Brazil, Indonesia, Nigeria, and Mexico are estimated to be responsible for nearly half of all human-induced methane emissions.
According to NASA, as of February, methane emissions are at 1929 ppb, whereas carbon dioxide emissions, as of now, are at 419.38 ppm. The former is measured in parts per billion, the latter in parts per million, but both have loaded up the atmosphere, resulting in warming. Without going into climate equivalencies between carbon dioxide and methane—because carbon dioxide remains our collective ghost haunting us—any reduction in methane emissions can go a long way in addressing at least a slice of the global emergency and individual misery.
This is especially important and relevant for India. As Sunitha explains, until recently, it was widely believed that trees, especially those in wet areas, only added to the methane in the atmosphere. However, their recent study has uncovered something surprising and important for India: trees in upland areas, such as those found in India’s Western and Eastern Ghats, can actually absorb methane from the air through their stem and branches.
“This means that these trees act like natural filters, helping to reduce the amount of methane in the atmosphere,” she says. This new understanding highlights the importance of protecting and expanding India’s forests, which can play a dual role in combating climate change by reducing both carbon dioxide and methane in the atmosphere.
For a country like India, she continues, where reforestation and forest conservation are already key strategies in climate action, “this finding suggests that our forests could be even more valuable in our efforts to slow down global warming. By recognising and utilising the natural ability of trees to absorb methane, India can make its reforestation and conservation efforts even more effective in creating a healthier environment and mitigating the effects of climate change.”
Trees are generous. In ways known and unknown, we’re entangled with the air and the atmosphere. We depend on the magnanimity of trees. Scientists are racing to understand methane dynamics in relation to global warming, and in particular, of methane emissions from trees. Their research has huge implications for the world and India’s efforts to reduce greenhouse gas concentrations and making urban environments sustainable and livable.
Tapan Kumar Adhya reckons India is handling methane emissions from agriculture, including livestock, pretty well. He is a professor at Kalinga Institute of Industrial Technology, Bhubaneswar. He is involved in research on reactive nitrogen in different sectors of the economy and his work focuses on greenhouse gas emissions and mitigation. Adhya says improved crop varieties that don’t contribute much to methane emissions and also good management practices keep agricultural, including livestock, methane emissions under control.
“As per India’s third Biennial Update Report, India’s methane emissions in 2016 (excluding Land Use, Land-use Change and Forestry (LULUCF)) were 409 million tonnes CO2e (Carbon dioxide equivalent) of which, 73.96 per cent was from agriculture sector, 14.46 per cent from waste, 10.62 per cent from the energy sector and 0.96 per cent from industrial processes and product use sector,” according to the July 24, 2023 note from the Ministry of Environment, Forest and Climate Change.
Initiatives like making farmers aware of proper fertiliser use and developing new technologies like isolating methanotrophs—methane oxidising bacteria—at institutions like Pune-based Agharkar Research Institute, have yielded results but this is not to say methane emissions have come down in agriculture. Rain-fed agriculture with alternate episodes of flooding and drying also helps. Feed rations for farm animals made of natural ingredients have helped. However, Adhya remembers the Gazipur landfill in Delhi going up in flames a while back, and says India needs proper waste management. Indian oil and natural gas wells and pipelines belch enormous amounts of methane.
In a study published in Aerosol and Air Quality Research (AAQR), in June 2024, researchers specifically estimated Indian livestock methane emissions to be 12.74 Tg yr–1. That translates to approximately 12.74 million tonnes a year or 1,031 million tonnes CO2e. The world’s methane emissions are estimated at 580 million tonnes. This calculation is arrived at in light of the fact that India is home to the world’s largest livestock herd. Enteric fermentation and manure account for higher methane emissions.
Researchers say that 100 out of 721 districts contribute nearly 40 per cent of total methane; Uttar Pradesh has the largest contribution to methane from livestock. Non-dairy cattle, draught animals, produce more methane than dairy cattle. Uttar Pradesh is followed, in descending order, by Rajasthan, Madhya Pradesh, Bihar, Maharashtra, Gujarat and West Bengal.
Saroj Kumar Sahu, lead and corresponding author of the paper, says getting access to high resolution activity data to estimate emissions is a real challenge in India. He is assistant professor at Berhampur University, Odisha and his research interests include estimating greenhouse gas emissions and air pollutants as well as urban air quality and environment.
Data about specific sectors is still more challenging. Hence, they could publish only from livestock, Sahu notes. “Methane emission data gap is the real challenge in India to support regional climate modelling. This is an attempt to address the data gap for one of India’s specific, traditional sources of methane,” he says.
Sahu also flags another thing about methane emissions: Sources of methane are diverse especially in India and to judge the actual methane load, identification of possible new sources (both natural and anthropogenic) and its quantification is important.
“Poor understanding of the diversity of the sources prevailing in a country like India or Asian countries adds to the problem,” he says.
Thirdly, since emissions are dynamic and change with source type and prevailing conditions, source-specific emission factors are vital to improve the estimation. This is in particular hard to get to because “your waste composition is changing with time as the waste that we are generating right now was not there in 10 years back.”
Sunitha’s journey into methane research began during her undergraduate years in environmental engineering at SJCE Mysore. In her final year, she worked on a project focused on developing low-cost treatment solutions for nutrient pollution in river water.
“I found the process of investigating and addressing real-world environmental challenges incredibly fulfilling,” she says. That led her to pursue a master's as the next step. She continued her studies at Edinburgh University, where she focused on the role of constructed farm wetlands as a treatment solution for diffuse water pollution from agriculture. During this time, she explored the use of gypsum and ochre, a waste product from cement manufacturing and mining, and its potential application in constructed wetlands.
“My research aimed to assess how gypsum and ochre could not only reduce methane emissions from these wetlands but also enhance their ability to treat agricultural diffuse pollution, that is, nutrient pollution,” she notes.
“What we discovered in 2013 was astounding—Amazonian trees were releasing large quantities of methane, some of the highest levels we have ever recorded."
That experience marked the beginning of her research into methane. Initially, her work was centred on pollution treatment, with a secondary focus on mitigating emissions. However, as she delved deeper, she says, “I became increasingly fascinated by methane itself—its role in various ecosystems, its environmental impacts, and the complexities of its behaviour in natural and anthropogenic environments.”
Since then, her research has evolved to focus primarily on methane in different ecosystems, particularly within natural forested landscapes. This journey, she adds, has been driven by a continuous desire to understand and address the challenges posed by methane in our environment. The path leading up to the latest Nature paper—and the discovery that trees suck up methane—is shot with aha! moments.
Researchers like Sunitha think of the world more as movements, flows, fluxes and exchanges rather than solidly material. In her case, it is methane as movement, as a flow from the soil to the atmosphere.
Since they began their measurements of methane emissions in 2013, there have been surprises. When they did the first set of measurements in the Amazon, they had had no idea that trees in the region would emit methane. However, based on measurements from the UK, they suspected the trees might, and so they set out to investigate whether this was true and, if so, to determine the scale of these emissions.
“What we discovered in 2013 was astounding—Amazonian trees were releasing large quantities of methane, some of the highest levels we have ever recorded. This finding sparked our curiosity, leading us to explore the regional and broader significance of these emissions across the Amazon,” Sunitha says.
In 2014, they conducted a much larger campaign that spanned three months. They lived on a boat and accessed 13 different flooded forests across the Brazilian Amazon, measuring emissions from over 2,500 trees.
“This extensive research revealed that trees are the largest emitters of methane in the Amazon, contributing as much methane as from the entire Arctic region,” she says.
With this critical information, they shifted their focus to understanding what causes these trees to emit methane, what drives these emissions, and what factors control them. “We have been gradually making progress in unraveling the processes responsible for these emissions, and now we are beginning to investigate how these emissions might change in the future as climate change continues to impact the region.”
To understand how these processes work they expanded their work beyond wetland forests and started exploring upland forests. They wanted to investigate the prevalence and significance of methane production within trees. As they ventured into this new area, they started noticing something unexpected—signals of methane uptake on the stems of trees.
“This finding ignited our curiosity, and we wanted to determine whether this process was consistent across different environments,” she says.
To do this, they conducted extensive measurements on a large number of trees in diverse ecosystems, including Brazil's tropical forests and Sweden's boreal forests. Additionally, Professor Vincent Gauci from Birmingham University and his team carried out similar measurements in upland forests in Panama and the UK. A consistent pattern emerged from these studies: trees were absorbing methane, particularly higher up on the trunk, indicating that they were taking up methane directly from the atmosphere.
To further understand this process, Sunitha conducted a detailed investigation at the process level. She removed wood cores from trees and incubated them in a controlled environment to observe whether they produced or oxidised methane.
“What we found was intriguing—while some wood cores did show signs of methane production, the majority exhibited methane uptake. This uptake was primarily driven by a specific type of methanotroph, microorganisms that consume low concentrations of methane from the atmosphere,” she says.
Sunitha explains that there are two types of methanotrophs—microorganisms that consume methane. One type thrives on high concentrations of methane, such as that produced in the soil. These methanotrophs are typically found on the stem surfaces near the soil, where methane emissions are highest due to the diffusion gradient with high methane concentrations in the soil and lower concentrations in the air. As you move up the tree, methane emissions decrease, and this type of methanotroph becomes less prevalent.
Higher up the tree, a different type of methanotroph is found, which relies on atmospheric methane. This type is thought to be responsible for the methane uptake or methane sink observed in upland trees. In upland soils, which are usually dry, methane sources are minimal. Occasionally, there may be small pockets of wet soil that the roots can access, but these contribute only small amounts of methane, usually detected at the base of trees. This aligns with what they have consistently observed in their measurements across various sites.
“What truly surprised us was that when we measured methane levels higher up the trees, we found evidence of methane uptake. Instead of the methane signal increasing, as expected when methane is being emitted, the signal was decreasing—indicating that the trees were consuming methane,” she says.
While the exact location within the tree where this process occurs, and whether the same microorganisms found in the soil are responsible, remains unknown, they have confirmed that this methane uptake process exists across different latitudes and a variety of tree species.
“This discovery of methane uptake in upland trees suggests a new and potentially significant terrestrial methane sink, expanding our understanding of methane dynamics in forested ecosystems,” she says.
Once they identified this methane uptake process and confirmed its existence across various latitudes, they sought to understand whether temperature played a role in this process. Their analysis indicated that trees in tropical regions, exposed to higher temperatures, oxidised more methane than those in cooler regions like Sweden.
“Trees not only sequester carbon but also play a significant role in absorbing methane, a potent greenhouse gas. This newly identified process means that trees serve as a previously unrecognised terrestrial methane sink."
To quantify the global significance of this methane uptake by trees, they combined this temperature-dependent relationship with a novel approach developed and applied by Oxford University using terrestrial laser scanning-derived allometry (changes in organisms in relation to proportional changes in body size) to estimate the woody surface area of trees worldwide. By applying their observations globally, they made a remarkable discovery—trees contribute to a global methane sink, absorbing up to 49.9 teragrams (Tg) of methane annually, approximately 50 million tonnes. That is about 10 per cent of the estimated global emissions of methane. It is also about four times the annual emissions from India’s cattle population. Rice fields are estimated to emit approximately 3.97 Tg each year.
Sunitha says the annual global methane uptake comfortably offsets India’s numbers. “This comparison highlights how substantial the methane absorption by trees is in balancing out emissions from these key agricultural activities.” In the context of climate change, their discovery has huge implications.
“Trees not only sequester carbon but also play a significant role in absorbing methane, a potent greenhouse gas,” she says. “This newly identified process means that trees serve as a previously unrecognised terrestrial methane sink, adding a similar quantity to the global methane budget as upland dry soils, which were traditionally considered the primary methane sinks on land.”
This discovery highlights, she says, the uncertainty that currently exists in our global methane budget. Until now, methane absorption on land was primarily attributed to upland soils, but the recognition of trees as an additional sink term indicates that the previous estimates may have been incomplete.
“This underscores the urgent need for accurate measurements across various ecosystems worldwide to fully account for these unexplored sinks and sources of methane. Only by narrowing these uncertainties can we become more confident in our global methane estimates, which are crucial for guiding effective climate mitigation strategies.” The recent discovery adds an estimated 10 per cent to the climate benefits provided by trees.
“This broader understanding of trees’ role in the methane cycle opens new avenues for nature-based climate solutions, particularly in the expansion and management of forests to maximise methane uptake,” she says.
The integration of this new knowledge into global climate models and mitigation efforts help greatly in developing accurate strategies to combat climate change.
Sunitha says the discovery that trees absorb methane was indeed surprising. While it’s known that trees absorb carbon dioxide (CO2) through photosynthesis, the role of trees in methane uptake wasn’t fully understood until recently. So, it opens up new dimensions in our understanding of forest ecosystems.
In the phyllosphere (leaves) of the trees, methanotrophic microorganisms are present. Several methanotrophs have been isolated and purified from phyllosphere and can also play an important role in large forest areas.
The purpose of methane absorption in trees is still being explored, but certain microorganisms living on and within the woody surfaces of trees—known as methanotrophs—play a crucial role. These microorganisms consume methane as an energy source. (There are methane-producing microorganisms known as methanogens, which thrive when there is fresh carbon available, usually provided by tree roots, and as long as the soil remains wet and warm, they produce substantial amounts of methane. While methanogens help produce methane, methanotrophs use methane, as in methane oxidation.)
The process of trees absorbing methane is incredibly beneficial for the broader environment. As Sunitha explains, by absorbing methane, trees act as natural biofilters, reducing the concentration of this potent greenhouse gas in the atmosphere,” she says.
This adds to forests' climate benefits, making them even more critical in the fight against climate change. While photosynthesis helps remove CO2, methane absorption by trees offers an additional, previously underappreciated, way that forests contribute to regulating the Earth’s climate, she adds.
Adhya, who is not involved with the study, says, that “this idea is now gaining ground that in tropical and sub-tropical forests, the trees can serve an active role in the cycling of methane.”
The thing to be mentioned here, he says, is that in the phyllosphere (leaves) of the trees, both methanotrophic and methylotrophic (a specific group of methanotroph which metabolizes methanol or 1-C compound as the source of their carbon requirement) microorganisms are present. Several methanotrophs have now been isolated and purified from phyllosphere and they can also play an important role especially in large forest areas, Adhya says.
The recent findings on trees’ ability to absorb methane, a potent greenhouse gas, suggest a strategic opportunity for India to enhance its climate mitigation strategies. India can optimise its reforestation efforts by selecting tree species that have high methane uptake capacities, particularly in the tropical and subtropical regions where these trees are most effective. By focusing on these species, India can not only sequester carbon but also reduce methane levels in the atmosphere, thereby doubling the climate benefits of its reforestation projects.
Additionally, careful planning is needed to avoid planting in areas prone to water saturation, as this could lead to increased methane emissions rather than absorption. Seasonality may also play a role, as some parts of India such as the Western Ghats, North East and West coast receive about 4000mm of rainfall in the monsoon season, which may switch the trees from absorbing methane to releasing methane.
By choosing tree species that contribute to methane absorption, Indian cities can reduce greenhouse gas concentrations, making urban environments more sustainable.
Sunitha feels further studies should identify the trees and their potential to absorb or release methane and how these emissions or uptake changes with seasonality to determine which trees are better suited for reforestation efforts in a particular environment or region.
Although they have not yet conducted specific studies on methane absorption in tree species in India, Sunitha suggests tree species within the Fabaceae family (legumes, peas and beans) may work in India because they have observed significant methane absorption capacities in them. The Fabaceae family is common in both India and Brazil.
“Given the promising results from our measurements in Brazilian forests, there is potential for similar outcomes in India, but further work is essential to determine this,” she says.
Another area where the new discovery could contribute is in urban tree planting initiatives. By choosing tree species that contribute to methane absorption, Indian cities can reduce greenhouse gas concentrations, making urban environments more sustainable. This approach would help India’s cities become critical players in the national strategy to combat climate change.
“By incorporating these strategies into its reforestation and urban greening efforts, India can enhance its progress towards its net-zero goal. The dual benefits of carbon sequestration and methane absorption will make these initiatives even more impactful, supporting India's broader environmental and climate objectives,” she says.
Sunitha is actively exploring funding opportunities to support research projects aimed at measuring methane emissions and uptake in the Western and Eastern Ghats of India. These two forest regions are particularly intriguing because they experience very different rainfall intensities and patterns, which could influence the behaviour of tree species in terms of methane dynamics. The Western Ghats, with its tropical, high-rainfall environment, might show different methane absorption and emission patterns compared to the drier Eastern Ghats.
“By studying these extensive forests, we hope to gain a deeper understanding of how different tree species, contribute to methane absorption or emission under varying environmental conditions, ” she says.
Such research could provide critical insights that inform reforestation strategies in India, ensuring that the right tree species are planted in the right conditions to maximise climate benefits.
“Ultimately, this could help refine our approach to using forests as a natural solution to mitigate methane emissions, enhancing India's contribution to global climate change mitigation efforts,” Sunitha says.
is a freelance journalist based in Andhra Pradesh.
