Researchers measure methane exchange on upland trees in Peru.
| Photo Credit: Vincent Gauci (CC BY-ND)

Tree bark in the world’s forests absorbs the greenhouse gas methane, my colleagues and I have demonstrated for the first time on a global scale – a discovery that could have big implications for tackling climate change.

As trees photosynthesise, their leaves take up carbon dioxide (CO₂) and lock it away as biomass in their trunks and branches providing a long-term store of carbon. But now, our large-scale study proves that there’s another way that trees absorb greenhouse gases – so forests can provide even more climate benefits than previously thought.

Methane has contributed about a third of the observed climate warming since preindustrial times. Concentrations of methane in the atmosphere have been rising rapidly for the best part of two decades.

That’s a real problem for Earth’s climate because methane traps much more heat in the atmosphere than the equivalent amount of CO₂. But while CO₂ can last in the atmosphere for hundreds of years, methane has a lifetime of around ten years.

This short atmospheric lifetime means that any changes to sources of methane or processes that remove methane from the atmosphere (known as methane sinks) can have rapid effects. If removal is enhanced, this can be a quick climate win helping to mitigate escalating climate change.

That’s why researchers are so interested in understanding how methane gets into the atmosphere and how different processes remove it. It’s why my team of ecologists and climate scientists have been studying the exchange of methane between tree bark, a surface that had previously been overlooked for its climate contribution, and the atmosphere.

Wetlands are known to be the primary natural source of methane – trees in swamps and floodplains can emit methane from the lower portions of their trunks. But methane exchange in trees growing on free-draining soils that don’t flood – that includes most of the world’s forests – has not been well-studied, until now.

We measured methane exchange on hundreds of tree stems in forests along a climate gradient spanning the Amazon and Panama, through to Sweden and forests near Oxford in the U.K. We used a simple plastic chamber that wrapped around the tree trunk which was then connected to a laser-based methane analyser.

At first, we looked for methane emissions from trees, and some do emit a small amount from their trunk base. But the surprise happened when we measured higher up the trunks: trees were taking up methane from the atmosphere and this methane removal grew stronger the higher up we went, with methane removal from the atmosphere dominating overall exchange.

Next, we investigated whether this was a globally important process. To do this we needed to calculate the global area of tree bark. Using a technique called terrestrial laser scanning, we mapped tree woody surfaces down to the finest twig.

We discovered that, if the bark from all the world’s trees were laid flat, it would cover all of Earth’s land surface. Potentially, this represents a vast area for gas exchange between tree bark and the atmosphere but this mechanism is still poorly understood.

An untapped sink

In total, our cautious first estimate is that trees take up between about 25 and 50 million tonnes of atmospheric methane each year, with most taken up by tropical forests.

This is similar to the only other land-based methane sink – soils – and it makes temperate and tropical trees 7%-12% better for climate than they are currently credited for.

But unlike soil, which isn’t changing in area, forests are contracting and expanding through deforestation and reforestation – these changes can influence atmospheric methane. If we reforest and plant trees in the right place, more methane could be drawn down from the atmosphere.

Clearly, decarbonisation of the global economy and energy system is the key way to address climate change. But, this capacity for tree bark to absorb methane offers another angle of attack as a nature-based climate solution.

There may be new ways to improve methane uptake in plantation forestry, by selecting trees that are particularly good at atmospheric methane removal or modifying tree bark microbial communities.

Nations could be given greater incentives to preserve existing natural forest and avoid further deforestation. Expensive reforestation projects may become more economically viable under reputable carbon offset schemes that consider methane.

This new evidence reinforces the importance of trees and forests for our climate system while demonstrating there is still much to learn about these valuable ecosystems.

Vincent Gauci is Professorial Fellow, School of Geography, Earth and Environmental Science, University of Birmingham. This article is republished from The Conversation.

For the last three years, Naveen Chandra has been spending most of his days running simulations at the Research Institute for Global Change in Japan. He is trying to recreate the last 50 years of the earth’s atmosphere on a supercomputer roughly the size of an auditorium.

Dr. Chandra has been trying to answer a question that came out of his team’s research. During 2019-2020, these researchers examined the concentration of methane in the atmosphere and how it changed with time. Until the 1990s, the concentration increased, then stabilised for a bit, and then started to increase again around 2007. According to recent estimates, the atmospheric concentration of methane today is three-times what it was 300 years ago.

Where is this methane coming from? That’s what they wanted to know.

Evolving understanding

Methane is the second most abundant anthropogenic greenhouse gas after carbon dioxide (CO2) but it warms the planet more. Over a century, methane has a global warming potential 28-times greater than CO2, and even higher over shorter periods like two decades.

It wasn’t until recently that policymakers began to focus on methane vis-a-vis addressing global warming. At the U.N. climate talks in 2021, member countries launched the ‘Global Methane Pledge’ to cut the gas’s emissions and slow the planet’s warming. Yet our understanding of methane also continues to evolve.

For instance, Dr. Chandra and his team recently reported that microbes have been the biggest sources of methane in the atmosphere, not the burning of fossil fuels.

The sources of methane

Scientists are increasingly recognising various sources of methane, most of which fit in two categories: biogenic and thermogenic. When fossil fuels such as natural gas or oil are extracted from deep within the earth’s crust, thermogenic methane is released. Biogenic methane comes from microbial action.

The microbes that produce methane are archaea — single-celled microorganisms distinct from bacteria and eukaryotes — and are called methanogens. They thrive in oxygen-deficient environments, such as the digestive tracts of animals, wetlands, rice paddies, landfills, and the sediments of lakes and oceans.

Methanogens play a crucial role in the global carbon cycle by converting organic matter into methane. While methane is a potent greenhouse gas, its production by methanogens is an essential part of natural ecosystems. But human activities like agriculture, dairy farming, and fossil fuel production have further increased methane emissions.

Both biogenic and thermogenic activities produce different isotopes of  methane. Tracking the isotopes is a way to track which sources are the most active.

Modelling with a supercomputer

According to Prabir Patra, principal scientist at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and one of the lead authors of the study, carbon-13 is key. (Atoms of this carbon isotope have 13 nucleons: 6 protons + 7 neutrons.)

If there are fewer carbon-13 atoms than a certain level in a group of 1,000 methane molecules, the methane is from a biological source. If the methane is from thermogenic sources, such as trapped fossil fuels or geological activities, there will be more carbon-13 atoms in 1,000 molecules.

Dr. Chandra and Dr. Patra worked with scientists from Austria, Japan, the Netherlands, and the U.S. to collect data from the 12 monitoring sites worldwide tracking atmospheric parameters since the 1990s. Then they sorted the methane isotope data by year and ran it through a program they had developed to recreate the atmosphere from 1980 to 2020 on a supercomputer.

“One year of data analysis takes about 4-5 hours,” Dr. Chandra said.

Data mismatch

Finally, the team compared their own results with two emissions inventories, called EDGAR and GAINS, and found some discrepancies. EDGAR had reported that methane emissions from oil and natural gas exploration had increased between 1990 and 2020. GAINS had recorded a large “unconventional” rise in emissions since 2006. Their findings disagreed with both inventories.

Dr. Patra said combining the numbers for all biogenic and thermogenic isotopes should match the total emissions in a year. They also took insights from other available data like, number of rice fields, wetlands, dairy farms, biomass burning and likewise sources of methane emission, and estimated the emissions by those sources. But when they ran their atmosphere models with this data, the year-wise total methane emissions overshot the total production.

In fact, the models said methane emissions from fossil fuels declined between 1990 and the 2000s and that they’ve been stable since. They also found microbes were producing more methane than fossil fuels.

Need for local data

One possible reason could be an increase in cattle-rearing in Latin America and more emissions from waste in South and Southeast Asia, Latin America, and Africa, the study’s authors wrote in their paper. They added that the number of wetlands worldwide had increased as well.

Studies in the past have pointed to microbes like anaerobic archaea as potentially top contributors of atmospheric methane using satellite data. But according to Dr. Patra, “Most studies which use satellites cannot measure the actual [changes over time] of methane.” Satellite data is interpreted using models “and thus are prone to uncertainties.” He said ground models are required to confirm these interpretations.

He added that their own atmospheric model was also only the beginning. The data for it came from observatories located in far-flung places. “If you really want to ask what is from the wetland, what is from the rice fields, we need measurements in those exact locations,” per Dr. Patra. “We don’t have that kind of observation at all anywhere in the world to make that kind of measurement. We can only speak for global emissions.”

But what we do know is: “If you want to reduce methane, anthropogenic activity should be first controlled. And we can clearly outline what is anthropogenic here. Waste and landfills, rice fields, enteric fermentation, oil and gas, and coal,” he said.

Monika Mondal is a freelance science and environment journalist.