The bark of trees in the world’s forests absorbs the greenhouse gas methane. My colleagues and I have demonstrated for the first time a discovery that could have major implications for combating climate change on a global scale. As trees photosynthesize, their leaves absorb carbon dioxide (CO₂) and trap it as biomass in their trunks and branches, providing long-term carbon storage.

But now our large-scale study proves that there is another way trees absorb greenhouse gases, meaning forests may provide more climate benefits than previously thought.

Methane has caused about a third of the observed climate warming since pre-industrial times. Methane concentrations in the atmosphere have been rising rapidly for most of the last two decades. This is a real problem for Earth’s climate because methane traps much more heat in the atmosphere than an equivalent amount of CO₂. However, CO₂ can persist in the atmosphere for hundreds of years, whereas methane lasts for about ten years.

This short lifespan in the atmosphere means that any changes to methane sources or the processes that remove methane from the atmosphere (known as methane sinks) could have rapid consequences. If withdrawals increase, this could be a rapid climate win that would help mitigate accelerating climate change.

That’s why researchers are so interested in understanding how methane enters the atmosphere and how various processes remove it. That’s why my team of ecologists and climate scientists is studying the exchange of methane between tree bark and the atmosphere, a surface that has previously been overlooked because of its impact on climate. Wetlands are known to be a significant source of natural methane; trees in marshes and floodplains can emit methane from their lower trunks.

However, as is the case in most of the world’s forests, methane exchange in trees growing in well-drained, non-flooded soils has not yet been studied sufficiently. We measured methane exchange in hundreds of tree trunks in forests along a climate gradient from the Amazon and Panama to Sweden, and in forests near Oxford in the UK.

We used a simple plastic camera that we rotated around the trunk of a tree, which we then connected to a laser methane analyzer. We first looked at methane emissions from the trees, and some of them emit small amounts of methane from the base of the trunk. But the surprise came when we measured higher up the trunks: the trees were absorbing methane from the atmosphere, and this removal of methane became stronger as we went up, with the removal of methane from the atmosphere dominating the overall change.

We then investigated whether this was a globally significant process. To do this, we needed to calculate the global area of ​​the tree’s bark. Using a technique called terrestrial laser scanning, we mapped the wood surfaces of the trees down to the smallest branch.

We discovered that if all the bark from all the trees in the world were laid flat, it would cover the entire surface of the Earth. This potentially represents a huge area for gas exchange between the bark and the atmosphere, but the mechanism is still not fully understood.

Sink without tap

Overall, our conservative first estimate is that trees absorb about 25 to 50 million tons of atmospheric methane each year, with tropical forests absorbing the majority. This is similar to the only other terrestrial methane sink, soil, and would make temperate and tropical trees 7 to 12 percent better off climate-wise than currently thought.

But unlike land, which doesn’t change in size, forests shrink and expand through deforestation and reforestation, and these changes can affect atmospheric methane. If we renew forests and plant trees in the right places, we can pull more methane out of the atmosphere.

Decarbonizing the global economy and energy system is clearly a key way to combat climate change. But bark’s ability to absorb methane offers a different angle of attack as a natural climate solution. There may be new ways to improve methane uptake in forestry plantations by selecting trees that are specifically good at removing atmospheric methane or by altering bark microbial communities.

Countries could receive greater incentives to protect existing natural forests and prevent further deforestation. Expensive reforestation projects could become more economically viable with robust carbon offset schemes that account for methane. This new evidence reinforces the importance of trees and forests to our climate system, while also showing that there is still much to learn about these valuable ecosystems.