Planting trees may not slow climate change as much as previously thought, a team of UK and US scientists has said.
Plans to cover the Earth in a trillion more trees have taken root in recent years, backed by everyone from Donald Trump to governments and oil companies. The idea is simple: to put the brakes on global warming and restore habitats.
However, the reality is more complicated, according to a team of researchers in the journal Science. They found that almost a third of the cooling effect from vast new forests absorbing carbon dioxide could be undone by the ways that more trees affect atmospheric chemistry and how much sunlight is reflected back into space.
“We’re not saying trees are bad. We’re saying trees have a part to play, but we need to think about how they affect the whole Earth system,” said Dr James Weber at the University of Sheffield, who led the study.
The implication, he said, is that countries may have to decarbonise their economies more deeply rather than banking too much on tree-planting. “Trees are not a substitute for carrying on as we are. We need to cut emissions at the same time.”
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The team assumed that the trees would be planted on land where they would thrive, adding up to an extra 750 million hectares of forest by 2095. About 4 billion hectares, 31 per cent of the planet’s land, is forested at present.
They then modelled how trees would have cooling and warming effects in two scenarios. In one, humanity’s emissions are high and global warming is a disastrous 4C this century; in the other, emissions are cut steeply and the world warms by 2C.
In both futures, the trees are projected to absorb between 5 and 6.5 billion tonnes of CO₂ a year by 2095. That would significantly slow climate change, given humanity emits about 40 billion tonnes of CO₂ a year.
However, in the hotter future, the warming effects of large new forests offset up to 31 per cent of the CO₂ removal. In the cooler future, up to 18 per cent of the cooling is undone by warming factors.
It seems counterintuitive that forests can raise temperatures, but this happens in several ways. A key one is how they change the albedo of the land (a measure of how reflective the surface is). That is seen most clearly in the boreal forest encircling the north of the planet, where dark trees planted on snowy land cause more of the sun’s energy to be absorbed rather than reflected back to space.
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Trees can also have a warming effect via the volatile organic compounds they release, which are responsible for the pine smell of Scandinavian forests. In the atmosphere, those compounds deplete a species of oxygen that would otherwise have destroyed methane, a powerful greenhouse gas. The compounds can also increase ozone, another significant greenhouse gas. Previous research has not looked in such depth at these changes to atmospheric chemistry.
The study examined new forests, restoring cleared forests and enhancements to existing ones. It did not factor in how forest fires might affect the benefits of more trees.
Dr Phil Williamson, an honorary associate professor at the University of East Anglia, who was not involved in the study, said it showed that governments needed to cut their reliance on meeting net-zero goals by removing CO₂ from the air with forestation. It also showed that they needed to increase the urgency with which they cut emissions, he said.
