Tree planting has been widely touted as a cost-effective way of reducing global warming, due to trees' ability to store large quantities of carbon from the atmosphere.
But, writing in the journal Nature Geoscience, an international group of scientists, including from the University of Cambridge, argue that tree planting at high latitudes will accelerate, rather than decelerate, global warming.
As the climate continues to warm, trees can be planted further and further north, and large-scale tree-planting projects in the Arctic have been championed by governments and corporations as a way to mitigate the worst effects of climate change.
However, when trees are planted in the wrong places - such as normally treeless tundra and mires, as well as large areas of the boreal forest with relatively open tree canopies - they can make global warming worse.
According to lead author Assistant Professor Jeppe Kristensen from Aarhus University in Denmark, the unique characteristics of Arctic and sub-Arctic ecosystems make them poorly suited for tree planting for climate mitigation.
"Soils in the Arctic store more carbon than all vegetation on Earth," said Kristensen. "These soils are vulnerable to disturbances, such as cultivation for forestry or agriculture, but also the penetration of tree roots. The semi-continuous daylight during the spring and early summer, when snow is still on the ground, also makes the energy balance in this region extremely sensitive to surface darkening, since green and brown trees will soak up more heat from the sun than white snow."
In addition, the regions surrounding the North Pole in North America, Asia and Scandinavia are prone to natural disturbances - such as wildfires and droughts - that kill off vegetation. Climate change makes these disturbances both more frequent and more severe.
"This is a risky place to be a tree, particularly as part of a homogeneous plantation that is more vulnerable to such disturbances," said Kristensen. "The carbon stored in these trees risks fuelling disturbances and getting released back to the atmosphere within a few decades."
The researchers say that tree planting at high latitudes is a prime example of a climate solution with a desired effect in one context but the opposite effect in another.
"The climate debate is very carbon-focused because the main way humans have modified the Earth's climate in the last century is through emitting greenhouse gases from burning fossil fuels," said Kristensen. "But at the core, climate change is the result of how much solar energy entering the atmosphere stays, and how much leaves again - Earth's so-called energy balance."
Greenhouse gases are one important determinant of how much heat can escape our planet's atmosphere. However, the researchers say that at high latitudes, how much sunlight is reflected back into space, without being converted into heat (known as the albedo effect), is more important than carbon storage for the total energy balance.
The researchers are calling for a more holistic view of ecosystems to identify truly meaningful nature-based solutions that do not compromise the overall goal: slowing down climate change.
"A holistic approach is not just a richer way of looking at the climate effects of nature-based solutions, but it's imperative if we're going to make a difference in the real world," said senior author Professor Marc Macias-Fauria, from the University of Cambridge's Scott Polar Research Institute.
However, the researchers recognise that there can be other reasons for planting trees, such as timber self-sufficiency, but these cases do not come with bonuses for climate mitigation.
"Forestry in the far North should be viewed like any other production system and compensate for its negative impact on the climate and biodiversity," said Macias-Fauria. "You can't have your cake and eat it, and you can't deceive the Earth. By selling northern afforestation as a climate solution, we're only fooling ourselves."
So how can we moderate global warming at high latitudes? The researchers suggest that working with local communities to support sustainable populations of large herbivores, such as caribou, could be a more viable nature-based solution to climate change in Arctic and subarctic regions than planting millions of trees.
"There is ample evidence that large herbivores affect plant communities and snow conditions in ways that result in net cooling," said Macias-Fauria. "This happens both directly, by keeping tundra landscapes open, and indirectly, through the effects of herbivore winter foraging, where they modify the snow and decrease its insulation capacity, reducing soil temperatures and permafrost thaw."
The researchers say it's vital to consider biodiversity and the livelihoods of local communities in the pursuit of nature-based climate solutions.
"Large herbivores can reduce climate-driven biodiversity loss in Arctic ecosystems and remain a fundamental food resource for local communities," said Macias-Fauria. "Biodiversity and local communities are not an added benefit to nature-based solutions: they are fundamental. Any nature-based solutions must be led by the communities who live at the front line of climate change."
Reference:
Jeppe Å. Kristensen et al. 'Tree planting is no climate solution at northern high latitudes.' Nature Geoscience (2024). DOI: 10.1038/s41561-024-01573-4
Explore more discoveries, innovations and research on climate and nature at the University of Cambridge: www.cam.ac.uk/climate-and-nature
