Tree planting in the Arctic could make global warming worse, say University of Cambridge and University of Århus experts

Planting trees in the Arctic could accelerate rather than combat global warming, a group of scientists led by the University of Cambridge and the University of Århus have warned.

The unique characteristics of Arctic and sub-Arctic ecosystems make them poorly suited for tree planting for climate mitigation, they say, and they point to the amount of sunlight being reflected back by snow in the region.

Our warming climate means it is now possible to plant trees further north. And governments and corporations have championed the idea of large-scale tree planting projects in the Arctic to mitigate climate change.

But the new research suggests if trees are planted in the wrong places - such as normally treeless tundra and mires, or in large areas of the boreal forest with relatively open tree canopies - they could have the opposite effect.

Assistant Professor Jeppe Kristensen, from Aarhus University in Denmark, said: “Soils in the Arctic store more carbon than all vegetation on Earth. 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.”

The team also notes that regions surrounding the North Pole in North America, Asia and Scandinavia are prone to natural disturbances such as wildfires and droughts, which kill off vegetation and climate change makes such events more frequent and 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 Prof Kristensen. “The carbon stored in these trees risks fuelling disturbances and getting released back to the atmosphere within a few decades.

“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. 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.”

While greenhouse gases are important in determining how much heat can escape the planet’s atmosphere, the researchers say that at high latitudes the amount of sunlight reflected back into space without being converted into heat - something known as the albedo effect -is more important than carbon storage for the total energy balance.

Prof Marc Macias-Fauria, from the University of Cambridge’s Scott Polar Research Institute, said: “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.

“Forestry in the far North should be viewed like any other production system and compensate for its negative impact on the climate and biodiversity. 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.”

Instead, the researchers advise working with local communities to support sustainable populations of large herbivores, such as caribou, as 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,” explained Prof 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.”

This nature-based climate solution has key benefits.

“Large herbivores can reduce climate-driven biodiversity loss in Arctic ecosystems and remain a fundamental food resource for local communities,” said Prof 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.”