An international group of scientists argues in the journal Nature Geoscience that planting trees at high latitudes will speed up, not slow down, global warming.
The direct and indirect effects of afforestation on climate forcing at high latitudes and their relative magnitudes over the lifetime of a plantation. a, Plantation establishment disrupts the previously intact soil, leading to increased decomposition of microbial carbon (1), which is exacerbated by enhanced soil insulation caused by increased snow trapping and reduced snow packing (2). Growing trees exude carbon from their roots accelerating the turnover of soil carbon by root-associated microbes (3). As the plantation matures, trees darken the surface and diminish the proportion of energy reflected to the atmosphere (4). When a plantation is disturbed, the albedo increases while carbon stored in biomass decreases (5). Image Credit: Laura Barbero-Palacios, Greenland Institute of Natural Resources
The ability of trees to store large amounts of carbon from the atmosphere has led many to advocate planting trees as an affordable solution to combat global warming.
Large-scale tree-planting initiatives in the Arctic have been promoted by governments as well as corporations as a means of reducing the worst effects of climate change. As the climate warms further, trees can be planted farther north.
However, planting trees in inappropriate locations can exacerbate global warming. Examples of these include areas of the boreal forest with relatively open tree canopies, as well as tundra and mires that are typically treeless.
The special features of Arctic and sub-Arctic ecosystems make them unsuitable for planting trees for climate mitigation, according to lead author Assistant Professor Jeppe Kristensen of Aarhus University in Denmark.
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.
Jeppe Kristensen, Assistant Professor, Aarhus University
Furthermore, natural disturbances like droughts and wildfires that destroy vegetation are common in the areas around the North Pole in Asia, Scandinavia, and North America. These disruptions are becoming more frequent and severe due to climate change.
Kristensen added, “This is a risky place to be a tree, particularly as part of a homogeneous plantation that is more vulnerable to such disturbances. The carbon stored in these trees risks fueling disturbances and getting released back to the atmosphere within a few decades.”
One example of a climate solution that has the desired effect in one context but the opposite effect in another, according to the researchers, is the planting of trees at high latitudes.
“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,” Kristensen added.
One significant factor influencing the amount of heat that can escape from the planet’s atmosphere is greenhouse gases. However, according to the researchers, the albedo effect—the amount of sunlight that is reflected back into space without being transformed into heat—is more significant for the overall energy balance at high latitudes than carbon storage.
Scientists are advocating for a more comprehensive understanding of ecosystems to find genuinely significant natural remedies that avoid jeopardizing the main objective, which is to slow 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.
Marc Macias-Fauria, Study Senior Author and Professor, Scott Polar Research Institute, University of Cambridge
The researchers acknowledge that there could be additional justifications for planting trees, such as timber self-sufficiency, but these situations do not offer benefits for mitigating climate change.
Macias-Fauria further added, “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.”
At high latitudes, how can global warming be mitigated? Instead of planting millions of trees to combat climate change in the Arctic and subarctic, the researchers propose collaborating with local communities to support sustainable populations of large herbivores like caribou.
“There is ample evidence that large herbivores affect plant communities and snow conditions in ways that result in net cooling. 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,” Macias-Fauria stated.
When looking for nature-based climate solutions, the researchers say it is critical to take biodiversity and local communities' livelihoods into account.
Macias-Fauria concluded, “Large herbivores can reduce climate-driven biodiversity loss in Arctic ecosystems and remain a fundamental food resource for local communities. 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 frontline of climate change.”
Journal Reference:
Kristensen, J. A. et. al. (2024) Tree planting is no climate solution at northern high latitudes. Nature Geoscience. doi.org/10.1038/s41561-024-01573-4