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 argue that tree planting at high latitudes will accelerate, rather than decelerate, global warming.
Plantation in Southern Greenland with a dark green surface reflecting only about 10% of the incoming solar energy compared to about 75% for the snow around it.
The group includes lead author of the report Assistant Professor Jeppe Kristensen, an Honorary Research Associate at the ECI and also the Leverhulme Centre for Nature Recovery (LCNR) at the University of Oxford, and also Prof Yadvinder Malhi, Ecosystems Programme Lead at the ECI and Director of the LCNR.
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 Jeppe Kristensen, Assistant Professor at 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. 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 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 soil carbon by microbes (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). b, The approximate relative magnitudes of the different responses to conversions are exemplified by the coloured lines (see Extended Data Tables 1–3 for justification). These magnitudes are expressed via the carbon dioxide-equivalent (CO2e), a commonly used metric to compare various forcings on the energy balance, showing the equivalent mass of CO2 needed to cause the same amount of global warming over a given period, often 100 years. See the main text for further explanation.
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. Jeppe 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 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.
Jeppe said:
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.”
Herd of musk oxen in the tundra with the Greenland Ice Sheet in the back.
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.
They say 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.
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.
They say that forestry in the far North should be viewed like any other production system and compensate for its negative impact on the climate and biodiversity.
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.
The researchers say it’s vital to consider biodiversity and the livelihoods of local communities in the pursuit of nature-based climate solutions.
Senior author Professor Marc Macias-Fauria, from the University of Cambridge’s Scott Polar Research Institute, said:
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.”
Read the report in full in Nature Geoscience: Tree planting is no climate solution at northern high latitudes
Four adult musk oxen and a calf in a grass-covered tundra.