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Across much of the world, planting more trees means more carbon is stored, and global warming is reduced. That’s the thinking behind recent proposals to plant more trees in Alaska, Greenland and Iceland.
But we recently published a paper in the journal Nature Geoscience in which we argue that tree planting is no climate solution at northern high latitudes. In fact, it does more harm than good.
If we are to address climate change, it is of course imperative to reduce the concentration of greenhouse gases in the atmosphere. However, other components of the Earth system also play important roles in determining if any given intervention will cause the planet to warm more or less.
One such component is known as “albedo”. It refers to how much solar radiation is reflected by the surface of our planet back into space. In the northern high latitudes, snow covers the ground during many months every year. Snow is white, which makes it reflect about three quarters of the solar energy hitting it when covering the tundra: it therefore has high albedo.
Trees and tall vegetation protrude from the snow blanket and darken Earth’s surface, lowering its albedo when covered in snow to average values below 50% and causing more snow to melt. In the far north, the warming effect of the lower albedo of trees exceeds the cooling effect of the carbon they take from the atmosphere by converting CO₂ into biomass. That is, when accounting for both albedo and the carbon the trees may end up taking out of the atmosphere and storing, tree planting in the far north actually ends up warming the climate.
Soil carbon released into the atmosphere
But there is more. Carbon in the Arctic resides mostly in the soil. There is more carbon in Arctic soils than in all the trees on Earth combined, and this includes every rainforest in the tropics. Growing trees in the Arctic could cause some of that carbon to be released.
That’s because, even in the unlikely best-case scenario where a tree plantation tries to minimise soil disturbance, growing trees still discharge sugars from their roots. This provides nearby microbes with the tools and energy they need to break down portions of the soil carbon accumulated over millennia. This process, where the turnover of old soil carbon is induced by adding new carbon from roots, is referred to as the priming effect.
The unavoidable outcome of an afforestation project in the Arctic is decades of release of large quantities of soil carbon to the atmosphere. That’s an unacceptable contribution to atmospheric greenhouse gases in the period where we most critically need reductions.
Forests in the far north also have a tendency for being disturbed by other factors. Wildfires, for instance, burn a large part of the boreal forest almost entirely every few decades or centuries. And when not burnt, insect pests and extreme weather tends to get rid of standing vegetation periodically. All these risks are increasing the warmer the Arctic gets.
Intensively managed forests could mitigate the risks to some extent, but such management isn’t feasible in remote areas on a massive scale. What’s more likely is that people will create large plantations of trees of the same age and species, which means they reach the age of prime vulnerability at the same time. Standing trees in the far north therefore not only contribute to further warming, but the carbon they store is vulnerable.
Last, but by no means least, beyond its effects on climate, high latitude afforestation can harm Arctic biodiversity and challenges traditional livelihoods such as reindeer herding and caribou hunting.
We can fool ourselves but not the Earth
Why, then, do people plant trees in the Arctic? Local people might want to ensure a supply or timber, for instance, or to reduce their dependency from imports. Ultimately, it is up to them to decide whether to do so or not.
But these initiatives should not be sold as a climate solution. It is not the first time that we’ve see carbon credits being traded without much due diligence, enabling initiatives to thrive despite doing little to help mitigate climate change. Since we won’t fool the Earth system but only ourselves, we urgently need to get better at accounting for the overall climate effects of our interventions and escape what has been called “carbon tunnel vision”; a point equally relevant far beyond the Arctic.
There are viable nature-based climate solutions in the Arctic and its surrounding regions, however. For instance, sustainable populations of large herbivores such as caribou or musk oxen can actually contribute towards cooling the climate.
This can happen both directly, by herbivores keeping tundra landscapes open, and indirectly, through the effects of herbivores foraging in snow, which decrease its insulation capacity and help reduce the temperature of the soil. Large herbivores also reduce climate-driven biodiversity loss in Arctic ecosystems and remain a fundamental food resource for local communities.
We don’t yet understand everything about how these large animals affect their ecosystems, but the evidence backing their effectiveness is more robust than that of many lavishly funded climate mitigation initiatives. Ultimately, though, any nature-based solution must be led by communities in the far north, who live at the front line of climate change.
Marc Macias-Fauria receives funding from Natural Environment Research Council (NERC, UK), National Science Foundation (NSF, USA), and the European Commission..
Jeppe Aagaard Kristensen receives funding from The Carlsberg Foundation.
