How Boreal Trees Could Shift Earth’s Energy Ledger

Each tree planted in Canada’s north is entered into an accounting system. Governments include it in their emissions calculations, banks trade the related carbon offset, and scientists debate what snow does to the overall balance.

Yet another factor is absent from that ledger: one that drifts several hundred metres overhead and can be seen whenever the sky is overcast.

Where the trees are planted could make this factor more important than both carbon and snow.

The dark-canopy problem

When dark conifer trees stand above snow-covered terrain, they absorb sunlight that exposed snow would otherwise reflect back into space. The energy taken in warms the ground surface.

Researchers refer to this process as snow masking. Across established boreal forest stands, it creates an additional warming effect of between 0.5 and 2.5 watts per square metre.

It may seem like a small figure. Across the immense northern forest, however, which accounts for about 30 per cent of global forest cover, that extra warmth quickly becomes significant.

Dr Enoch Ofosu, a postdoctoral researcher at the University of Waterloo (UWaterloo), and his co-authors aimed to address that missing part of the calculation.

Their work compared the warming effect with an opposing influence that has long been visible but overlooked.

What forests release into the air

The distinctive sharp, resinous scent of a pine woodland comes from compounds produced by trees and continually emitted from their foliage. Known to scientists as BVOCs, these emissions increase as temperatures rise.

Once in the atmosphere, the compounds react with oxygen and create minute particles known as secondary organic aerosols. These particles provide the nuclei around which cloud droplets form. More nuclei produce more droplets in each cloud, while smaller droplets reflect sunlight more effectively than larger ones.

This cloud brightening caused by a greater number of smaller droplets is known as the Twomey effect.

Above boreal forests, the resulting cooling may be sufficient to offset the warming from snow masking and, at its strongest, may exceed it several times over. Carbon-only accounting generally fails to capture this effect.

Why the figures differ

Why is the issue still unresolved? The cooling calculation comes from a modelling study reliant on atmospheric chemistry, cloud-droplet formation and existing pollution concentrations. Climate models continue to find each of these stages difficult to represent accurately.

Satellite observations over snow and ice contain substantial noise, while monitoring stations throughout the boreal region are limited. The warming effect, by contrast, has been measured well.

The cooling mechanism is genuine, but its magnitude varies with temperature, background aerosol levels, tree species and season.

Assigning a single net figure to the entire northern biome is still not possible. Before Ofosu’s review brought these connected issues together, tree-planting policy generally considered the cloud effect only as a secondary concern.

The wrong tree in the wrong place

The implications for policy are substantial. Canada’s 2 Billion Trees Program, commitments from more than 140 countries and corporate offset markets all rest on the assumption that boreal trees deliver climate benefits. The review indicates that planting at certain sites could instead worsen the problem.

These include peatlands, permafrost regions and places with long periods of deep snow. In such settings, dark forest canopies increase winter warming beyond what the cloud-cooling effect can counteract.

Put trees in appropriate locations - on degraded land at lower latitudes, in mixtures that include deciduous species, and with honest monitoring - and those same trees can provide a net benefit.

Warming strengthens the feedback loop

The BVOC mechanism has a further complication. Higher temperatures prompt trees to emit more of these compounds: approximately 5.4 to 19.8 per cent more for every degree Celsius. In principle, as the climate gets warmer, the cooling pathway becomes more powerful.

This might appear to be an inbuilt safety mechanism. It operates only where the air remains relatively clean, however. Polluted air already contains ample particles that can seed clouds, leaving little capacity for additional forest-produced particles. Cleaner air means a stronger brake.

What policy has overlooked

Before this review, no single framework had combined carbon, snow reflectivity and cloud effects for northern afforestation. Most policies governing tree planting still rely on carbon accounting alone.

The team’s tool assesses each proposed location to determine whether planting is likely to cool the climate, warm it or produce an uncertain outcome. Many schemes would prefer not to acknowledge that final category, but it is often the accurate one.

It also suggests that some promised forests should not be planted at all - or, at minimum, not at the locations currently proposed.

Where evidence is limited

The cloud-cooling estimate is based on climate models rather than direct observation, and the results differ considerably from one model to another.

There are too few monitoring stations across the boreal zone, and satellite measurements over snow remain too uncertain, to verify those projections. The cooling component is still the least well constrained.

What needs to change

Boreal forests are not an assured solution to climate change. The cooling clouds they may promote could match, or even exceed, the warming caused by their dark canopies. But the outcome depends wholly on the planting location, the species chosen and whether the effects are actually monitored.

Earth’s energy budget is already moving in the wrong direction. Its imbalance has approximately doubled since the early 2000s. Carbon credits that disregard snow and clouds could increase that imbalance further.

The appropriate route is more limited than many policies suggest: sites with less snow and no peat, mixed stands of conifers and decididuous trees, and monitoring of surface reflectivity and cloud changes alongside carbon. It also requires co-design with Indigenous communities, whose knowledge includes long records of boreal disturbance and recovery.

Without these safeguards, planting additional trees in the north could imperceptibly shift the planet’s energy ledger in the wrong direction, removing the very climate benefit the forests were intended to deliver.