Not every forest cools the Earth – avoid reforestation in northern regions, says study

Whistler, Canada [credit: Thom Quine @ wikipedia]

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Another one for carbon obsessives to ponder. By ‘northern regions’ they mean areas subject to regular winter sunlight-reflecting snowfall: ‘in Siberia, Canada, Alaska and large parts of North America, large-scale reforestation does not usually have a cooling effect on the climate.’ Those kind of regions obviously don’t need cooling anyway. Planting more trees anywhere can gain so-called carbon credits, but it’s not that simple according to this study’s results.
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The researchers calculated the biochemical and biophysical temperature effects that reforestation would have by 2100 for the three scenarios – and how these would impact the global climate system, says EurekAlert.

This was based on the assumption that, for all three scenarios, forests would be reforested to their maximum potential between 2015 and 2070 and that the forest area would then remain constant for 30 years.

No urban areas, vegetation-free or ice-covered regions were to be reforested, and reforestation on agricultural land would be kept to a minimum so as not to jeopardise global food security.

In carrying out the simulation, the researchers used a climate model that incorporates all the components of the climate system, including the atmosphere, oceans and land.

To ensure that the calculated effects were actually attributable to reforestation and not to random weather fluctuations, the researchers ran the model five times on the ETH ‘Euler’ supercomputer, with slightly different starting conditions. The simulations took around four months and generated 300 terabytes of data.

The amount of land required could be halved
The results were astounding: although the reforested area differed by 450 million hectares, two of the scenarios examined achieved almost the same global cooling effect. The difference corresponds to an area roughly the size of all EU countries combined.

‘The fact that we can achieve the same cooling effect with significantly less land shows that where we plant is more important than how much we plant,’ explains Nora Fahrenbach, a doctoral student in Jnglin Wills’ group and lead author of the study.

The reason for these efficiency gains lies in the geographical location and in the contradictory effects of biophysical and biochemical processes at different latitudes.

While the reforestation scenario proposed by Jean-François Bastin’s team envisages vast forest areas in northern latitudes, more efficient approaches focus on regions where trees can better exert their cooling effect.
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Local interventions with global consequences
The researchers also demonstrated that reforestation influences atmospheric and oceanic circulation. This means that a new forest can alter temperatures and precipitation in regions thousands of kilometres away.

Surprisingly, these non-local effects varied dramatically between the three scenarios. Whether a region became warmer or cooler depended not only on the trees in that location, but also on where else on Earth additional forests were planted. This clearly shows that reforestation has not only a local impact, but also global consequences.

Full article here.
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Image: Whistler, Canada [credit: Thom Quine @ Wikipedia]