Planting trees has been positioned as a way to combat global warming for many years. But a new study reveals a much more complex picture: By increasing temperatures, afforestation actually appears to do the opposite. Moreover, the scale of this impact will be quite large in the coming decades.

Authors of new works in the magazine Nature Communication He studied how tree planting typical for a particular region affects changes in the Earth’s ability to reflect and absorb carbon dioxide. They concluded that, due to the dark color of the vegetation, new trees on average are warming the planet, even though they bind a certain amount of CO2.

To model the influence of trees on the capabilities of certain regions of our planet, researchers divided the world map into dozens of subregions and assigned them the average reflectance observed in forests in these regions today. Scientists assumed that in case of new plantings the same trees will prevail there, while others often die and will not take root well. For example, the probability of birch and larch spreading in the tundra is high, but for relatively heat-loving mid-latitude trees it is almost zero. In tropical regions, larch and birch trees have almost no chance.

Modeling revealed that under continental conditions, on average, areas where trees were planted increased warming rather than weakened it. Yes, they bind significant amounts of carbon dioxide. But trees normally reflect more than 20 percent (and often less) of the solar energy that falls on their leaves. On the contrary, grassy vegetation, tundra or sand reflects dozens of solar energy into space. So if you calculate the total impact of planting trees on the climate, it turns out that for every square kilometer covered with forest it is equivalent to an additional 12 thousand tons of CO2 on average. That is, in 50 percent of the cases the “warming” effect of the trees was above the 12,000 limit, and in half it was lower.

It turned out to be extremely interesting that under some conditions in the simulation, trees still did not increase warming. The fact is that in dry climatic zones there are very few clouds, so every square meter there receives several thousand (for deserts – more) kilowatt-hours of solar energy per square meter. A 10-20 percent reduction in reflective surface capacity when trees are planted there means an increase of hundreds of kilowatt-hours per square meter per year in the Earth’s energy budget. It is interesting that such extreme cases are actually common: in Israel, pine trees are planted on the edge of the Negev desert, and the once dry savannas of Africa are gradually covered with shrubs and trees.

Due to the high rainfall in the equatorial climate regions, clouds cover the sky for most of the year. On a clear day, usually only 10 percent of solar radiation passes through the clouds. Therefore, the “warming” effect from forest expansion is much weaker there than at higher latitudes (e.g. Russia) where there is little precipitation and cloudiness. And it is much weaker than in regions with a dry tropical climate.

It follows that planting trees in the Amazon, the Congo, and the forests of Southeast Asia does not increase warming, but increases it enormously, even if there are broadleaf trees in the savannas and northern tundra. Fortunately, the total share of solar energy reaching the surface in winter in the tundra is negligible (there is usually a short daylight or polar night), and in summer the reflectivity of broad-leaved trees is not much different from that of larch.

The authors of the article proposed special software to calculate where planting trees is “worth it” and where it is “not worth it.” However, in practice, the effectiveness of such a tool may be limited. The thing is that planting trees, the most “cooling” of the planet, is possible only in those regions where most of the trees on our planet already grow. The most active population growth is there. So no matter what you do, radically increasing their numbers in the humid equatorial climate will not work.

On the contrary, there are almost no trees in the dry tropical climate and tundra. The global greening process that Naked Science previously mentioned is happening most actively here. Based on the fact that eight or nine thousand years ago, during the optimum climatic period of the Holocene, forests extended to the Arctic Ocean, and there is almost no flat tundra on the Eurasian continent, the expansion of forests in this century should be expected. Entry into the tundra region will be very rapid. According to the authors of the new article, similar processes may occur in the savannas of the Sahel, a region where the “warming” effect of trees is maximum.

From all this we can conclude that by 2100 the natural spread of trees across the planet as part of global greening will significantly accelerate overall warming.