In recent weeks, a new study by researchers at ETH Zurich has hit the headlines worldwide (Bastin et al. 2019). It is about trees. The researchers asked themselves the question: how much carbon could we store if we planted trees everywhere in the world where the land is not already used for agriculture or cities? Since the leaves of trees extract carbon in the form of carbon dioxide – CO 2 – from the air and then release the oxygen – O 2 – again, this is a great climate protection measure. The researchers estimated 200 billion tons of carbon could be stored in this way – provided we plant over a trillion trees.

The media impact of the new study was mainly based on the statement in the ETH press release that planting trees could offset two thirds of the man-made CO 2 increase in the atmosphere to date. To be able to largely compensate for the consequences of more than two centuries of industrial development with such a simple and hardly controversial measure – that sounds like a dream! And it was immediately welcomed by those who still dream of climate mitigation that doesn’t hurt anyone.

Unfortunately, it’s also too good to be true. Because apples are compared to oranges and important feedbacks in the Earth system are forgotten. With a few basic facts about the CO 2 increase in our atmosphere this is easy to understand. Mankind is currently blowing 11 billion tonnes of carbon (gigatonnes C, abbreviated GtC) into the air every year in the form of CO 2 – and the trend is rising. These 11 GtC correspond to 40 gigatons of CO 2 , because the CO 2 molecule is 3.7 times heavier than only the C atom. Since 1850, the total has been 640 GtC – of which 31 % is land use (mostly deforestation), 67 % fossil energy and 2 % other sources. All these figures are from the Global Carbon Project, an international research consortium dedicated to the monitoring of greenhouse gases.

The result is that the amount of CO 2 in our air has risen by half and is thus higher than it has been for at least 3 million years (Willeit et al. 2019). This is the main reason for the ongoing global warming. The greenhouse effect of CO 2 has been known since the 19th century; it is physically understood and completely undisputed in science.

Room for more trees? Sheep grazing on deforested land in New Zealand. (Photo S.R.)

But: this CO 2 increase in the air is only equivalent to a total of just under 300 GtC, although we emitted 640 GtC! This means that, fortunately, only less than half of our emissions remained in the atmosphere, the rest was absorbed by oceans and forests. Which incidentally proves that the CO 2 increase in the atmosphere was caused entirely by humans. The additional CO 2 does not come from the ocean or anywhere else from nature. The opposite is true: the natural Earth system absorbs part of our CO 2 burden from the atmosphere.

Conversely, this also means that if we extract 200 GtC from the atmosphere, the amount in the atmosphere does not decrease by 200 GtC, but by much less, because oceans and forests also buffer this. This, too, has already been examined in more detail in the scientific literature. Jones et al. 2016 found that the amount of carbon removed from the atmosphere amounts to only 60% or less of the negative emissions, when these are implemented on the background of a mitigation scenario (RCP2.6).

We can also compare the “negative emissions” from tree planting to our other emissions. The 200 GtC would be less than one third of the 640 GtC total emissions, not two thirds. And the authors of the new study say that it would take fifty to one hundred years for the thousand billion trees to store 200 GtC – an average of 2 to 4 GtC per year, compared to our current emissions of 11 GtC per year. That’s about one-fifth to one-third – and this proportion will decrease if emissions continue to grow. This sounds quite different from the prospect of solving two-thirds of the climate problem with trees. And precisely because reforestation takes a very long time, it should be taboo today to cut down mature, species-rich forests, which are large carbon reservoirs and a valuable treasure trove of biological diversity.

There is another problem that the authors do not mention: a considerable part of the lands eligible for planting are in the far north in Alaska, Canada, Finland and Siberia. Although it is possible to store carbon there with trees, albeit very slowly, this would be counterproductive for the climate. For in snowy regions, forests are much darker than snow-covered unwooded areas. While the latter reflect a lot of solar radiation back into space, the forests absorb it and thus increase global warming instead of reducing it (Bala et al. 2007, Perugini et al. 2017). And increased regional warming of the Arctic permafrost areas in particular would be a terrible mistake: permafrost contains more carbon than all trees on earth together, around 1,400 GtC. We’d be fools to wake this sleeping giant.

And there are other question marks. Using high-resolution satellite maps and Google Earth, the researchers have analyzed where there is a suitable place for forests where none is currently growing, leaving out farmland and cities. With the help of machine learning technology, natural areas around the world were evaluated to determine the climate and soil conditions under which forests can thrive. The free and suitable land areas found in this way amount to 1.8 billion hectares – as much as the combined area of China and the USA.

But for many of these areas, there are probably good reasons why there is currently no forest. Often they are simply grazing lands – the authors respond that they have only assumed loose tree cover there, which could even be beneficial for grazing animals. The Dutch or Irish pastures would then resemble a savannah. Nevertheless, there are likely to be considerable obstacles of very different kinds on many of these areas, which are not apparent from the bird’s-eye view of the satellites. The authors of the study also write that it is unclear how much of the areas found would actually be available for planting.

Therefore, I’d still consider it optimistic to assume that half of the calculated theoretical planting potential can be realized in practice. Then we’re talking of 1-2 GtC of negative emissions per year. But that is precisely what we will need urgently in the future. The current global CO 2 emissions can be reduced by 80-90 % through transforming our energy, heating and transport systems – but there will remain a rest that will be hard get rid of (e.g. from agriculture, industrial processes and long-haul flights) and that we will have to offset in order to stabilize the global climate.

The study by the ETH researchers has another important result that has hardly been reported. Without effective climate protection, progressive warming will lead to a massive loss of existing forest cover, especially in the tropics. At the same time, the models are not yet able to make reliable statements on how forests can cope with new extremes, fire, thawing permafrost, insects, fungi and diseases in a changing climate.

Global warming threatens massive forest losses (red), especially in the tropics. Fig. 3 from Bastin et al., Science 2019

The massive planting of trees worldwide is therefore a project that we should tackle quickly. We should not do that with monocultures but carefully, close to nature and sustainably, in order to reap various additional benefits of forests on local climate, biodiversity, water cycle and even as a food source. But we must not fall for illusions about how many billions of tons of CO 2 this will take out of the atmosphere. And certainly not for the illusion that this will buy us time before abandoning fossil fuel use. On the contrary, we need a rapid end to fossil energy use precisely because we want to preserve the world’s existing forests.

Links

Would a large-scale tree restoration effort stop climate change? Forest expert Marcus Lindner from EFI points to the fires in Russia and the success story in China.

How to erase 100 years of carbon emissions? Plant trees-lots of them. National Geographic shows the importance of indigenous peoples as guardians of the forest.

Restoring forests as a means to many ends The commentary in Science on the Bastin study revolves around the question of how sustainable reforestation can be designed with multiple benefits beyond mere carbon storage.

Tree planting ‘has mind-blowing potential’ to tackle climate crisis Guardian

Analysis: How ‘natural climate solutions’ can reduce the need for BECCS Last year Carbon Brief prepared this analysis of how much carbon mitigation potential may be expected from “green” solutions like tree planting or biochar.