Citizen Action Monitor

Tree-planting study that’s being promoted as “best climate change solution available” has serious flaws

Critic of study says “… we must not fall for illusions about how many billions of tons of CO2 tree-planting will take out of the atmosphere.” 

No 2491 Posted by fw, July 18, 2019

Stefan Rahmstorf

“In recent weeks, a new study by researchers at ETH Zurich has hit the headlines worldwide. 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 from the air … 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. … 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.”Stefan Rahmstorf, Real Climate

Stefan Rahmstorf is a physicist, oceanographer and climatologist. As noted in the above excerpt, the immediate, widespread, positive response to the ETH Zurich research caught his attention, prompting him to write a critique of the study.

Among the publications that promoted the Swiss “tree-planting” story was National Geographic’s July 4 piece entitled How to erase 100 years of carbon emissions? Plant trees—lots of them. Included in its article were these claims by co-authors of the ETH Zurich report:

Tom Crowther, a researcher at ETH Zürich, and senior author of the study, stated: “Our study shows clearly that forest restoration is the best climate change solution available today.” Co-author Jean-Francois Bastin added: “Anyone can plant a tree and we can start doing it tomorrow. Reforestation can buy us time to cut our carbon emissions.

Canada’s Below 2C website reposted the National Geographic article under a different title — Is Planting Trees The New Carbon Fighting Technology? – and with this introductory editorial:

“One has to wonder why it has taken so long to discover that tree-planting done on a massive global scale is likely to become one of the most effective tools in the carbon-fighting toolbox. A new study just out in Science journal claims that reforestation is “our most effective climate change solution to date.” This is mind-boggling. We are spending precious time and billions on technologies to suck carbon dioxide out the air we breathe. And trees can do this—for nothing.”

Below is my heavily abridged and edited repost of Rahmstorf’s critique. Why an abridged/edited version? Both my partner and I found chunks of Rahmstorf’s writing quite confusing. (Admittedly, our unfamiliarity with climate science may have contributed to our confusion).

In particular, we were particularly perplexed by Rahmstorf’s opening segment, which, in his words, “With a few basic facts about the CO2 increase in our atmosphere, this is easy to understand.” Rahmstorf begins the segment with the opening sentence of paragraph 3 “Unfortunately, it’s also too good to be true. … and ends with the closing sentence of paragraph 5: “And precisely because reforestation takes a very long time …” Easy to understand for fellow scientists perhaps, but hardly easy for those unprepared to navigate a discussion of the thousands of billions of gigatonne (GtC) measures of both carbon and carbon dioxide.

In an effort to make my repost of Rahmstorf’s critique accessible for others as untutored as myself, it is presented in three segments:

First Segment — I begin with the opening of Rahmstorf’s article, including some of the more challenging bits. To help readers navigate these bits, I have included my added subheadings and highlighted text;

Second Segment — I follow with an abridgement and edited selection of eight flaws that Rahmstorf identified as problems with ETH Zurich’s research ; and

Third Segment — I conclude with a sample of four responses from readers of Rahmstorf’s article who share their own criticisms of the Swiss study. And with this group of four, I also share a remark from Rahmstorf to a reader’s comment.

Alternatively, read Rahmstorf’s complete article and 30-plus readers comments by clicking on the following linked title. Hopefully, you will find it more understandable than I did.

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First Segment – Opening of Rahmstorf’s article

Can planting trees save our climate? by Stefan Rahmstorf, Real Climate, July 16, 2019

Tree leaves extract CO2 from the air. So let’s plant lots and lots of trees and bury carbon in the ground. Voila, emissions problem solved, right?

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 CO2 from the air and then release the oxygen O2 again, this is a great climate protection measure.

By planting over a trillion trees, about 200 billion tons of carbon could be absorbed in the trunk, branches, leaves and roots  

The researchers estimated 200 billion tons of carbon could be stored in this way – provided we plant over a trillion trees.

Those dreaming of a climate mitigation jumped on the tree-planting bandwagon and the news went viral

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 CO2 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.

To understand why the tree-planting news is too good to be true, first know basic facts about CO2 increase

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 CO2 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 CO2 – and the trend is rising.
  • These 11 GtC correspond to 40 gigatonnes of CO2, because the CO2molecule 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.

CO2 level is higher now than it has been for 3 million years, and is the main cause for global warming

The result is that the amount of CO2 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 CO2 has been known since the 19th century; it is physically understood and completely undisputed in science.

Although 640 GtC of CO2 was emitted since 1850, only 300 GtC of CO2 went into the air, the rest were absorbed by oceans and forests

But: this CO2 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 CO2 increase in the atmosphere was caused entirely by humans. The additional CO2 does not come from the ocean or anywhere else from nature. The opposite is true: the natural Earth system absorbs part of our CO2 burden from the atmosphere.

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Second Segment — 8 flaws that Rahmstorf identifies with ETH Zurich’s research

1/ The authors of the new [Zurich] study say that it would take 50 to 100 years for a trillion trees to store 200 GtC [of carbon] – an average of 2 to 4 GtC [of carbon] per year, compared to our current emissions of 11 GtC per year of carbon. That’s a removal of only about one-fifth to one-third of our annual carbon emissions.

2/ “Moreover, this proportion will decrease even further if our carbon emissions continue to grow. …This sounds quite different from the prospect of solving two-thirds of the climate problem with trees.”

3/ “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 [snow-covered unwooded areas] reflect a lot of solar radiation back into space, the forests absorb it and thus increase global warming instead of reducing it.”

4/ “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.”

5/ The Swiss study used high-resolution satellite maps and Google Earth to identify locations where there is a suitable place for forests where none is currently growing, leaving out farmland and cities. And 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 landsthe authors respond that they have only assumed loose tree cover there, which could even be beneficial for grazing animals. 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.

6/ The authors of the study also write that it is unclear how much of the areas found would actually be available for planting. Therefore, Rahmstorf still considers it optimistic to assume that half of the calculated theoretical planting potential can be realized in practice. In that case, we’re talking about one trillion trees removing only about 1-2 GtC of emissions per year.

And the current global CO2 emissions can be reduced by 80-90 % through transforming our energy, heating and transport systems – but there will remain the rest that will be hard [to] get rid of (e.g. from agriculture, industrial processes and long-haul flights) and that will have to [be] offset in order to stabilize the global climate.

7/ Although “the study by the Swiss 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. (*It’s not clear whether Rahmstorf is referring to the Swiss study models or models by other researchers].

8/ Rahmstorf concludes that although the massive planting of trees worldwide should be undertaken quickly “to reap various additional benefits of forests on local climate, biodiversity, water cycle and even as a food source,”  “We must not fall for illusions about how many billions of tons of CO2 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.”

*****

Third Segment — Four responses from readers, plus one from Rahmstorf

Stefan Rahmstorf writes:

… my main point here is that the claim by your first author – as quoted by National Geographic – that “reforestation can buy us time to cut our carbon emissions” is the wrong conclusion. We need to cut our carbon emissions right now, and giving politicians the message that in the light of your study we now have more time is dangerous. It was tweeted enthusiastically here in Germany by those that resist emissions reductions. -Stefan]

John Pollack writes:

In response to Rahmstorf: “Without effective climate protection, progressive warming will lead to a massive loss of existing forest cover, especially in the tropics.”

How true! But it’s even more difficult than that. Worldwide, the climate will be changing across existing and newly-planted forests. The result will be that the trees composing the current forest will often be poorly adapted for the future climate. This means greater tree mortality, insect and disease problems, fires due to a higher percentage of dead wood in forests, etc. So, even if the future climate in an area still allows a forest, the individual trees composing the forest will be on a rapid replacement cycle, with diminished capacity for carbon storage.

Jef writes:

It is all about healthy forest not trees. The difference between trees and forest is massive. All effort should go toward preserving forests we have and reclaiming/expanding on them. You can’t plant a forest but we must do the best we can. That and stopping the madness of consumption.

Nemesis writes:

I see a huge gap between certain geoengineering plans (that’s exactly what large scale tree planting etc means) and real world fossil fuel policy. As long as there is no significant CO2 emissions reduction (and that means a radical systemic shift of vast proportions, avoided for many decades so far), any geoengineering scheme will fail for certain. It seems like the powers that be hop from one technofix idea to the next just to avoid any substantial system change, that game is going on for many decades now and the longer it goes on, the harder it will be for any engineering plan to be successful.

… all over Europe right now, millions of trees are dying because of the ongoing severe drought resp insects and diseases, new planted trees are dying as well as young trees are most vulnerable.

Doug Alder writes:

If the IPCC’s estimate of only 1-12 years remaining in which to reduce by 50% the amount of CO2 we have injected into the atmosphere since the start of the Industrial revolution then there simply isn’t enough time remaining for the trees to mature enough to have reached their maximum carbon sequestration. That’s not to say we shouldn’t do this, we should, but we need faster solutions now.

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