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Planting trees and restoring forests is not going to stop climate breakdown. “We need a rapid end to fossil energy use precisely because we want to preserve the world’s existing forests”

By now you will have seen the headlines. The image above is just a small selection. They are based on a paper published in Science titled, “The global tree restoration potential”. While the paper generated wildly optimistic headlines, it also generated a fair bit of criticism.

The paper was written by Jean-Francois Bastin (Crowther Lab, ETH-Zürich), Yelena Finegold (FAO), Claude Garcia (ETH-Zürich, CIRAD) , Danilo Mollicone (FAO), Marcelo Rezende (FAO), Devin Routh (Crowther Lab, ETH-Zürich), Constantin M. Zohner (Crowther Lab, ETH-Zürich), and Thomas W. Crowther (Crowther Lab, ETH-Zürich).

Funding and public relations

The funding for the paper came from DOB Ecology, Plant-for-the-Planet, and the German Federal Ministry for Economic Cooperation and Development. Data collection was partially funded by Germany’s International Climate Initiative.

Plant-for-the-Planet is an NGO set up in 2007 by a 9-year-old called Felix Finkbeiner. The organisation’s goal is “to fight the climate crisis by planting trees around the world”. Since September 2018, Finkbeiner has worked at ETH–Zürich, under Thomas Crowther.

In February 2018, ETH-Zürich announced that Thomas Crowther had won research funding of potentially more than €17 million over a period of 13 years from DOB Ecology.

DOB Ecology is a Dutch Foundation set up by the De de Rijcke family, after the sale in 2002 of the family business, discount pharmacy Kruidvat (which also owns Superdrug in the UK, and 50% of Rossmann in Germany).

On its website, DOB Ecology explains that,

We are establishing a unique research group to generate global maps of ecological information to guide local, national and international forest restoration.

One of the conditions of the money from DOB Ecology was that Crowther set up a digital marketing team to communicate the results and the science to the public. For the launch of the paper in Nature, Crowther hired a PR firm called Greenhouse.

Greenhouse explains that it “translated the scientific paper into an impactful, digestible narrative that would work for global media and among the general public, while maintaining the essence of the research”. About 700 media pieces were published, according to Greenhouse.

Greenhouse also worked with “key social media accounts”, such as UN Environment, Friends of the Earth, and Christiana Figueres. The hashtag #TreePotential reached 4.8 million people in the five days following the publication of the paper.

This was clearly a successful PR exercise, but was the message conveyed based on sound science? Unfortunately not.

The best climate change solution?

In their research, the authors asked: What area of the planet could be planted with trees (excluding land already used for agriculture or cities) and how much carbon would the trees store?

They found that,

[T]here is room for an extra 0.9 billion hectares of canopy cover, which could store 205 gigatonnes of carbon in areas that would naturally support woodlands and forests. This highlights global tree restoration as our most effective climate change solution to date.

A press release put out by ETH-Zürich states that covering 900 million hectares of land with trees “could ultimately capture two thirds of human-made carbon emissions”.

The press release quotes Thomas Crowther as saying that, “Our study shows clearly that forest restoration is the best climate change solution available today.”

Myles Allen, Professor of Geosystem Science at the University of Oxford, comments that,

Restoration of trees may be “among the most effective strategies”, but it is very far indeed from “the best climate change solution available,” and a long way behind reducing fossil fuel emissions to net zero. The additional 200 billion tonnes of carbon the study highlights represents less than one third of human-induced carbon dioxide emissions to date, and less than 20 years of emissions at the current rate. So, yes, heroic reforestation can help, but it is time to stop suggesting there is a “nature-based solution” to ongoing fossil fuel use. There isn’t. Sorry.

The paper makes no mention of burning fossil fuels as the cause of climate breakdown.

Comparing apples to oranges

Stefan Rahmstorf, writing on the Real Climate website, describes the paper’s claims as “too good to be true”.

Rahmstorf is a physicist and oceanographer, who has worked at the Potsdam Institute for Climate Impact Research since 1996.

Rahmstorf points out that the ETH-Zürich paper compares apples to oranges and forgots “important feedbacks in the Earth system”. Since 1850, 640 gigatonnes of carbon (GtC) have been emitted to the atmosphere. (According to Global Carbon Project, 67% from burning fossil fuels, 31% from land use, and 2% from other sources.)

Although we have emitted 640 GtC, the CO2 increase in the atmosphere is equivalent to just under 300 GtC, Rahmstorf explains. The rest was absorbed by oceans and forests.

Rahmstorf adds that,

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.

Rahmstorf compares the “negative emissions” from tree planting to our other emissions. The 205 GtC stored in newly planted trees is less than one-third of the 640 GtC total emissions, not two-thirds as claimed in ETH’s press release.

Simon Lewis, Professor of Global Change Science at University College London, questions the figure of 205 GtC that could be stored in trees:

The estimate that 900 million hectares restoration can store an addition 205 billion tones of carbon is too high and not supported by either previous studies or climate models (e.g. Lewis et al 2019 Science, Arora & Montenegro 2011, Nature Geoscience).

Trees take decades to grow

In their Nature paper, the authors acknowledge that, “the carbon capture associated with global restoration could not be instantaneous because it would take several decades for forests to reach maturity”.

Rahmstorf points out that if it takes 50 to 100 years for the trees to store 205 GtC, that’s an average of 2 to 4 GtC per year. (That’s assuming that all the trees are planted immediately, and that they all survive.)

Every year we emit 11 GtC to the atmosphere. So, if we could plant 900 million hectares with trees tomorrow, that would store between one-fifth and one-third of current emissions. Of course, if emissions continue to grow, the proportion will decrease.

Another problem is that a large part of the land theoretically available for planting is in Alaska, Canada, Finland, and Siberia. Rahmstorf explains that planting trees in the snowy regions of the far north would be counterproductive for the climate. Snow reflects a lot of solar radiation back into space, while forests are much darker and absorb the heat.

Rahmstorf adds that,

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.

Land rights matter

The ETH-Zürich paper acknowledges that their research has not taken into account whose land it is that they are considering planting trees on:

Of course, it remains unclear what proportion of this land is public or privately owned, and so we cannot identify how much land is truly available for restoration.

The land rights of local communities and Indigenous Peoples are thus swept out of sight. ETH-Zürich make no mention of the importance of land rights in their paper. Neither do they mention how a process of free, prior and informed consent could be carried out over the 900 million hectares of land that they are proposing to cover in trees.

Rahmstorf notes that the researchers used satellite maps, Google Earth, and machine learning technology to analyse where the land is suitable for growing trees. “But for many of these areas,” Rahmstorf writes, “there are probably good reasons why there is currently no forest.”

Rahmstorf notes that the obstacles to tree planting are “not apparent from the bird’s-eye view of the satellites”. Rahmstorf considers it optimistic to assume that half of the area could actually be planted. That means we’re looking at between 1 and 2 GtC of negative emissions per year.

Forest restoration or industrial tree plantations?

Despite his criticisms of the paper, Rahmstorf is in favour of “massive planting of trees worldwide”. He writes that “The current global CO2 emissions can be reduced by 80-90% through transforming our energy, heating and transport systems.” But that leaves 10-20% from agriculture, industrial processes, and long-haul flights. The 1 to 2 GtC of negative emissions from tree planting could offset these emissions, according to Rahmstorf.

Rahmstorf argues against planting monocultures, and in favour of planting “carefully, close to nature and sustainably”. But whether this is likely to happen in reality is an open question.

A recent paper in Nature, looks into countries pledges to restore forests, including those under the Bonn Challenge, an initiative that was launched in 2011 by the German government and IUCN. The Bonn Challenge aims to restore 350 million hectares of forest by 2030.

The authors of the Nature paper, Simon Lewis, Charlotte Wheeler Edward Mitchard, and Alexander Koch, found that “almost half of the pledged area is set to become plantations of commercial trees”.

They list four serious concerns with this focus on commercial plantations:

  • “Plantations hold little more carbon, on average, than the land cleared to plant them.”
  • “Drastically increasing the area plantations could undercut their profitability – the reason that nations are prioritizing them.” Countries are proposing 157 million hectares of new plantations, which would probably result in a collapse of pulp and paper prices.
  • “Policymakers are misinterpreting the term ‘forest restoration’.” Part of the problem is the FAO’s definition of a forest: more than 0.5 hectares in area, trees at least five metres high, and more than 10% canopy cover. This fails to differentiate between industrial tree plantations and forests.
  • “Reports often mix up the process of regeneration to natural forest with the resulting land-cover type.” Land can be labelled as forest before the forest is mature. Climate calculations assume that the trees grow to maturity, but there is no guarantee that the trees will still be there in 50 or 100 years’ time.

The ETH-Zürich paper compares its country-level results with country’s commitments under the Bonn Challenge. The authors found that “Approximately 10% of countries have committed to restoring an area of land that considerably exceeds the total area that is available for restoration.” But 43% of countries have committed to restore an area that is less than half the area available, according to ETH-Zürich’s results.

But ETH-Zürich makes no mention of the fact that almost half of these commitments are for industrial tree plantations.

Massive deforestation in the tropics – due to climate change?

Rahmstorf notes an import result in the ETH-Zürich researchers’ study that got little media coverage. As the global climate heats up, tropical forests in particular are facing massive deforestation. The authors illustrate this with a map of expected losses in potential tree cover by 2050, under the “business as usual” climate change scenario:

Simon Lewis questions this finding:

The idea that climate change risks losing areas of tropical forests before 2050 is also not consistent with the results from climate models or observational studies for today’s forests. The authors make a critical error by ignoring the central role of carbon dioxide fertilisation, which counters much of the negative impacts of higher temperatures. Today, the immediate major threat to tropical forests is deforestation by people and out of control fires, not the more subtle impacts of higher temperatures.

But there is an increasing collection of evidence that as the climate warms, forests are more vulnerable to fires, droughts are more severe and more frequent, and tropical forests in particular are increasingly at risk of going up in smoke. Thus releasing the carbon stored in the soil and trees to the atmosphere.

Fossil fuels

Simon Lewis focusses on emissions from fossil fuels in his comment on the ETH-Zürich paper:

To curb climate change means keeping fossil carbon out of the atmosphere. That means ending the burning of fossil fuels and the dumping of carbon in the atmosphere. New forests can play a role in mopping up some residual carbon emissions, but the only way to stabilise the climate is for greenhouse gas emissions to reach net zero, which means dramatic cuts in emissions from fossil fuels and deforestation.

In a series of tweets in response to some of the criticism the paper received, Thomas Crowther acknowledges that “to address climate change, we need both drastic cuts to emissions and significant carbon drawdown. Restoring trees is an extremely powerful drawdown solution, but cuts to emissions are still essential.”

It’s a pity he didn’t write that in the paper itself. The paper makes no mention of emissions from burning fossil fuels.

And in the ETH-Zürich press release, Crowther said that forest restoration is the “best climate change solution”. That is simply not true.

Rahmstorf concludes that,

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.


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  1. Annually 15 million hectares of global forests are lost. In addition 320 million ha (size of India) are damaged by forest fires each year. Out of these fires 90 %, are caused by human activities. If we (through education) can reduce this human stupidity (causing wildfires) to only 50 %, then we reduce scheduled tree planting and use this money for forest management i.e. protection of existing forests. Here in Finland, 20 % of the entire human population are forest owners. Massive forest education has taught everybody to love the forest.