Remember the headlines: Tree planting is our “most effective climate change solution”? Well, it would take 200 years, and we can only plant 10% of the area, says Tom Crowther of ETH-Zürich

“The restoration of trees remains among the most effective strategies for climate change mitigation.” That’s the first line from the abstract of a July 2019 paper published in Science magazine.

The abstract continues as follows,

We mapped the global potential tree coverage to show that 4.4 billion hectares of canopy cover could exist under the current climate. Excluding existing trees and agricultural and urban areas, we found that there 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.

Five of the authors of the paper work at the Crowther Lab, based in the Swiss Federal Institute of Technology in Zürich (ETH-Zürich). The other three authors work at the UN Food and Agriculture Organisation.

Greenhouse PR

The Crowther Lab hired a PR company called Greenhouse to publicise the findings.

The result was hundreds of media reports, with wildly optimistic headlines. Here’s a small sample:

• Planting 1 trillion trees could stop climate change, argues study, Deutsche Welle, 4 July 2019.

• How to erase 100 years of carbon emissions? Plant trees – lots of them, National Geographic, 4 July 2019.

• Planting a trillion trees could be the “most effective solution” to climate change, study says, CBC News, 5 July 2019.

• Tree planting ‘has mind-blowing potential’ to tackle climate crisis, The Guardian, 4 July 2019

The paper also generated a fair bit of criticism from climate scientists. REDD-Monitor summarised the criticism here:

“It would be over 200 years”

In September 2019, The Economist put out a video titled, “Climate Change: can nature repair the planet?”

The video includes an interview with Thomas Crowther of ETH-Zürich. Crowther makes clear that he does not stand by the claims made in the reporting about the ETH-Zürich paper.

Nor does he stand by the claims made in the paper itself.

Here’s the transcript of the short interview with Sarah Collinson, a video journalist with Economist Films:

Sarah Collinson, Economist Films: We’re about to call Professor Tom Crowther, who said that, “Tree planting has mind-blowing potential to tackle climate crisis”. So we want to speak to him to find out how, and see whether it doesn’t sound too good to be true.

Thomas Crowther, ETH-Zürich: So, I’m Tom Crowther. I’m an assistant professor of ecosystem ecology at ETH-Zürich. And we, about a year ago here, started the Crowther Lab where we sort of bring together a lot of perspectives to have a holistic understanding of where all the carbon on earth is and how we can manage ecosystems to store more or less carbon in those areas.

Collinson: Your recent study. Why is that new and important?

Crowther: So, simply put, we just made a very high resolution model of where trees could exist on the planet. And by quantifying how much carbon is in that land we can actually show that restoring forests can capture far more carbon than we ever previously thought.

What we found is actually forests have the potential to capture 200 gigatons of carbon. Since the start of the industrial revolution, we’ve got about 300 gigatons in the atmosphere.

So, it would be very slow, it would be over 200 years, but still we’re talking about a really, really powerful carbon drawdown solution.

Collinson: These numbers, you know, sound great, but how does that translate to real world, real life?

Crowther: I’m so glad you asked these questions, because the big media explosion was pretty devastating, it wasn’t really, I mean it was great, it got loads of people interested, but it also led to loads of confusion.

We would never, ever propose that tree planting works everywhere.

We’re just showing the full potential. It’s not like 200 gigatons are really going to come out of the atmosphere immediately, it would be amazing if we even reached 10% of that full potential.

So we need all of the possible climate change solutions in concert.

But I do believe trees are more powerful than anything else I know of. Because they are such a low-tech, easy access solution, anyone can get involved in it, and you feel good about yourself when you give a dollar to plant a tree.

It’s more important than the simple carbon sequestered by trees, it’s the advocacy that it promotes. I don’t believe we will address climate change until we all feel positive and engaged in the challenge, and I think it’s a beautiful way to open the door to get us engaged.

Stop burning fossil fuels

Collinson could have embarrassed Crowther by quoting what’s written in the paper in Science magazine. Or she could have referred to the press release that ETH-Zürich put out, in which Crowther says,

“We all knew that restoring forests could play a part in tackling climate change, but we didn’t really know how big the impact would be. Our study shows clearly that forest restoration is the best climate change solution available today.”

Instead Collinson turns to Oliver Morton, one of her senior editors at The Economist.

Morton didn’t mince his words:

Yeah, well I mean there were some headlines and Crowther said “the single most important thing people can do about climate change is to plant trees.”

That’s not really the … the single most important thing people can do about climate change is to find ways to move humanity, humankind off the use of fossil fuels.

Which isn’t to say planting trees isn’t good, but by and large getting off fossil fuels is very important.

But I think that’s true of almost anything anyone says about any single act, it’s a good thing but there’s no magic bullet.

The key thing to take into consideration when people talk about planting trees is what else is going on on the land where they want to plant the trees.

The other thing is that sometimes planting trees you plant the wrong sort of trees. In some parts of the world planting trees makes the world warmer.

An article in Nature, published the day after The Economist’s video, reported that,

Two months on, Crowther is chastened by the furore his paper created among scientists who thought its message might encourage the public to relax about curbing carbon emissions as long as they planted enough trees. “We certainly did our communications a little bit wrong, and we’ve learned from that,” he says. “I want to be extremely clear that cutting greenhouse-gas emissions is absolutely essential if we are going to have any chance to stop climate change.”

REDD-Monitor is looking forward to the Crowther Lab’s next paper.

Call me a ridiculous optimist, but I’m hoping that the headlines generated by the PR company that the Crowther Lab hires next time around will read something like this:

• Leaving fossil fuels in the ground would stop climate change, argues study, Deutsche Welle, 4 July 2020.

• How to erase 100 years of carbon emissions? Stop burning fossil fuels – starting now, National Geographic, 4 July 2020.

• A legally binding international agreement to limit fossil fuel use would be the “most effective solution” to climate change, study says, CBC News, 5 July 2020.

• The real answer to climate change is to leave fossil fuels in the ground, The Guardian, 4 July 2020

Oh wait, the last headline is a 2007 George Monbiot article. It reminds me of another Monbiot headline, also from 2007: “Hurray! We’re Going Backwards!”

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