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Can we really plant 500 billion trees?

Can we really plant 500 billion trees?
28 Aug
2019
The lead author of a scientific study, which claimed that planting trees has mind-blowing potential to tackle the climate crisis, reflects on the mixed response to the report

Not every scientific study gets the attention it deserves, but every now and then one captures the public imagination. That’s what happened when a group of scientists demonstrated that the Earth is home to 1.7bn hectares of treeless land (around the size of the US), which they say could support an additional 500 billion native trees without affecting cities or agriculture. If planted, the researchers, led by Dr Thomas Crowther of ETH Zurich, claim that once mature, these trees could remove around two-thirds of all the carbon emissions produced by humans in the industrial era. The estimates were produced by combining satellite images of Earth’s surface with environmental data about where trees can grow and how much carbon they can store.

While the frenzy that accompanied the release of the report was largely positive (Crowther says that donations to restoration projects spiked in its wake), there were criticisms. Geographical caught up with Crowther to discuss the response the report generated...

Some commenters to the report suggested that planting 500 billion trees isn’t economically viable.

‘Well, the study was never intended to include an economic analysis but that’s certainly the next step. I’m optimistic that it can be done. We really want to start looking into the economics of global-scale restoration. I hope this will inspire more government support, but I don’t think that’s what’s going to make the difference. I really think its going to be industry and the general public donating.’

Even if that many trees were planted, it’s likely to be a long time before the amount of carbon predicted could realistically be sequestered.

‘Certainly. As with all climate change solutions we have at the moment, it’s a very long-term solution. It’s only by the end of the century if not into the next century that we’d start to see that amount of carbon sequestered. The critical thing is that we need to get going yesterday.’

Some observers had doubts about the figures, pointing in particular to the boreal forests of the Northern hemisphere and their reduced potential
to absorb carbon.

‘True, but this doesn’t affect the overall figures. Boreal forests grow much more slowly than tropical forests, we’re well aware of that. On top of that, forests up there could actually warm the planet because the surface created by boreal forest is much darker than the snow that would otherwise be there. A lot more of the sun’s energy gets absorbed. But simply in terms of carbon capture, none of that changes any of our numbers. It just highlights the need to really understand the ecology before you do any restoration.’

What would you say is the real message people should take from your report?

‘That this is only the start. I would reiterate that all we’ve done is identify this as an incredibly powerful carbon draw-down solution, but it only benefits local biodiversity and sustainability when it’s done ecologically right.’

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