Last year, the journal Science published a study that made a bold - and elegantly simple - claim: To mitigate climate change, plant a trillion new trees.
Authored by a team of scientists from various research institutions in Europe and the Food and Agriculture Organization of the United Nations, the study attracted considerable mainstream media coverage.
Soon after, tree-planting initiatives across the globe bloomed. Ethiopia announced it would plant 350 million trees in a single day and India promised to plant 220 million. The US unveiled a plan to establish forests in Asian and African cities. Companies ranging from Biocarbon Engineering to EasyJet to Warner Music turned the spotlights on their tree-planting initiatives.
The excitement was understandable. The idea that we could negate the effects of centuries of deforestation and keep the planet cool enough to survive simply by planting some trees sounded really good.
The study found that a trillion new trees could store 205 billion metric tonnes of carbon—the equivalent of 25 percent of the current atmospheric carbon pool and enough to help keep us under a 1.5-degree Celsius global temperature rise. Climate action, meet your magic bullet.
Yes, we need to plant trees. Close to one billion hectares (2.5 billion acres) globally is estimated to be available for some kind of forest restoration. If only it were that simple.
To succeed in the fight against climate change we have to do two big things: Stop emitting carbon dioxide and remove the excess carbon dioxide we have already emitted. Restoring forests is the best way to do that second part - but not all restoration is created equal.
In the buzz surrounding the study published in Science, what got scant attention was the cost of planting a trillion trees. With conservation needs already facing a $350bn annual gap between what we are spending and what is needed to secure ecosystems, planting and stewarding a trillion new trees will require mobilising huge amounts of money - something the world does not seem brave enough to do. According to the paper, we would have to reforest approximately 0.9 billion hectares (2.2 billion acres) of land - an area the size of China - to reach their magic number, and at an average cost of $3,000 per hectare, the invoice for this gardening project is prohibitively expensive.
But there is a more realistic way to replace the trees we have destroyed: Help nature run its course.
It is a real, science-based strategy known as assisted natural regeneration. It is low-tech, high-yield, highly scalable, and 70 percent cheaper than planting new saplings.
The premise of assisted natural regeneration is that the most economical way to restore and protect forests is to acknowledge nature's resilience, remove barriers to natural regeneration and—where necessary—accelerate it. Given time, trees regrow and forests come back. Assisted natural regeneration simply supports and accelerates the process. What does it look like in practice?
Examples include stopping fires from burning young trees that are naturally regrowing, dispersing seed mixes in degraded areas close to intact forests, and developing national policies that incentivise intensifying agriculture in some areas in order to let others naturally regenerate.
One of the most exciting assisted natural regeneration strategies is called applied nucleation, also known as "tree islands," which involves planting only a very small number of trees that attract birds and other seed dispersers, which can spread seeds around the tree islands. Gradually, these tree islands turn into intact forests.
If it is such an obvious and effective tactic, why has it not caught on yet? First, it does not have the PR appeal of a person lowering a young sapling into the ground. Second, until recently, assisted natural regeneration was not seen as a solution that could work on a large scale. But advances in our ability to model and predict natural processes - and an unlikely and unexpected test case in Brazil - showed otherwise.
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Brazil's Atlantic Forest stretches across 34 million hectares (84 million acres) of the country's coastal southeast. As large as it is, it is a fraction of what it used to be, having lost nearly three-quarters of its original extent to deforestation.
Over the past two decades, though, rural populations there thinned, with people in farming communities abandoning their land to move to cities to find work, while well-organised local groups ensured enforcement of a Brazilian law aimed at curbing deforestation.
What happened next was remarkable: Between 1996 and 2015, nearly three million hectares (7.4 million acres) of the area was found to have regenerated naturally - without a single sapling being planted.
This did not escape the notice of conservationists. Researchers from the International Institute of Sustainability (IIS) in Rio de Janeiro analysed this regeneration and found that one-third of the degraded Atlantic Forest - some 21.6 million hectares (53.4 million acres) - could eventually be restored if assisted natural regeneration is applied. It was the first real evidence that this method could be scaled up.
Seizing on these findings, Conservation International launched what is on track to be the largest tropical restoration project in history in the Brazilian Amazon. Working with local and international partners, the organisation helped protect and nurture a portion of the Amazon rainforest so it could rebound without interference - and it has started to do so.
Now, Conservation International and IIS are leading efforts to identify other areas of the world where assisted natural regeneration is likely to be ecologically and socially feasible, and it is now estimated that, of the billion or so hectares of forest around the world that have been destroyed or degraded, fully one-third is suitable for assisted natural regeneration.
What that means is that all that land, if protected around the edges from logging, fires, farming and grazing, then left to its own devices, could come back to life—bringing with it all the benefits that forests provide, from water filtration to biodiversity to climate regulation. And that is without threatening food security—critical to our exploding world population—or sticking a single (expensive) sapling in the ground.
So what needs to happen now?
First, the research community must pay closer attention to what nature has been doing for millennia to focus its efforts on actions that support that process.
Second, science and indigenous knowledge must be brought together to show governments where assisted natural regeneration is possible, and inform policies to unlock it.
Third, banking and development communities need to create financial incentives to spur investment in reforestation.
Fourth, corporate actors should put protection above profit so that mistreated land is given space to recover - which in the long run is good for their bottom line.
Let us be clear: Assisted natural regeneration is not the only way forward. We still need to plant new trees where it is necessary, and in ways that respect local ecology and local cultures.
But if we can see to all of the above, Mother Nature will have a much easier time doing what she does best—naturally.