So, about that “trillion trees” study…

Climate and Energy

Last week, the journal Science published a new study on forest restoration and the role it could play in reducing atmospheric greenhouse gases. The findings generated a lot of eye-grabbing headlines, many of which were incomplete, hyperbolic, or downright misleading. It’s a complicated study with important findings and caveats. This blog will dig into a few nuances that were buried under a trillion trees. As we’ll see, forests are part of the solution but far from a cure-all.

What the study says

The study looks at potential from global forests for pulling GHGs from the atmosphere. In particular, it asks: to what extent could increasing global forests reduce GHG emissions? To answer this question, it estimates the maximum additional canopy cover the planet is capable of supporting. (The findings are generally robust, see here for methodological criticism.)

The authors calculate that the planet can support 4.4 billion total hectares of canopy cover—essentially a measure of the number of trees. (For reference, Canada’s total landmass is about 1 billion hectares.)

We can further disaggregate this number:

  • Existing forest canopy cover
  • Total potential new forest canopy cover
  • Total potential new forest canopy cover on land that is currently unproductive (i.e. not cropland)

That mix looks a little like this:

Source: Bastin et al., 2019

The authors’ focus (and the media’s) is unproductive land that is suitable for new canopy cover, which they put at about 900 million hectares globally. Over 50% of this restoration potential resides in six countries. Canada (78.4 million hectares) ranks third after Russia and the United States.

According to the study, this additional canopy cover land could store a total of 205 gigatonnes of carbon once mature. This number is technically correct, but pulling one tonne of carbon from the atmosphere causes other sinks (namely the ocean) to release carbon. See comments from Canadell and Hausfather for elaboration.

Still, 205 gigatonnes is a big number. For context, global GHG emissions from industry and fossil fuels hit a record 36 gigatonnes in 2018.

Some caveats

The study abstracts a lot of the practical and technical challenges of achieving another 900 million hectares of canopy cover. It doesn’t offer solutions to them. Getting to 900 million would be tough, maintaining it might be tougher.

First, the study is silent on the economics. Reforesting 900 million hectares of land would be an effective climate solution, but this study isn’t clear if it would be cost­-effective.

Second, the study doesn’t differentiate privately- and publicly-owned land. If enough private landowners are reluctant, a lot of that 900 million hectares may simply be inaccessible for reforestation initiatives. (This is less of a problem in Canada.)

Third, this sequestration is not instantaneous. It would take decades for the forests to mature, with the constant risk of fire, invasive species, and land degradation along the way. Overshooting 2 degrees of warming within that timescale is quite possible.

Finally, climate change will affect the carbon storage potential of forests for the worse. There is new evidence that rising global temperatures may actually weaken the carbon-storing abilities of forests. The study is unequivocal: further climate change will reduce both tree coverage and the ability of our remaining forests to absorb and store CO2.

From the ground up

The authors give us plenty of food for thought, but how to encourage this reforestation? It’s not going to happen on its own.

There are plenty of options on the policy menu:

All of these ideas have their advantages and disadvantages. Regardless of which options we pursue, it’s essential we have strong accounting and monitoring systems, and standardized baselines (i.e. what would have happened without the offset).

Branch out

This study is a lot of things. Here’s what it’s not: an excuse to avoid reducing emissions at the source. Forests may buy us some time, but they won’t address the root of the climate change problem.

We can think of the balance of CO2 and other greenhouse gases in the atmosphere like a bathtub that’s both filling up and draining at the same time. It doesn’t matter how quickly the tub is draining if the water is coming in even faster.

Reforestation should absolutely be a piece of our climate strategy. But it’s exactly that: one piece, of many. Trees are still our best bet for pulling carbon directly from the atmosphere. They’re still not a substitute for a price on carbon or methane regulations or policies to enhance other carbon sinks. If we want to stop the tub from overflowing, we still need to turn off the tap.


  1. KS

    Thanks for this – a much more nuanced view of what is needed. Although forest mitigation is key to keeping warming below 1.5C, it alone is not the magic bullet. We need to see massive emissions reductions across all economic sectors. We can’t continue down the same path we’re on expecting to find easy outs.

  2. Bill

    After reading this I am still unclear whether “this additional canopy cover land could store a total of 205 gigatonnes of carbon once mature” means 205 GT per year or a one-time capture of 205 GT. My understanding is that trees capture a lot of CO2 while they are growing but when they die they release a large proportion of that as they rot. But not all, and depending on the type of ecosystem that the tree dies in, a fraction of the CO2 is permanently sequestered. I would like to know if the 205 GT is the take-up during the growth of the new trees or the annual permanent sequestration. If it is the former, then that’s a bit depressing because all these trees over their lifetime would only absorb 5.6 years of “GHG emissions from industry and fossil fuels” which are 36 GT.

    • Brendan Frank

      Hi Bill,
      Thanks for the comment. The 205 Gt figure is over the entire lifetime of the forests, not annually. And yes, those forests will eventually return most of the carbon to the atmosphere.

    • OT

      Bill, thanks for your comment and for clarifying something that I think often gets misinterpreted. One correction though, is that the 36 GT of GHG emissions is a measure of CO2, not carbon. In terms of carbon the annual emissions are about 10 GT so I believe the additional canopy discussed in the article would equate to about 20 years’ emissions at the current rate. It doesn’t solve the problem but maybe it could buy some time, like putting a bucket under a leaking pipe while you go looking for the shutoff valve.

  3. Thomas

    Good blog post, but I think you missed some solid tree puns. “What the study says” could have been “root of the issue” or “the heart-wood of the matter”. “Some caveats” could have been “out on a limb” or “potential widowmakers”.

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