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What is the CDR capacity of trees?

There are good reasons to look to tree planting and forestry management to remove CO2 from the atmosphere. In the absence of land use by humans, it is estimated that biomass could absorb 450 to 500 Pg more carbon than they store today. This is enough to counter all of human emissions to date. At the current emissions rate of about 10 Pg C/year (equivalent 44 Pg CO2/yr), this budget would keep the planet's carbon accounting budget balanced for several years to come.

While this is a lovely story, such an approach does not leave any room for human activity, and thus should not be considered anywhere near practical. A less extreme approach to a nature-based strategy would be to set aside money to retire some (but not all) land for rewinding, providing both climate and ecosystem benefits in return. Today the cheapest tree planting projects cost out at an equivalent of less than $20/ton of CO2 captured, making this approach extremely alluring at first glance. Land, however, is a scarce resource. While industrial CDR technologies get cheaper with scale (thanks to technological learning), tree planting gets more expensive, as less productive or less accessible forests must be reached to expand the activity's footprint. The forest measurement company Pachama suggests a carbon credit price of $50-$100/ton is needed to sustainably develop trees into carbon offset programs. To scale to the level of sequestering a Gt/year, a price between $100-200/ton may be necessary, on par with the cost of industrial CDR.

Recent research suggests even this may be optimistic, at least at the scales needed to bend the planet's climate trajectory. Previous studies have famously estimated that the carbon restoration potential of trees on a rewilded planet could be over 200 Gt. However, if we constrain the problem further by allowing only existing forests to be rewilded (leaving farmland available to humans), and taking into consideration the natural carbon turnover cycle of forests, available capacity drops dramatically, to just 44 Gt carbon. This estimate could rise slightly by taking into consideration improvements possible from afforestation (grown of new forests) and adding accounting for underground carbon storage (in roots, and soil). However, realistically it likely represents an over-estimate of what is economically possible. The majority of the planet's forest regrowth potential is in tropical regions such as Brazil, Indonesia, or the Congo, where it will be technically and politically challenging to both pay for reforestation and verify that the trees we plant continue to thrive.

Forests also require decades to grow, such that the most optimistic estimate of the rate of carbon removal might be 2 Gt CO2/year sequestered, with a politically and economically accessible opportunity likely less than 1 Gt CO2/year. To be clear, this is a lot of carbon, and growing forests offers benefits beyond just CDR. But this is also nowhere near the IPCC's current goal for removal of 6 Gt CO2/year by 2050 in order to limit the planet's temperature increase to just 1.5°C. Tree growth is good on its own merits, and should be seen as a complement to (but not a replacement for) all of the other methods of CDR under consideration today.

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