Planting trees is a well-known method for mitigating climate change. The raw cost of planting trees is relatively low, estimated at $10 to $20 per ton of CO2 captured. This is a lower cost than any point source carbon capture method, and an order of magnitude smaller than estimates for the future cost of direct air capture (DAC). It almost certainly could get cheaper still, with additional technology investment. We should be doing more of it, but we aren't. The problems are not technical at all, but accounting.
The first challenge with tree planting is, ironically, that trees have value on their own. A tree planted for carbon capture could also be harvested and used for paper. It might deliver fruit, or nuts, or provide shade for livestock. There are many reasons for planting a tree beyond the carbon it captures, which perversely means that there is no universal price that would spur people to plant trees. Many landowners would plant trees without any help at all from outside carbon credits, but if credits are in the offing they won't share that information in advance for fear of not qualifying for the credits. It is impossible to tell the difference between an incentive program that actually encourages tree planting, and one that simply produces reports saying that planting increased. By contrast, CO2 captured by DAC has almost no use whatsoever, which weirdly makes the incentives much easier to track.
Even more terrifying to would be providers of credits is, again perversely, the living stock of trees. Owners of existing forests might feel compelled to report that they will cut down existing trees unless they are paid a credit as well. "Nice forest, pity if something should happen to it" is probably an entirely viable business model, and completely supported by the incentives. This will inevitably happen a few times, just to prove the point, and in fact it probably is.
An added problem is that tree planting is not permanent. Trees can be harvested, and there is no way to be certain that a tree planted today for its carbon credits won't be chopped down tomorrow. After all, the carbon stops accruing at significant rates when the tree matures, so the credit and incentives will expire as well. Worse, there is no guarantee that this tree won't be lost in a forest fire, a problem which appears to be aggravated in many reasons by climate change itself. Or if a credit program creates an incentive to burn down the forest in order to collect more incentives later... this all looks very bad.
Finally, not all tree planting is of equal value. Trees will mature faster in the tropics than in temperate zones, and so the net present value of trees will differ by region, or by the tree species planted. Trees in arctic zones actually undermine the fight against global warming, because they tend to slough off snow in the winter to expose their dark green leaves to the sun, thereby reducing the reflectivity of the earth's surface. A tree planted in Siberia will actually hurt, not help.
All of these can be modeled, of course, but stacking risk upon risk drives up the effective cost of any tree planting program. Sites can be selected for geographic appropriateness, at a cost of overhead. Applicants for credits can be selected for their worthiness, at the cost of bureaucracy, and inevitably some public corruption as well. The cost of management should be assumed to be high, and a certain fraction of programs should be assumed to be total losses. But what fraction? If we were to pay the equivalent of $15/ton for tree planting, the actual cost to society will be higher because of this "deadweight". But how much higher? 2X? 5X? 10X?
Ultimately, our best solution for incentivizing behavior is capitalism, and capitalism seems woefully inadequate to the task. Carbon credits for tree planting will unquestionably reward organizations for past misconduct and potential future harm. Some sort of government intervention is needed, and this probably looks best like a Georgist land tax, which would levy an assessment on land based on its carbon sequestration potential The more potential for carbon capture on the land, the higher the assessment would be, so that land the trade off between land-left-fallow and land-for-production is considered and balanced. Such a land use tax could actually be economically efficient, but taxes are unpopular.
We do have policy levers that might unravel the mixed incentives for tree planting, and these should prove cheaper economically than industrial solutions such as DAC. They aren't cheaper politically, though. We don't care quite enough to change things yet, but maybe in the future we will.
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