Enlarging forests is one of the big topics in climate change policy. The literature on carbon uptake by trees and soils is immense, and one of the few functioning global agreements is REDD, Reducing Emissions from Deforestation and Forest Degradation, which has funneled nearly $200 million into programs to reduce greenhouse gas concentrations by bulking up forests. On a smaller scale, many of the carbon offsets people purchase when they do things like air travel go to pay for tree planting. The idea is that forests store carbon, and more carbon in the forest means less in the atmosphere.
Before going under the hood to look at the actual carbon flows involved, I should take a moment to mention a couple of important problems with REDD and similar programs. First, whatever their impacts on atmospheric carbon, forests happen to be places where lots of people live. To manage forests strictly for their carbon fixing potential frequently means disrupting traditional ways of life or even turning forest dwellers into carbon policy refugees. Read, for instance, this recent report by Oxfam of the expulsion of thousands of forest inhabitants in Uganda.
Second, forests are more than carbon repositories. They are habitats for plant and animal species, sources for food and medicine, bulwarks against soil loss, and intricate water management systems. Alas, single-minded policies to promote tree growth often result in turning forests into plantations of fast-growing trees. In many regions, for instance, reforestation projects favor eucalyptus trees, which, although native to Australia, grow rapidly in a variety of climates. And so one sees a profusion of eucalyptus monocultures which allow their growers to sell lots of carbon credits, but often have negative effects on the other functions that forests should perform.
Still, what about all that carbon that forest projects claim to fix? Aren’t there significant climate benefits to weigh in the balance?
Answering questions like this is where we begin to see big payoffs from moving beyond immediate carbon flows and thinking about carbon cyclically. In a way, this misconception is the same as the pollution one I described previously, but it involves carbon leaving the atmosphere rather than entering it.
Consider what happens when you purchase a carbon offset and someone plants a tree. As advertised, the tree draws carbon from the atmosphere as it grows. At some point, however, the tree stops growing: either it is harvested, or it burns in a forest fire, or it dies from some other cause. Then most of its carbon makes its way back to the atmosphere. (Some may settle in the soil, but there is carbon exchange between soil, water and atmosphere too.)
One immediate effect of this series of events is that some carbon temporarily makes its home in the tree rather than in the atmosphere. This could be beneficial if we are trying to buy time for other policies to take effect, but it is essentially building up a carbon debt that will have to be repaid later on, ready or not. Whatever you think of this transaction, it is not a simple reduction of greenhouse gases, as your offset coupon may claim.
More to the point, the best way to think about carbon storage in forests is to compare steady states before and after the tree plantings. Carbon is fluxing from atmosphere to forest and from forest to atmosphere, but what matters is the average amount of carbon fixed in the forest over time. If a forest used to store X tons of carbon but now stores 2X in perpetuity, it does indeed represent a reduction in greenhouse gases. This is what I referred to as tweaking the carbon cycle in an earlier post.
But how do we find out how much today’s actions, like planting trees, alters the steady state carbon storage of a forest? Good question. It’s not easy at all. First, we have to know quite a bit about forest ecosystem dynamics. For instance, fires, storms and other disturbances, which burn up or knock down lots of trees, are part of the equation. Another consideration is whether particular forest interventions, like the planting of eucalyptus monocultures, are sustainable. (Degradation of mycorrhizae, on which trees depend for nutrient uptake, may result from changes in forest composition.) An enormous uncertainty, however, is climate change itself. The various habitats that make up this planet are not going to be the same in fifty years as they are today. Many regions will lose forest cover no matter what we try to do about it. Planting more trees in such locations is simply useless.
And finally, of course, there is us. To guarantee that a forest will arrive at a new, higher steady state of carbon storage means to guarantee that people will not cut or burn its trees for generations to come. And how do we guarantee this?
In short, projects that claim to offset the use of fossil fuels by growing more forests are promising more than they can deliver. This is not an argument “against forests” or their protection, but it suggests that there are no loopholes that enable us to live within our carbon budgets but allow us to continue burning fossil fuels.