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Does the new Duke study prove that tree-based offsets are no good?


I’ll spare you the suspense, and start with the answer: no, it does not.

Now the full story. A few weeks ago, I pointed out some of the well-known problems with carbon offsets from tree-planting projects. These include issues of permanence, timing, and measurability.

Coincidentally, only a few days later some researchers at the Nicholas School of the Environment and Earth Sciences at Duke University released some findings from a highly-regarded 10-year study of forest growth. According to accounts in the popular press, the main conclusion is that we can’t count on trees to soak up the excess carbon dioxide we’re putting in the atmosphere. The study got a smattering of attention from various green blogs, which generally took the result to be another nail in the coffin of tree-planting projects.

The problem with this reading of the Duke results is that the study doesn’t have anything to do with carbon offsets. If I’m reading it correctly (always a big if), the study is entirely concerned with whether forests will act as a natural corrective to global warming, even in the absence of any human-directed efforts to reforest the planet.

To understand why trees might act as a natural corrective, you have to consider the concept of feedback loops. Global warming is notorious for its many positive feedback loops, which are mechanisms by which climate change becomes self-reinforcing. For example, ice is a highly reflective substance, so glaciers help to cool the planet by reflecting sunlight back into space. A hotter globe causes ice to melt. Less ice means more sunlight hitting the earth. More sunlight means more warming, meaning less ice…and so on. There are many such positive feedback loops in global warming.

There are also some hypothesized negative feedback loops, mechanisms by which global warming could act as a brake on itself. One conjectured loop is the notion that a more carbon-rich atmosphere might stimulate plant growth. The plants would absorb some of the excess carbon, thus acting as a natural buffer against climate change. Such negative feedback loops are particularly popular among global warming denialists, who like to claim that climate change will fix itself if we just calm down and ignore it.

The Duke study casts doubt on this fantasy scenario. The problem, researchers found, is that plants can only make use of excess carbon in the atmosphere if they also have access to lots of water and nutrients. But global warming tends to cause drought, so it now seems likely that the excess carbon will be unavailable to plants.

This is a disappointing result, but it has little to do with tree-planting projects or avoided deforestation projects. The Duke study doesn’t imply that trees can’t sequester carbon. It just suggests that natural tree growth won’t keep pace with our emissions.

Of course, tree-planting projects still suffer from all of the problems I mentioned previously. The overall picture hasn’t changed one way or the other.

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