An important study from in last week’s Nature concluded that the mountain pine beetle infestation that have devastated the forest industry out here has also converted western Canadian forest from a carbon sink to a carbon source. In the simplest possible terms, the beetles kill the trees, decreasing carbon uptake (photosynthesis) and increase carbon loss (decomposition). This is not a new suggestion. The Kurtz et al. paper is, however, the most complete accounting to date.
A lot of people are labelling this a ‘positive feedback’ from climate change (e.g. like how warming melts sea ice, which reduces reflection of solar radiation, which accelerates the warming). The logic is that warmer weather promotes the beetle outbreak, which releases carbon from the forest, which further warms the climate.
The assertion appears correct. But be wary of the hyperbole. Not all positive feedbacks are made equal. There are only so many pines, the beetles can only do so much damage. At some point, they run out of trees to eat, and the infestation recedes. As the authors of the Nature study have said, the betters have almost eaten themselves out of house and home. And re-forestation initiatives could return the forests to being a carbon sink.
The type of positive feedback that should be included in carbon cycle assessments? Yes. Runaway positive feedback that will send the atmosphere to 1500 ppm? Probably not.
There is a sadly ironic policy twist to the beetle infestation. For one, the forests now might pump out more carbon than the entire BC economy, throwing a wrench into regional GHG reduction agreements. The Canadian government lobbied hard to include forest carbon sinks under the Kyoto process, under the expectation that our boreal forest would provide a big carbon credit and reduce the need to address emissions themselves. Safe to say, that stance has softened in recent years.