Reliability of the Forest
This is the third in a series of blogs on forests taking up the carbon dioxide that comes from the burning of fossil fuels. I started with a discussion of the pine beetle infestation that is currently killing many trees in the western U.S. and Canada. An important point is that disruptions to forests are often a major source of carbon dioxide to the atmosphere. Therefore, whether or not the pine beetle infestation is exacerbated by climate change, the impact of the infestation on climate change is definitive. I give a set of links to primary references below.
The forests and soils of the world store an enormous amount of carbon. In the spring the plants take up carbon dioxide, and in the fall carbon dioxide is released. In the figure shown, the uptake of carbon dioxide is slightly larger than the release of carbon dioxide (1 billion tons of carbon is taken up in the net). This stands in comparison with about 5 billion tons of carbon dioxide released by fossil fuel burning. The transfer of carbon dioxide from the atmosphere into trees and soils by biological processes is an important removal process. Our accounting and management of terrestrial ecosystems is an important part of our strategies to manage carbon dioxide in the atmosphere.
Carbon Budget in the Earth’s Climate System
Forests are an important part of the carbon management puzzle; however, forests cannot be relied on as the place where we place our excess carbon dioxide. There is a limited ability for forests to take up carbon dioxide, and forests are, often, in direct competition with other uses of land on which forests grow. In particular, deforestation, especially to turn land over to agriculture, is an important source of carbon dioxide to the atmosphere. Avoiding deforestation is important way to reduce emissions.
In the first blog of the series I introduced a paper by Dale et al. that discusses the importance of disruptions to forests. Climate Change and Forest Disturbances: Dale et al. 2001
The destruction of trees in western North America by the pine beetle is such a disruption. In the limited geographical region studied by Kurz et al., Nature, 2008
, they found the forest changed from taking up 0.59 Megatons of C per year to releasing 17.6 Megaton of C per year. The emissions associated with the pine beetle infestation are comparable, in Canada, to both the forest fire emissions and the emissions from the transportation sector. This demonstrates the slow take up of carbon dioxide by a growing forest in contrast to the rapid release of carbon dioxide if the forest dies quickly. This poses a great challenge to carbon management – many years of carbon accumulation can be released very shortly. Therefore, for the long term, the forests are not a robust place to store excess carbon dioxide. Again, this contributes to my conclusion that we will be compelled to develop technology to remove carbon dioxide from the atmosphere and store it someplace more permanent.
Over the past few years we have experienced other disruptions to the forest which provide examples of the impact of the disruptions. Chambers et al. in Science in 2007
calculated the amount of carbon dioxide released by forests killed and damaged by hurricane Katrina. They calculated a total biomass loss of 105 Teragrams C from the damaged forests, which is comparable to the net annual uptake of carbon dioxide by forests in the U.S. In this case, we see an entire year’s worth of carbon take up lost to a single disruptive event. Another type of disruption to forests is drought. In a paper published by Peter’s et al. 2007
, the uptake by forests in the North America was reduced by about 50% during the extensive 2002 drought.
The disruptions described above all lead to increasing the amount of carbon dioxide in the atmosphere. It is true that climate change through the increase of carbon dioxide and the warming of higher latitudes also accelerates growth and, therefore, uptake of carbon dioxide. Nemani et al.
calculated that the take up of carbon dioxide has increased by about 6 % between 1982 and 1999, with interestingly, the largest increase in tropical ecosystems. (Some have argued that the increases in, say, Canada and Siberia, should provide a natural buffer to warming, and hence, are more important to managing carbon dioxide.) Kurz et al., Nature, 2008
compare the loss due to pine beetles with this increased use of carbon dioxide by the trees, and again, find that the loss associated with the disruption to the forest is comparable to increased uptake.
From the perspective of climate change it is important to manage the carbon that is stored in trees. We need to keep the carbon that is in the trees, in the trees. However, to imagine that we truly control the amount of carbon dioxide accumulating in the atmosphere with the uptake by trees is naïve. The trees are not, in the long term, a reliable place to store this carbon dioxide. A single disruptive event to a large forest counters many years of carbon uptake. Additionally, there are ecosystem impacts. If it is a goal to limit warming and stabilize the climate, then we need to reduce emissions. We also, however, need to figure out how to remove the carbon dioxide that has already been emitted.
Primary References:Kurz et al., Nature, 2008 Chambers et al. in Science in 2007 Logan and Powell, American Entomologist, 2001 Nemani et al. Peter’s et al. 2007 Climate Change and Forest Disturbances: Dale et al. 2001 from class readings Effects of Climate Change on Range of Pine Beetles: Carrol et al. 2003 Pine Beetle Symposium 2003
Previous Pine Beetle Blogs: Beetles and the Climate Climate and the Beetles
Previous Blogs on Phenology and Ranges of Trees Series of Blogs in 2008 of Spring Coming Earlier Trees Moving North