Carbon Offsets Up in Smoke

Abstract

The carbon cycles that you probably learned about in school look quite complicated, don’t they? Lots of boxes and arrows flying in different directions. But, in reality, the carbon cycle, as it occurs out in the real world, is actually more complex than one would infer from looking at the flow chart. For example, the ocean has played an important role in slowing down climate change by removing carbon dioxide from the atmosphere, about 1⁄4 of excess carbon dioxide during the past few centuries. In the past, some people have assumed that this buffering capacity would prevent serious effects of anthropogenic carbon. However, as the ocean has absorbed carbon, it has become more acidic. This increased acidity may have detrimental effects on coral reefs and on organisms like bivalves that build shells based on calcium carbonate. The ability of ocean waters to absorb excess greenhouse gasses also decreases with acidity, meaning that this buffer is likely to be less important in buffering carbon dioxide as atmospheric carbon dioxide continues to increase. Other simplistic assumptions made about carbon dynamics have also been shown to be, well, too simplistic. Greenhouse and chamber studies of plant growth under increasing carbon dioxide have been used to assume that photosynthesis and primary productivity would increase, acting to buffer carbon dioxide concentrations. However, in real ecosystems, most plants are not limited by carbon concentrations in the air. Rather, water availability often limits plant growth. Global changes wrought by increasing carbon dioxide are increasing the length and severity of droughts in many parts of the world, with devastating effects to humans and other biota. Most ecologists now realize that increasing carbon will not be buffered by faster plant growth. It has always been politically difficult to directly limit industrial carbon emissions, even when the technology exists. Shifting to a carrot (rather than a stick) approach has led to the trading of carbon credits. Carbon credits allow companies to offset carbon dioxide emissions by fostering forest growth through conservation and replanting. You can also offset your carbon dioxide produced by travel by paying for trees to be planted. These new forests will presumably capture atmospheric carbon in accreting biomass. However, this scheme is vulnerable to loss of biomass to ecological disturbances. (It also has the potential for encouraging mismanagement of areas that are naturally dominated by shrubs and grasses.) However, this idea has hit a few potholes out in real landscapes. A recent study by the nonprofit CarbonPlan found that wildfires in California and elsewhere in the American West have depleted about 95% of the carbon credits in forestry projects. Such projects assume that trees will continue growing for 100 years. Instead, fires have affected forests far sooner. Future wildfires are likely to completely deplete current carbon credits, according to the study. Indeed, wildfires are burning forests at higher frequencies than in the past. Climate change is involved in this shift, as longer droughts increase the likelihood of fire. Thus, climate change is likely one of the main drivers of increased fire frequencies and intensities. This feedback may mean that carbon credits, while economically attractive, may be ecologically unrealistic.