Separating human and natural changes

Figure 8.3 illustrates a simulation of the different sources of change in carbon storage from 1950 through 2000 for the USA. Felzer et al. (2004) estimated that the USA was a net carbon sink but there were multiple factors, some offsetting and others interacting, that explain the net effect. The factors involve feedbacks from natural systems, natural variability and those that could be attributed to direct management. For example, land-use change and fertilization of crops (with nitrogen) are related directly to management decisions. Climate, shown to have a varying effect, is both naturally variable and may be changing because of human influence - sorting how much is natural variability and how much is due to human influence is a complex issue and not completely resolvable, particularly at smaller scales. Here the scale is near continental, but to create incentives for carbon management the scale needs to be at the level of parcels owned by specific individuals or companies. Increased tropospheric ozone damage is mainly due to increased precursor emissions from anthropogenic sources but these emissions are from energy use, over which the forest or farm manager has no direct control. Note that Felzer et al. (2004) also show an interaction effect between nitrogen fertilization and ozone damage: there is increased damage from ozone when there is nitrogen fertilization, that cannot be attributed to management alone or to the earth system feedback alone. In general, interactive effects are likely to be more important at smaller scales. The type of vegetation grown will interact with climate and CO2 concentrations. When there are fundamental interactions of this type it is not possible to clearly attribute carbon changes to one or the other factor. Thus, attempts to base policy on whether the change in carbon is due to direct management, natural variability or some indirect anthropogenic factors are futile.

We clearly have a complex policy environment and a complex natural system with multiple feedbacks and interactions

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Fig. 8.3. Simulated effects on carbon storage in the USA attributable to different sources. (From Felzer et al., 2004.)

between management and nature. Is there a way out of this complexity? We turn now to imagining how a very simple emissions trading system could work, if we could escape the inertia of recent negotiations and policy thinking and add some more information on the rough magnitude of sinks potential.

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