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Implications

With this brief background of the constraints imposed on indices by physical reality, let us examine the implications concerning relative national responsibilities for choosing different kinds of indices. The entire landscape of nations and gases - historical, present, and future - is complex. Before addressing the total picture in the next chapter, therefore, we can take a cue from atmospheric scientists, who often make use of greatly simplified models of the world (only one-dimensional or two-dimensional, for example), in their quest to understand and predict the behaviour of extremely complex systems.

The one-nation, one-pollutant model

The most obvious measure of a nation's responsibility for greenhouse gas emissions is simply its present emissions. This measure has the clear benefit of being relatively easily determined and being the most responsive to control efforts. For these and other reasons, it has many advantages as an index.

A problem with this measure, however, is that it does not completely reflect physical reality. The extra greenhouse warming that occurs at any time is actually due to the cumulative amount of greenhouse gases remaining at that time, rather than to the emissions that year. That year's emissions are important only to the extent that they add to the accumulation.

The amount of greenhouse gases remaining in the atmosphere at any one year due to a nation's emissions has been termed the 'natural debt' (Smith 1989b, 1991). A national debt is built by borrowing financial resources from the future, but the natural debt is built by borrowing assimilative capacity of the atmosphere from the future, through the release of greenhouse gases faster than they can be naturally removed. Just as with the national debt, borrowing on the natural debt has allowed nations to build up their infrastructure and economic wealth faster than would have occurred otherwise. Like the national debt, however, if the natural debt becomes too large, problems are created. Just as with a financial debt, therefore, it does not seem unfair to ask nations to pay off the natural debt in the same proportion as it was borrowed.

Figure 2.10 Gross and depleted natural debts of US CO₂ from fossil fuels

Figure 2.10 shows the relationship between current CO₂ emissions from fossil fuels and natural debt for the United States. (It leaves out other greenhouse gases and other sources of CO₂.) Current (1990) emissions are roughly 1.3 Gt (billion tonnes carbon as CO₂) per year. Cumulative emissions since 1950, however, are approximately 41 Gt. Of this, approximately 70 per cent still remains in the atmosphere, making the US natural debt to be 29 Gt, or some 116 tonnes per living US resident. This natural debt might well be considered to be a reasonable measure of US responsibility (see Chapters 3 and 4). Tables 2.3 and 2.4 list the 1950-86 natural debts for the 62 largest nations in the world (with populations over ten million).

Table 2.3 Current and historical carbon emissions and population data by country

Data for the 62 most populous nations (approximately 10 million or larger in 1986). The lost column shows the natural debts, as defined in the text. The emissions data reflect carbon dioxide from fossil fuel combustion and cement manufacture (data mainly from Marland et al.1988).

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