Conclusion: indices do matter

Table 2.7 on page 48 shows a summary of the various values in the one pollutant, two-nation model and illustrates the wide range of answers that could be derived from use of indices that vary only in two parameters current versus cumulative and per nation versus per capita. It is useful to note that these values are much more sensitive to the choice of index, than to the choice of emissions scenario. Assume, for example, that the USA was able to reduce its per capita carbon emissions use by 1.5 per cent each year for the 39-year period, reaching a total emission rate more than 30 per cent below that of 1986 (and bringing the per capita emissions down by more than 40 per cent). How much difference would this rather heroic effort make in the final ratios? It would be very little for cumulative per capita indices. As shown in Table 2.5, the difference in ratios for cumulative atmospheric carbon would be less than 15 per cent (14 versus 16). Conversely, if the USA were to continue to expand its fossil fuel use by 0.5 per cent, as it has been doing recently, the ratio would only rise to 17 by 2025.

Table 2.6 Greenhouse gas indices according to time horizon for India and the USA

Note that including methane and using short time horizons tend to make the US contributions seem relatively smaller, i.e., the USA/lndia ratio goes down.

Based on coefficients in Table 2.1 and emissions in WRI (1990), Table 24.1. CO2 emissions from fossil fuels and cement production only. Numbers in brackets refer to direct warming only (no indirect effects). See Table 2.1.

Note that including methane and using short time horizons tend to make the US contributions seem relatively smaller, i.e., the USA/India ratio goes down.

Based on coefficients in Table 2.1 and emissions in WRI (1990), Table 24.1. CO₂ emissions from fossil fuels and cement production only. Numbers in brackets refer to aired warming only (no indirect effects). See Table 2.1.

Figure 2.13 Sensitivity of global warming potentials to time horizon

Table 2.7 Ratio of responsibilities: USA/India

Based on data in Table 2.5, no-emissions-growth scenario for the USA. CO₂ emissions from fossil fuels and cement production.

Any of these indices might be used to compare relative responsibilities for greenhouse gas emissions. In this book, natural debt indices are recommended, those three in the lower right quadrant, which are based on cumulative emissions per capita.

Some have argued that the need to choose a time horizon greatly diminishes the value of greenhouse-gas indices (Hammond et al. 1990). This argument misses the point, which is that when making choices between options with different patterns of consequences over time, there is no way not to choose a time horizon (Environment 1991). It may be done implicitly, but it is always involved in the choice made. Even if the choice is not to count future effects at all, a time horizon is implicit: an infinite discount rate. It is far better to bring the issue out into the open, make an explicit choice, explain the rationale, and allow it to become part of the review and negotiating processes.

There are many other possibilities to vary indices to reflect the real world of multiple criteria, greenhouse gases, and nations. We want to emphasize here, however, that the choice of index does indeed make a difference, sometimes a very large difference, and thus must be chosen with care to be relevant to the problem at hand and scientifically justifiable, as well as useful for policy.