Most decoupling indicators are country specific and do not usually address the cross-border flow of environmental externalities. However, material flow accounts and the Ecological Footprint methods do address this issue explicitly and could be used to construct decoupling indicators capable of tracking such trends. Often quoted in this respect are the greenhouse gas (GHG) emissions associated with a country's imports and exports (OECD 2003). A different type of example can be found in the fishery sector, when fisheries are not confined within national boundaries,4 necessitating decoupling indicators developed at an appropriate level of aggregation.
The most direct manner of displaying decoupling between an environmental pressure and an economic driving force is to plot two indexed (e.g., 1980 = 100) time series on the same graph, as in Figure 13.1. From such a graph, it is immediately clear whether the economic driving force is growing or shrinking, whether decoupling (absolute or relative) is occurring, when it started, and whether it continues. This method is used by the OECD in displaying overall trends for all OECD countries and for the three main OECD regions (i.e., Europe, North America, and Pacific). Some of these qualities are lost if decoupling is presented as a single line (i.e., a time series of the ratio of environmental pressure to driving force), although the idea of improvement in efficiency or intensity is better communicated this way.
However, neither of these presentations lends itself well to numerical displays of decoupling trends for a large number of countries. To compare decoupling between countries, the ratio of the value of the decoupling indicator at the end and the start of a given time period may be defined as follows:
^ ' end or period ^ ' start of period where EP = environmental pressure and DF = driving force.
If the ratio is less than 1, decoupling has occurred during the period, although it does not indicate whether decoupling was absolute of relative. To avoid displaying (on a bar graph) small values when decoupling is significant, a decoupling factor is defined as follows:
Decoupling factor = 1 — Decoupling ratio.
The decoupling factor is zero or negative in the absence of decoupling and has a maximum value of 1 when environmental pressure reaches zero.5 Decoupling factors for CO2 in OECD countries for the interval 1990 to 1999 are displayed in Figure 13.2.
When several pollutants have similar effects, aggregation can reduce the information load. For example, to construct a single indicator accounting for the overall effect of all six GHGs on the climate system, conversion factors (based on the relative radiative force of the individual gases) are used to construct the decoupling indicator "GHG emissions from all sources per unit of GDP." Similarly straightforward procedures have been used elsewhere to construct indicators for acidification, toxic contamination, ozone depletion, or low-level ozone formation. Such aggregated indicators can often be linked to an appropriate driving force to obtain aggregated decoupling indicators.
In other cases, more complex aggregation procedures may be needed. One approach used to produce a weighted index of local air pollution is based on the health effects of
exposure to specific levels of individual pollutants. A variety of other aggregated environmental indicators have been proposed in recent years, some of which are used for communication (e.g., various urban air pollution indices) or policymaking (e.g., the indicators associated with the Dutch National Environmental Policy Plan). Other indicators push the aggregation even further, using a common unit (such as tons) to aggregate measures across a variety of environmental and natural resource issues. Nevertheless, aggregated indicators are still far from being universally accepted.
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