Approach and Method

The basic concepts are sustainability as a characteristic of dynamic systems that maintain themselves over time (not a fixed endpoint that can be defined) and environmental sustainability as a long-term maintenance of valued environmental resources in an evolving human context. The overall picture created by the index does not define sus-tainability but instead provides a gauge of a country's present environmental quality and capacity to maintain or improve conditions in the future. The ESI is still under development in both methodological improvements and data collection methods.

ESI must cope with several challenges commonly encountered in the computation of composite indices: variable selection, missing data treatment, aggregation, and weighting methods.

At the top level of aggregation, the ESI loosely uses the driving force, pressure, state, impact, response (DPSIR) framework. It centers on the state of environmental systems, both natural and managed. Then it measures pressures on those systems, including natural resource depletion and pollution rates. The ESI also includes impacts as human vulnerability to environmental change and responses as a society's capacity to cope with environmental stresses and each country's contribution to global stewardship. Thus, the broad categories "environmental systems," "reducing environmental stresses," "reducing human vulnerability," "social and institutional capacity," and "global stewardship" form the core components of the ESI. Below this level of aggregation each of these five components encompasses three to six indicators of environmental sustain-ability. These twenty-one indicators, in total, are considered the building blocks of environmental sustainability. Each indicator builds on two to twelve data sets, for a total of seventy-six underlying variables (Figure 23.1, Table 23.1).

The issues reflected in the indicators and the underlying variables were chosen through an extensive review of the environmental literature, assessment and analysis of available data, and broad-based consultation with policymakers, scientists, and indicator experts. The seventy-six variables cover issues that are local in scope as well as those

The components summarize the + indicator values in 5 thematic categories

The components summarize the + indicator values in 5 thematic categories

5 components

5 components

21 indicators

The ESI is the equally > weighted average of these 21 indicators

The ESI is the equally > weighted average of these 21 indicators

76 variables

Figure 23.1. ESI architecture: aggregation scheme (Esty et al. 2005, modified).

Table 23.1. ESI components and indicators.

5 Components

21 Indicators

Environmental systems

Air quality

Biodiversity

Land

Water quality

Water quantity

Reducing stresses

Reducing air pollution

Reducing ecosystem stresses

Reducing population growth

Reducing waste and consumption pressures

Reducing water stress

Natural resource management

Reducing human vulnerability

Environmental health

Basic human sustenance

Reducing environment-related natural disaster

vulnerability

Social and institutional capacity

Environmental governance

Eco-efficiency

Private sector responsiveness

Science and technology

Global stewardship

Participation in international collaborative efforts

Greenhouse gas emissions

Reducing transboundary environmental pressures

that are global in scale. Although countries at different levels of development and with diverse national priorities may choose to focus on different aspects of environmental sustainability, all of the issues included in the ESI are of some relevance to all countries. Despite the great diversity of national priorities and circumstances, a uniform weighting of the twenty-one indicators was chosen in order to keep the aggregation easy to understand.

At the lowest level of aggregation, each indicator is itself an equally weighted sum of the two to twelve underlying variables. The variables are standardized by the means of z scores. The z scores for each variable are constructed by subtracting the mean from the observation and dividing the result by the standard deviation of the variable. They preserve the relative position of each country for each variable while providing a neutral way to aggregate the variable into indicators. The ESI score is then calculated as an equally weighted average of the twenty-one indicator scores.

It is obvious that because of the structure of the ESI (unevenly distributed seventy-six variables into twenty-one indicators and five elements), the individual variable weights vary in their contribution to the overall ESI score in proportion to the number of variables in a given indicator. This hidden weighting implies that the relative contribution of variables to the total ESI score ranges from 2 percent for an indicator with only two variables (e.g., greenhouse gas emissions) to 0.3 percent for an indicator with twelve variables (e.g., environmental governance). By giving each variable within an indicator the same weight and weighting each of the twenty-one indicators equally, ESI provides an imperfect but clear starting point for analysis. An interactive version of the ESI, which will allow the user to adjust the indicator or component weights and calculate a new score, is planned in order to improve the policy utility of the ESI.

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