Other useful metrics

Many other environmental metrics have been defined and some are shown in Table 6. They include the hydrogen/carbon ratio in fuel used (e.g. natural gas, CH₄, generates more energy per kilogram of carbon emitted to the atmosphere than fuel oil), intensity of use of material (e.g. a ton of steel in an automobile is less efficient in people transport that one ton of steel in a train), and others.

Table 6

Some useful indices of environmental performance (after Wernick and Ausubel, 1995)

Name	Dimensions	Higher values indicate
Hydrogen to carbon ratio	Mass H / mass C	lower carbon emissions per unit of energy generated
Intensity of material use	$ of GDP/kg of material	economic usefulness of material
Intensity of carbon use	$of GDP/kg of carbon emitted	decarbonization of process/product
Fertilizer productivity	Tons produce/ton of fertilizers used	lower emissions of fertilizers to envir’t
Recycled material use	Mass of recycled material/ (recycled plus virgin material)	Conservation of primary materials
Restoration of forest resources	Mass of forest growth/forest products harvested	Global carbon balance, less ecosystem disruption
Dissipation index	1- mass of materials dissipated into environment during production / mass materials produced	Efficiency of use of materials; also avoidance of contamination

Ecological footprints

Ecological footprints is one indicator which deals with land use, something often neglected by other indicators. (An alternative name and similar approach is described in Hertwich et al 1996 as a SPI or 'sustainability indicator'.) It measures the total land area required to support the production of a service, product, or lifestyle. This is a useful measure, as it can be used to estimate the sustainability of given activities for a given land area. 'Ecological footprints' have been calculated for typical citizens of different countries around the world (see Wackernagel 1997). It is simple then to determine how close a society is to sustainable consumption levels by multiplying the total population by the available land. The problem with the 'ecological footprint' is in the data available to calculate the footprints, and its inability to take account of variables not related to land area.

Multi-point models

Cumulative energy demand, Global Warming, MIPS and Ecological footprints are all attempts to find a tangible measure as a representation of environmental impact. However, the demand for LCA's to compare across different kinds of impacts and environmental media has required the development of models with less tangible units, or 'eco-scores'.

There are two approaches to LCA impact models (initially described in Braunschweig et al 1996) which are:

• the bottom-up approach taken in international standards, in which the impact model simplifies the inventory, and the indicators which are relevant to the study are chosen as part of the LCA. (The EcoPoints model mentioned below falls into this category.)

• the top-down approach which begins with a definition of the end result of the assessment by asking: 'what do we mean by "environment" when we are developing environmental indicators?' (This approach is used by the EPS and Eco-indicator models.)

Eco-indicator 95

This method was developed by PRé Consultants for the Dutch government in conjunction with a large range of manufacturing companies and research agencies (Goedkoop et al 1995). Impacts are grouped in the categories of human health, ecosystem health and resources. The impacts are calculated based on the best available scientific knowledge, and these three areas are then weighted against each other based on the distance to target principle. This is one of the most widely used indicators, especiallyin the design area with Eco-indicator 95 being incorporated into a range of software products and published in a simple 100 common materials and processes guide for designers. This is a comprehensive indicator; however, like many impact models, it deals poorly with land use and biodiversity impacts. It also weighs acidification highly, which makes it a difficult model to use in the Australian situation where acidification is not a major impact.

Eco-indicator 99

Eco-indicator 99 (EI99) is a complete rebuild of the Eco-indicator 95 method with substantial improvements in the modelling of damage occuring from emissions. This is done by taking greater account of the probable fate of the emission (where the emission is likely ultimately to have an impact, and the sensitivity of that media). It is a top-down model which begins by identifying three environmental damage end-points: human health, ecosystem quality and resources ( in the same way as Eco-indicator 95). The model then analyses the damage paths from the inventory, through to the predicted damage. This is done by using fate analysis to detemine the sensitivity of the receiving environment (in Europe) to predict the damage. The modelling approach for EI99 and fate analysis is shown in Figure 3.