The classifications adopted in the model provide the basis for the collection of data and the organization of the variables in the model. They provide the operational significance for terms such as 'energy', 'environmental emissions' or 'economic activity' and are chosen so as to make the most of the data available on energy use and environmental emissions. The following terms and classifications are included in the model; the letter codes shown, e.g. J, are those adopted in the new version of the model (MDM88) containing the energy sub-model. The environmental emphasis in this new version is on air emissions.
Economic activity indicators for environmental effects—Fuel users (FU)
It is convenient to define a much shorter list of economic activities than is usual in a SAM, concentrating on those which are of more direct relevance to environmental protection. The following categories of fuel users are chosen to account for the most serious air pollutants, with extra detail provided by a disaggregation of each type of fuel use into the principal fuels being consumed.
1 Power generation
2 Oil refineries and other own use of energy
3 Iron and steel
4 Non-metallic mineral products (e.g. bricks, glass)
6 Other industry
7 Rail transport
8 Road transport
9 Water transport 10 Air transport
11 Domestic use
12 Other final demand, including agriculture, commercial, government and construction
Energy types (J)
Another important classification is that of energy types or fuels. Here a balance must be struck between the full detail available, e.g. including coke and creosote as separate categories, and the requirement that the model is primarily concerned with long-term projections, when items such as coke consumed may be negligible. Another criterion for the classification is the emission characteristics of different fuels. The categories adopted for energy carriers are as follows.
1 Coal and coke (solid fuels)
2 Motor spirit
4 Gas oil
5 Fuel oil
6 Other refined oils
7 Gas (natural gas, coke oven gas and town gas)
8 Electricity (all secondary use, plus net trade and pumped hydro)
9 Nuclear electricity (primary supply)
10 Other fuel, including renewable (primary supply)
Emissions or pollutants (EM)
These are generated by many types of economic activity, in particular chemical-intensive agriculture, the burning of fossil fuels for electricity or for transport and the processing of metals and chemicals. The time-series emissions data in official statistics (DEPWS) are mainly for flows into the atmosphere. Data on stocks of pollutants are often not available; however, although these are important for land and water pollutants, they are less important for air pollutants such as NOX and CO where flows are more important. Although the environmental consequences of CO2 emissions are stock related (i.e. are related to the ambient concentrations of the gas in the atmosphere) the absence of spatial or temporal differentiation in the effects simplifies the stock-flow relationship so that using flow data is not such a problem. (In contrast, to take one example, the role of volatile organic compounds in generating low-level ozone depends on the place and time of emissions.) Since the model is initially concerned with the economic consequences of achieving given environmental targets, in particular a given level of CO2 abatement, and not the targets themselves, it is sufficient to include flows of emissions to the atmosphere, especially those associated with the burning of fossil fuels. The following categories cover the main emissions for which data are readily available.
1 Carbon dioxide CO2
2 Sulphur dioxide SO2
3 Nitrous oxides NOX
4 Carbon monoxide CO
6 Black smoke
7 Volatile organic compounds VOC
8 Nuclear—emissions to air
9 Lead—emissions to air 10 Chlorofluorocarbon CFC
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