Gross Domestic Product (GDP) is a key indicator to measure macro-economic activity in the IMAGE 2.2 model. It is also used as a welfare indicator. GDP formally measures expenditures on private household consumption, investment and government purchases of goods and services, along with the balance of exports and imports of merchandise and non-factor services. Other economic growth indicators, such as private consumptions or service value added, are strongly related to GDP.

In IMAGE 2.2, growth of GDP (or related economic indicators) plays an important role in both the formulation for energy demand and food demand. The historic data for GDP used in IMAGE are taken from World Bank's World Development Indicators (World Bank, 1998). The future growth trajectories are determined using the WorldScan macro-economic model. GDP is expressed per capita; the total GDP for each world region is obtained by multiplying the GDP per capita with the total population.

	Population
	unit: million persons dimension: region

The size and structure of a population is determined by three fundamental demographic processes: birth, death and migration. The number of births is determined by the fertility rates, the number of deaths by the age-specific mortality rates. Migration is taken into account exogenously by the total number of net migrants per year (i.e. immigration flow minus emigration). These aspects are included in a standard cohort-component model of 101 age groups and the two sexes.

	Secondary energy use
	unit: PJ/yr (Petajoule per year) dimension: region, secondary energy carrier

Secondary energy use shows the total demand for secondary energy in each region. Secondary energy use is equal to the amount of energy consumed by the end-user and does not include the energy lost in the production, processing and delivery of energy carriers. Neither does it include the use of feedstocks and non-energy use.

In the TIMER model the demand for secondary energy is derived from the demand for energy services multiplied by time-dependent conversion efficiencies. Unless potential investments are constrained or there are delays in actual investments, the demand for final energy is fully satisfied and thus equals its use. A description of the energy cariers is given under the information for the total box of secondary energy indicators.

	Total primary energy use
	unit: PJ/yr (Gigajoule per year) dimension: region, primary energy carrier

Total primary energy use shows the use of all primary energy carriers for each region. Primary energy use is defined as the sum of all energy consumed, including losses at various stages of energy upgrading and processing. It also includes non-energy use and feedstocks.

The following categories are distinguished: coal, oil, gaseous fuel, modern biofuel (in the form of bio-liquid fuels BLF or bio-gaseous fuels BGF), traditional fuel (wood, straw, dung, charcoal, etc.), non-fossil electricity generation options (nuclear, wind, solar, etc.) and hydropower. The definition of the primary energy carriers corresponds to those used by the International Energy Agency (IEA). The distinction between the two categories of liquid fuels (heavy and light) is not made for primary energy use - and all crude oil use has been indicated as 'heavy oil' (the distinction is only relevant for secondary fuels). The categories bio-liquid fuels and bio-gaseous fuels are aggregated into the category modern biofuels.

In TIMER, use of primary energy carriers is calculated from the secondary energy use and includes the energy losses in the system in the chain from primary fuel production to secondary fuel use. The most important losses are associated with the generation of electricity and are calculated in the electric power generation submodel. The conversion efficiency from fuel-based thermal power plants is based on exogenous time, region and fuel dependent data and assumptions. The conversion efficiencies for other electricity generation options (hydropower, nuclear, wind, solar, etc.) are set at unity. For fossil fuel production the conversion losses are among other due to refining, transformation and interregional transport.

	Cumulative production
	unit: 1000 PJ (thousand Petajoule) dimension: region, primary energy carrier

Cumulative production shows for all regions the cumulative energy production for fossil fuel resources, starting in 1971. Cumulative production is one of the cost determinants for two reasons. First, the capital-output ratio is assumed to rise non-linearly with the ratio of cumulative production and initial resource. Secondly, innovations in fossil fuel exploration and exploitation are driven by production through the progress ratio which indicates how much the capital-output ration will decline per doubling of cumulative production. Cumulative production of the fossil fuels also gives an indication of the cumulative carbon emissions.