Appendix A: Description of the IMAGE 2.4 model
Introduction
The IMAGE model is an integrated assessment modelling framework, consisting of a set of linked and
integrated models (Bouwman et al., 2006a). Together the framework describes important elements in
the long-term dynamics of global environmental change, such as air pollution, climate change, and
land-use change. Important subcomponents of the model are the global energy model TIMER, the land
use and land cover submodels of IMAGE, the detailed description of the carbon cycle and the climate
model MAGICC that is included as the climate model within IMAGE. Part of the framework is also
the FAIR model. The latter model is used to create climate policy-scenarios on the basis of available
information in the other parts of IMAGE. The FAIR model distributes the necessary global emission
reductions following a least-cost approach, using regional MAC curves derived from IMAGE for the
different emissions sources (den Elzen and Lucas, 2005; den Elzen et al., 2008). Socio-economic
processes are calculated in IMAGE for 24 or 26 regions (depending on the submodel); most
environmental parameters, however, are calculated at a 0.5 x 0.5 grid.
Energy and associated emissions
The energy submodel of IMAGE (called TIMER) looks into long-term trends in the energy system.
The model describes the demand and supply of 9 final energy carriers and 10 primary energy carriers
for 26 world regions. The model focuses on several dynamic relationships within the energy system,
such as inertia, learning-by-doing, depletion and trade among the different regions. The demand submodel of TIMER determines demand for fuels and electricity in five sectors (industry, transport,
residential, services and other) based on structural change, autonomous and price-induced change in
energy intensity ('energy conservation') and price-based fuel substitution. The demand for electricity is
fulfilled by fossil-fuel or bio-energy based thermal power, hydropower, nuclear power and solar or
wind. Fossil-fuel or bio-energy fired electricity production can all be combined with carbon-captureand-storage techniques. All technologies are chosen on the basis of relative costs (using multinomial
logit equations). Moreover, substitution only occurs for new investments (so that capital lifetime plays
an important role in the introduction of new technologies) The long-term costs of all energy forms
(fossil fuels, renewables) are described in terms of depletion and technological development.
Emissions are calculated by multiplying the data on energy consumption and production with fuel- and
region specific emission factors that are derived from the EDGAR database.
Land use and associated emissions
The land-cover and land-use submodels in IMAGE simulate the change in land use and land cover at
0.5 x 0.5 degrees (driven by demands for food, timber and biofuels, and changes in climate). For
demand for agricultural production the IMAGE model often uses scenarios from agro-economic
models. A crop module based on the FAO agro-ecological zones approach computes the spatially
explicit yields of the different crop groups and the grass, and the areas used for their production, as
determined by climate and soil quality. Where expansion of agricultural land is required, a rule-based
“suitability map” determines the grid cells selected (on the basis of the grid cell’s potential crop yield,
its proximity to other agricultural areas and to water bodies). The earth system also includes a natural
vegetation model to compute changes in vegetation in response to climate change. Land-use related
emissions occur from both land use (e.g. methane emissions from animals) and land use change
(deforestation).The former are calculated by explicit emission factors based on the EDGAR model.
The latter depend on the carbon fluxes between vegetation, carbon stocks in soils and the atmosphere
(thus, regrowth of forests lead to a temporal net carbon uptake).
Climate system
Emissions from land use and energy and industrial sources are used to calculate changes in
concentrations of greenhouse gases and air pollutants. These drive the MAGICC-6 model that is
included in IMAGE to calculate changes in global mean temperature (Meinshausen et al., 2011).
The MAGICC-6 model is calibrated on the basis of complex climate models. Next, changes in
temperature and precipitation are calculated at grid scale by means of pattern scaling (Schlesinger et
al., 2000). IMAGE accounts for feedbacks within the system, such as temperature, precipitation and
atmospheric CO2 feedbacks on the selection of crop types, and the migration of ecosystems. This
allows for calculating changes in crop and grass yields and, as a consequence, the location of different
types of agriculture, changes in net primary productivity and migration of natural ecosystems.