Parts of Agricultural economy/Description
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Model description of Agricultural economy
The MAGNET model (Woltjer et al., 2014) is based on the standard GTAP model (Hertel, 1997), which is a multi-regional, static, applied computable general equilibrium (CGE) model based on neoclassical microeconomic theory. Although the model covers the entire economy, there is a special focus on agricultural sectors. It is a further development of GTAP regarding land use, household consumption, livestock, food, feed and energy crop production, and emission reduction from deforestation or afforestation.
Demand and supply
Household demand for agricultural products is calculated based on changes in income, income elasticities, preference shift, price elasticities, cross-price elasticities, and the commodity prices arising from changes in the supply side. Demand and supply are balanced via prices to reach equilibrium. Income elasticities for agricultural commodities are consistent with FAO estimates (Britz, 2003), and dynamically depend on purchasing power parity (PPP) corrected GDP per capita. The supply of all commodities is modelled by an input–output structure that explicitly links the production of goods and services for final consumption via different processing stages back to primary products (crops and livestock products) and resources. At each production level, input of labour, capital, and intermediate input or resources (e.g., land) can be substituted for one another. For example, labour, capital and land are input factors in crop production, and substitution of these production factors is driven by changes in their relative prices. If the price of one input factor increases, it is substituted by other factors, following the price elasticity of substitution.
Regional aggregation and trade
MAGNET is flexible in its regional aggregation (140 regions). In linking with IMAGE, MAGNET distinguishes 28 individual large world regions, closely matching the regions in IMAGE (Figure IMAGE regions). Slightly more detail is provided the European regions in order to properly model the EU single market. Similar to most other CGE models, MAGNET assumes that products traded internationally are differentiated according to country of origin. Thus, domestic and foreign products are not identical, but are imperfect substitutes (Armington assumption; Armington, 1969).
In addition to the standard GTAP model, MAGNET includes a dynamic land-supply function (Van Meijl et al., 2006) that accounts for the availability and suitability of land for agricultural use, based on information from IMAGE (see below). A nested land-use structure accounts for the differences in substitutability of the various types of land use (Huang et al., 2004; Van Meijl et al., 2006). In addition, MAGNET includes international and EU agricultural policies, such as production quota and export/import tariffs (Helming et al., 2010).
MAGNET includes ethanol and biodiesel as first-generation biofuels made from wheat, sugar cane, maize, and oilseeds (Banse et al., 2008) and the use of by-products (DDGS, oilcakes) from biofuel production in the livestock sector. Second-generation biofuels are also included, with the potential amount of residues available from IMAGE/TIMER (Daioglou et al., 2016).
MAGNET distinguishes the livestock commodities of beef cattle, dairy cattle, other cattle (sheep & goats), dairy cattle, poultry, and pig and other animal products. The first three are the ruminant sectors which are grass and crop fed, while the poultry and pigs sectors are crop fed. Modelling the livestock sector includes different feedstuffs, such as feed crops, co-products from biofuels (oil cakes from rapeseed-based biofuel, or distillers grain from wheat-based biofuels), and grass (Woltjer, 2011). Grass may be substituted by feed from crops for ruminants.
In MAGNET, land supply is calculated using a land-supply curve that relates the area in use for agriculture to the land price. Total land supply includes all land that is potentially available for agriculture, where crop production is possible under soil and climatic conditions, and where no other restrictions apply such as urban or protected area designations (see also Component Land-use allocation). In the IMAGE model, total land supply for each region is obtained from potential crop productivity and land availability on a resolution of 5x5 arcminutes (Mandryk et al., 2015). The supply curve depends on total land supply, current agricultural area, current land price, and estimated price elasticity of land supply in the starting year. Regions differ with regard to the proportion of land in use, and with regard to change in land prices in relation to changes in agricultural land use.
Reduced land availability
By restricting land supply in IMAGE and MAGNET, the models can assess scenarios with additional protected areas, or reduced emissions from deforestation and forest degradation (REDD). These areas are excluded from the land supply curve in MAGNET, leading to lower elasticities, less land-use change and higher prices, and are also excluded from the allocation of agricultural land in IMAGE (e.g., Overmars et al., 2014, Doelman et al., 2018).
Intensification of crop and pasture production
Crop and pasture yields in MAGNET may change as a result of the following four processes:
- autonomous technological change (external scenario assumption);
- intensification due to the substitution of production factors (endogenous);
- climate change (from IMAGE);
- change in agricultural area affecting crop yields (such as, decreasing average yields due to expansion into less suitable regions; from IMAGE).
Biophysical yield effects due to climate and area changes are calculated by the IMAGE crop model and communicated to MAGNET. Likewise, also the potential yields and thus the yield gap can be assessed with the crop model in IMAGE. External assumptions on autonomous technological changes are mostly based on FAO projections (Alexandratos and Bruinsma, 2012), which describe per region and commodity, the assumed future changes in yields for a wide range of crop types. In MAGNET, the biophysical yield changes are combined with the autonomous technological change to give the total exogenous yield change. In addition, during the simulation period, MAGNET calculates an endogenous intensification as a result of price-driven substitution between labour, land and capital. Projections of crop yield increase in IMAGE-MAGNET and other global agricultural models were evaluated recently (Van Zeist et al., 2020). In IMAGE, regional yield changes due to autonomous technological change and endogenous intensification according to MAGNET are used in the spatially explicit allocation of land use (Component Land-use allocation).