Management Practices, one of the six independent factors determining agricultural yields in the model, characterizes the impact of land management--in particular, fertilization--on cropland yields. This factor (γ) is parametrized by GDP according to the following equation:
- δ (Incremental Effect of GDP on Cropland Management Practices) = 7.0
- ε (Elasticity of Effect of GPD on Cropland Management Practices) = 1.5E-8
- GDPpc represents Gross Domestic Product per Capita
The effect of input-neutral technologies on agricultural yields is also coupled to GDP growth, in this case logarithmically:
- Δ (Reference Input-Neutral Technological Change in Agriculture) = 0.3
- The argument of the natural log is present GDP per capita relative to 1900.
Each of these factors is evaluated as a function of per capita GDP and is used to calculate agricultural yields. Their product is also used to scale Fertilizer Consumption (until saturation, defined as 3.0E-3 ton/ha/year). This derived quantity is plotted below in red. As usual, the effects of high and low population scenarios are shown in the shaded region. Historical data from the FAO on fertilizer consumption--also shown in grey--is used for model validation (not calibration).
Fertilizer Consumption is used in the emissions sector of the model as input for land use emissions.