# _3.2.2.1 Crop growth simulation models_

The two approaches commonly used for analyzing the impact of climate change on agriculture in this group of models are a) the crop suitability approach and b) the production function approach.

# _a) Crop suitability approach_

This approach is also referred to as the agro-ecological zoning (AEZ) approach, which is used to assess the suitability of various land and biophysical attributes for crop production. In this approach, crop characteristics, existing technology, and soil and climate factors, as determinants of suitability for crop production, are included (FAO 1996). By combining these variables, the model enabled the identification and distribution of potential crop producing lands. As the model includes climate as one determinant of agricultural land suitability for crop production, it can be used to predict the impact of changing climatic variables on potential agricultural outputs and cropping patterns (Du Toit et al. 2001; Xiao 2002).

The AEZ framework contains three basic elements (FAOSTAT 2002). The first is land utilization types (LUTs), which are selected agricultural production systems with defined input and management relationships, and crop specific environmental requirements and adaptability characteristics. The second is agro-referenced climate, soil and terrain data, which are combined into a land resource database. The third element is the procedure for calculating potential yields by matching crop/LUT environmental requirements with the environmental characteristics captured in the database. These models were developed to look at potential production capacity across various ecological zones by using a simulation of crop yields rather than measured crop yields (Mendelsohn & Tiwari 2000).

Xiao et al. (2002) used the AEZ approach to estimate the area and spatial distribution of global potential croplands under different climate change predictions. The Xiao et al. (2002) study indicated that the area of global potential croplands is about (32.91)x10<sup>6</sup> km<sup>2</sup> under contemporary climate, with a tendency to increase substantially over the period of 1977–2100 as a result of global warming. In this study, developed countries accounted for most of the increase in global potential croplands, while developing countries showed little change in area of cropland. A similar study by the Food and Agriculture Organization (FAOSTAT 2002) showed that a temperature increase of 3<sup>o</sup> C, paired with a 10% increase in rainfall, would lead to about 4% more cultivable rainfed land. The cultivable land in developed countries would increase by 25% whereas it would decrease by 11% in developing countries, which clearly indicates the uneven distribution of climate benefits.

Adaptation to changing climatic conditions can be addressed within this model by generating comparative static scenarios with changes in technological parameters (Mendelsohn & Tiwari 2000). The disadvantage of the AEZ methodology is that it is not possible to predict final outcomes without explicitly modeling all the relevant components and thus the omission of one major factor would substantially affect the model’s predictions (Mendelsohn & Tiwari 2000).

# _b) The production function approach_

The production function approach to analyzing the impacts of climate change on agriculture is based on an empirical or experimental production function that measures the relationship between agricultural production and climate change (Mendelsohn et al. 1994). In this approach, a production function, which includes environmental variables such as temperature, rainfall and carbon dioxide as inputs into production, is estimated. Based on this estimated production 9