Page 139 - Climate Change and Food Systems

chapter 4: an overview of climate change impact on crop production and its variability in europe, related uncertainties and research challenges
Lobell et al., 2013, Teixeira et al., 2013). Another major development has been more work on crop modelling for large areas, in particular making fairly detailed crop models operational for use at regional (Tao et al., 2009) and global scales (e.g. Bondeau, 2007).
However, substantial progress will still be needed in data gathering, improvement of
crop models and other impact assessment techniques in order to meet the demands for integrated assessments at different scales. In both the Agricultural Model Intercomparison and Improvement Project (AgMIP) and MACSUR projects, efforts are underway in this direction.
2.4 Current use of crop simulation for assessing effects of climate and adaptation
Many factors will shape future crop productivity, including changes in climate and atmospheric concentration of CO2 and other gases such as ozone, as well as improvements in agronomic management and technology (adaptation).
However, in most biophysical impact assessment studies of CC on crop production, only a few factors influencing crop yields are addressed. These are: changes in climatic variables (most notably temperature and precipitation); CO2 concentration; and, to a lesser extent, technical development or adaptation options (White et al., 2011).
Before discussing the ways these factors
are usually treated in simulation model-based impact assessment studies, we offer here a
brief account of how knowledge of modelled processes regarding some critical factors and their interactions has developed.
Experimental progress has enabled researchers to incorporate and evaluate atmospheric CO2 concentration impact functions within crop
models. Combined with temperature and water balance routines, this has enabled crop simulation techniques to be used for CC impact assessments (Asseng et al., 2011; Lobell et al., 2013; Teixeira
et al., 2013). Three different approaches have been used to simulate the photosynthesis response to increasing CO2 concentrations (see Kersebaum and Nendel, 2014). Effects on transpiration are seen as an empirical reduction in transpiration
with enhanced CO2, or by a reduction in stomatal conductance (Tubiello and Ewert, 2002). Crop models have been widely tested using FACE experiments and with elevated CO2 in open-top chambers (Ewert et al. 1999; Nendel et al., 2009).
All crop models consider temperature effects on various ecophysiological processes, including phenology, light utilization, photosynthesis and respiration, dry matter allocation to different plant organs and evapotranspiration. However, as of yet, very few models consider heat stress effects with maximum temperatures above certain thresholds – e.g. accelerated leaf senescence or effects on floret mortality /spikelet fertility of various cereals (see discussion below).
All widely applied crop models include consideration of water balance and the impact
of crop water shortage. However, there are distinct differences in how various models
treat the simulation of soil water dynamics (van Ittersum et al., 2003). Only a few models consider excess water and oxygen stress impacts on crop growth (e.g. Supit et al., 1994). An increasing number of models include the impact of crop nitrogen stress on crop growth and nitrogen use efficiency (e.g. Kersebaum, 2007).
Most crop model-based CC impact studies deal with several of the factors below, but for a fairly limited number of crops and regions (White et al., 2011).
Temperature
Temperature increases have multiple effects on crop growth and yield formation depending on
the crop growth stage in which they occur. Higher temperatures usually accelerate rates of crop development, resulting in a shortened growing period, and typically – but not always – in lower crop yields (e.g. Nonhebel, 1996; Batts et al., 1997; Hatfield, 2011). Increased temperatures can prolong the vegetation period and reduce frost
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