Water balance and weather extremes are key to many agricultural and forestry impacts. Decreases in precipitation are predicted by more than 90% of climate model simulations by the end of the 21st century for the northern and southern sub-tropics (IPCC, 2007a). Increases in precipitation extremes are also very likely in the major agricultural production areas in Southern and Eastern Asia, in East Australia and in Northern Europe (Christensen et al., 2007). It should be noted that climate change impact models for food, feed and fibre do not yet include these recent findings on projected patterns of change in precipitation.
The current climate, soil and terrain suitability for a range of rain-fed crops and pasture types has been estimated by Fischer et al. (2002b) (see Figure 5.1a). Globally, some 3.6 billion ha (about 27% of the Earth’s land surface) are too dry for rain-fed agriculture. Considering water availability, only about 1.8% of these dry zones are suitable for producing cereal crops under irrigation (Fischer et al., 2002b).
Figure 5.1. (a) Current suitability for rain-fed crops (excluding forest ecosystems) (after Fischer et al., 2002b). SI = suitability index; (b) Ensemble mean percentage change of annual mean runoff between present (1981 to 2000) and 2100 (Nohara et al., 2006).
Changes in annual mean runoff are indicative of the mean water availability for vegetation. Projected changes between now and 2100 (see Chapter 3) show some consistent runoff patterns: increases in high latitudes and the wet tropics, and decreases in mid-latitudes and some parts of the dry tropics (Figure 5.1b). Declines in water availability are therefore projected to affect some of the areas currently suitable for rain-fed crops (e.g., in the Mediterranean basin, Central America and sub-tropical regions of Africa and Australia). Extreme increases in precipitation (Christensen et al., 2007) also are very likely in major agricultural production areas (e.g., in Southern and Eastern Asia and in Northern Europe).