5.4.3.2 Impacts of gradual temperature change

A survey of experimental data worldwide suggested that a mild warming generally increases grassland productivity, with the strongest positive responses at high latitudes (Rustad et al., 2001). Productivity and plant species composition in rangelands are highly correlated with precipitation (Knapp and Smith, 2001) and recent findings from IPCC (2007b) (see Figure 5.1) show projected declines in rainfall in some major grassland and rangeland areas (e.g., South America, South and North Africa, western Asia, Australia and southern Europe). Elevated CO₂ can reduce soil water depletion in different native and semi-native temperate and Mediterranean grassland (Morgan et al., 2004). However, increased variability in rainfall may create more severe soil moisture limitation and reduced productivity (Laporte et al., 2002; Fay et al., 2003; Luscher et al., 2005). Other impacts occur directly on livestock through the increase in the thermal heat load (see Section 5.4.3.1).

Table 5.3 summarises the impacts on grasslands for different temperature changes. Warming up to 2°C suggests positive impacts on pasture and livestock productivity in humid temperate regions. By contrast, negative impacts are predicted in arid and semiarid regions. It should be noted that there are very few impact studies for tropical grasslands and rangelands.