IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group II: Impacts, Adaptation and Vulnerability Interactions of elevated CO2 with temperature and precipitation

Many recent studies confirm and extend the TAR findings that temperature and precipitation changes in future decades will modify, and often limit, direct CO2 effects on plants. For instance, high temperature during flowering may lower CO2 effects by reducing grain number, size and quality (Thomas et al., 2003; Baker, 2004; Caldwell et al., 2005). Increased temperatures may also reduce CO2 effects indirectly, by increasing water demand. Rain-fed wheat grown at 450 ppm CO2 demonstrated yield increases with temperature increases of up to 0.8°C, but declines with temperature increases beyond 1.5°C; additional irrigation was needed to counterbalance these negative effects (Xiao et al., 2005). In pastures, elevated CO2 together with increases in temperature, precipitation and N deposition resulted in increased primary production, with changes in species distribution and litter composition (Shaw et al., 2002; Zavaleta et al., 2003; Aranjuelo et al., 2005; Henry et al., 2005). Future CO2 levels may favour C3 plants over C4 (Ziska, 2003), yet the opposite is expected under associated temperature increases; the net effects remain uncertain.

Importantly, climate impacts on crops may significantly depend on the precipitation scenario considered. In particular, since more than 80% of total agricultural land, and close to 100% of pasture land, is rain-fed, general circulation model (GCM) dependent changes in precipitation will often shape both the direction and magnitude of the overall impacts (Olesen and Bindi, 2002; Tubiello et al., 2002; Reilly et al., 2003). In general, changes in precipitation and, especially, in evaporation-precipitation ratios modify ecosystem function, particularly in marginal areas. Higher water-use efficiency and greater root densities under elevated CO2 in field and forestry systems may, in some cases, alleviate drought pressures, yet their large-scale implications are not well understood (Schäfer et al., 2002; Wullschleger et al., 2002; Norby et al., 2004; Centritto, 2005).