5.4.1.3 Increased frequency of extreme events

The TAR has already reported on studies that document additional negative impacts of increased climate variability on plant production under climate change, beyond those estimated from changes in mean variables alone. More studies since the TAR have more firmly established such issues (Porter and Semenov, 2005); they are described in detail in Sections 5.4.2 to 5.4.7. Understanding links between increased frequency of extreme climate events and ecosystem disturbance (fires, pest outbreaks, etc.) is particularly important to quantify impacts (Volney and Fleming, 2000; Carroll et al., 2004; Hogg and Bernier, 2005). Although a few models since the TAR have started to incorporate effects of climate variability on plant production, most studies continue to include only effects on changes in mean variables.

5.4.1.4 Impacts on weed and insect pests, diseases and animal health

The importance of weeds and insect pests, and disease interactions with climate change, was reviewed in the TAR. New research confirms and extends these findings, including competition between C₃ and C₄ species (Ziska, 2003; Ziska and George, 2004). In particular, CO₂-temperature interactions are recognised as a key factor in determining plant damage from pests in future decades, though few quantitative analyses exist to date; CO₂-precipitation interactions will be likewise important (Stacey and Fellows, 2002; Chen et al., 2004; Salinari et al., 2006; Zvereva and Kozlov, 2006). Most studies continue to investigate pest damage as a separate function of either CO₂ (Chakraborty and Datta, 2003; Agrell et al., 2004; Chen et al., 2005a, 2005b) or temperature (Bale et al., 2002; Cocu et al., 2005; Salinari et al., 2006). For instance, recent warming trends in the U.S. and Canada have led to earlier spring activity of insects and proliferation of some species, such as the mountain pine beetle (Crozier and Dwyer, 2006; see also Chapter 1). Importantly, increased climate extremes may promote plant disease and pest outbreaks (Alig et al., 2004; Gan, 2004). Finally, new studies, since the TAR, are focusing on the spread of animal diseases and pests from low to mid-latitudes due to warming, a continuance of trends already under way (see Section 5.2). For instance, models project that bluetongue, which mostly affects sheep, and occasionally goat and deer, would spread from the tropics to mid-latitudes (Anon, 2006; van Wuijckhuise et al., 2006). Likewise, White et al. (2003) simulated, under climate change, increased vulnerability of the Australian beef industry to the cattle tick (Boophilus microplus). Most assessment studies do not explicitly consider either pest-plant dynamics or impacts on livestock health as a function of CO₂ and climate combined.