Climate change will affect plant pests and diseases in the same way it affects infectious disease agents. In other words, the range of many insects will expand or change, and new combinations of pests and diseases may emerge as natural ecosystems respond to altered temperature and precipitation profiles. Any increase in the frequency or severity of extreme weather events, including droughts, heat waves, windstorms, or floods, could also disrupt the predator-prey relationships that normally keep pest populations in check. An explosion of the rodent population that damaged the grain crop in Zimbabwe in 1994, after 6 years of drought had eliminated many rodent predators, shows how altered climate conditions can intensify pest problems. The effect of climate on pests may add to the effect of other factors such as the overuse of pesticides and the loss of biodiversity that already contribute to plant pest and disease outbreaks .
The ingenuity of farmers, breeders, and agricultural engineers, and the natural resilience of biological systems, will help buffer many of the negative effects of climate change on agriculture. However, experts believe that over the longer term, the accumulated stresses of sustained climate change stand a good chance of disrupting agro-ecosystems and reducing global food productivity.
The regions thought to be most vulnerable to productivity declines are semiarid and arid areas where rain-fed, nonirrigated agriculture predominates. Unfortunately, many of these areas as in sub-Saharan Africa, South and East Asia, and on some Pacific islands are already hard-pressed agriculturally and suffer from high rates of malnutrition. In Senegal, for example, one study predicts a 30-percent yield decline with a 4