Climate change will drive dramatic changes in the abundance and distribution of species. Assessing the impacts of climate change on our agricultural systems is essential for mitigation planning. Here, I combine projections from the UK Hadley Centre's HadRM03 climate change model for Southern Britain with a general mechanistic model of the interaction between climate, temperate grass physiology and cereal aphid population dynamics. Aphids are one of the largest and most important groups of crop pests and disease vectors worldwide but particularly in temperate regions. The model predicts an increasingly dramatic decline in cereal aphid abundance from the 1961 to 1990 baseline with increasing CO2 emissions: low emissions=−5%, medium low=−12%, medium high=−61%, and high=−92%. Of the six climate variables used in the model, changes in temperature and rainfall were the most important across all emissions scenarios and, counter-intuitively, the direct impact of elevated CO2 actually declines as emissions increase. The results suggest that the pest status of cereal aphids in Southern Britain will significantly decline by the end of this century.