The purpose of the study was to assess the effect that global warming and changes in atmospheric CO2 concentration would have on grassland production within Scotland. This required the development of a mathematical model of herbage production that was responsive to climatic factors and changes in CO2 levels. A model of pure grass and grass white clover swards is described, and this has been used to assess the effects that the predicted increases in temperature, rainfall and CO2 might have on grass and white clover production. It is projected that global warming will increase the length of the growing season by between 12 and 37 days for every 1°C rise in annual mean daily temperature. The indications are that global warming will have little effect on annual production of grass, either from pure grass or grass white clover swards. On the other hand, white clover as a percentage of total herbage production is estimated to increase from 32% to 46% for a 2°C temperature rise. Nevertheless, increasing concentrations of CO2 is predicted to increase the yields of grass and white clover under both current climatic conditions and the global warming scenario.