This paper discusses the importance of testing models which may be used to forecast the impact of climate on society. Model testing using sensitivity analysis and validation techniques is illustrated with two models: (1) the YIELD model which simulates the impact of climate on crop yields of several major crops, and (2) the International Futures SimulatiOn model which can be used to simulate the impact of crop yield changes on the world food system. The problems of linking such models to each other are also discussed. 1. Forecasting the impact of climate on food systems Changes in climate can have dramatic effects on food systems. Climatic variation triggers starvation, migration, economic disruption, and land use change, for the climatefood link is one of the most ancient, critical, and persistent problems of the physical environment. Hence, the anticipation of climatic change and the associated impacts on human activity are important forecasting issues which link the natural and social sciences.
For example, the possibility that increased carbon dioxide (CO2), produced by fossil fuel burning will lead to an increase in average global temperatures is an important forecasting and policy issue. In order to decide what we should do about carbon dioxide emissions, we need to make a series of forecasts. First of all, we need to predict the consumption of fossil fuels for the forseeable future and to decide what contribution this will make to the concentration of carbon dioxide in the atmosphere. This assessment requires both a socioeconomic forecast of energy use, and a technical calculation of the carbon content of different fuels. Secondly, climate modelers can estimate the effect of increased atmospheric CO2 on global and regional temperature, precipitation, and other variables. Crop yield modelers can use temperature and precipitation data to assess the impact of a CO2 climate change on crop production. The crop yield and production results can then be used to make a forecast of the associated changes in food distribution and consumption, economic conditions, and other social factors. If the forecast changes are serious enough, either globally or nationally, policy makers may decide to control fossil fuel burning or prepare other responses to the impacts that