The baseline is not really year 1990. It is the average yield simulated under current climate (1970-2000 baseline). Since total production is determined by many other factors beyond the effects of climate change, total production changes are not used in the assessment. The assessment is based on emissions and climate scenarios. Scenarios are devices for analyzing situations in which outcomes are uncertain. Much internally consistent assumption is built into each set of scenarios. Therefore the results cannot be interpreted as the predictions of the future. Instead, they provide the projections of the effect of climate change under a wide range of possible futures.