A magnetic tape containing 50 years of daily weather records was modified to synthesize new records representing three different types of temperature change. These data were used as input to a phenological response model for maize in a series of computer simulation studies. Results show cooling as much as 2°F through one type of change could reduce the freeze-free season by one month and severely affect the ability to grow corn and other warm season crops in the northern half of the U.S. Corn Belt. The same decrease with another type of change would not affect the freeze-free season. A trend for warming would increase the incidence of freezing in early planted fields and may result in spurious speculation that the climate is cooling.