The US Corn Belt supports agroecosystems that flourish in a temperate climate regime that could see significant changes in the next few decades. Because Wisconsin is situated on the northern, cooler fringes of this region, it may be the beneficiary of a warmer climate that could help support higher corn and soybean yields. Here we show that trends in precipitation and temperature during the growing season from 1976–2006 explained 40% and 35% of county corn and soybean yield trends, respectively. Using county level yield information combined with climate data, we determined that both corn and soybean yield trends were enhanced in counties that experienced a trend towards cooler and wetter conditions during the summer. Our results suggest that for each additional degree ( °C) of future warming during summer months, corn and soybean yields could potentially decrease by 13% and 16%, respectively, whereas if modest increases in total summer precipitation (i.e. 50 mm) were to occur, yields may be boosted by 5–10%, counteracting a portion of the negative effects associated with increased temperature. While northern US Corn Belt regions such as Wisconsin may benefit from a warmer climate regime and management changes that lengthen the crop-growing period in spring and autumn, mid- to high-latitude crop productivity may be challenged by additional summertime warming unless adaptive measures are taken.