Is agricultural production becoming more or less sensitive to extreme heat?

"Extreme heat is the single best predictor of corn and soybean yields in the United States. While average yields have risen continuously since World War II, we find no evidence that relative tolerance to extreme heat has improved between 1950 and 2005. Climate change forecasts project a sharp increase in extreme heat by the end of the century, with the potential to significantly reduce yields under current technologies.Published: Michael J. Roberts & Wolfram Schlenker, 2010."Is Agricultural Production Becoming More or Less Sensitive to Extreme Heat? Evidence from U.S. Corn and Soybean Yields,"NBER Chapters,in: The Design and Implementation of U.S. Climate PolicyNational Bureau of Economic Research, Inc"--National Bureau of Economic Research web site.