I get it that a 15F increase in global temperatures would not be good for agriculture. Of course, I think 15F is absurd, at least from anthropogenic CO2.
However, for the types of warming we are seeing (in the tenths of a degree), such warming has always been a harbinger of prosperity through history. The medieval warm period in Europe was a time of expanding populations driven by increasing harvests. When the medieval warm period ended and decades of cooler weather ensued, the Great Famine resulted — a famine which many blame for weakening the population and making later plague outbreaks more severe.
I read a lot of history, and take a number of history courses (both on tape and live). Its so funny when the professor gets to these events, because she/he always has to preface his remarks with “I know you have been tought that warming is universally bad, but…”
2009 may rank as a below average year for American agriculture, not because of heat, but because of late frosts and an unusually cool summer.