Worrying about global warming and its potential effects on our lives has become a national pastime. Climate extremes – floods, droughts, hurricanes, and tornados – are often blamed on global warming and taken as harbingers of a changing climate. Increasing levels of carbon dioxide and other greenhouse gases that trap long-wave radiation from the earth's surface (the greenhouse affect) may lead to higher temperatures and changes in rainfall amounts and distribution. Some areas may get more rainfall while others get less. Some reports predict that the Midwest will be one area that will see a decline in summer rainfall over the next 100 years. These predictions are made with large-scale computer models of the atmosphere; models that are related to those used to produce the daily weather forecast. It is difficult to be sure that these models accurately depict all the complexities of the world-wide climate which is one reason why there is still debate about the potential impact of the increased greenhouse effect.

Will global warming affect corn and soybean yields in Kentucky in 2009? It is impossible to answer this question. As every farmer knows, there is a lot of variation in the weather from one year to the next in Kentucky (see Figure 1). For example, there was more than 15 inches of rain in the wettest summers at Henderson, KY (3 out of 31 years), but only about 5 inches (2 out of 31 years) in the driest years. Maximum temperature was a boiling hot 94 degrees in 1980 but it was almost chilly at less than 84 degrees as recently as 2004. These year-to-year fluctuations are much larger than the small yearly changes predicted by the global warming models. The 31 years of data from Henderson and other locations in western and central Kentucky don’t show any evidence that summers are getting any warmer, wetter or drier.

Yields in 2009 will depend upon the weather this summer. If we have plenty of rain yields will probably be high, if we don’t yields will be low. We don’t need global warming to get this variation. At the time this article when to press (Feb. 12, 2009) the long-range National Weather Service forecast for this summer is not very helpful, giving an equal chance of above- or below-normal precipitation and air temperature for June, July and August in Kentucky. No one knows for sure what the weather will do this summer, but if it turns out to be a bad year we shouldn’t blame it on global warming.