Many past entries have noted the complexity of global climate change issues, and the extent to which key issues are and are not well understood. Past entries have also noted the necessity of examining the entire "picture", and not obsessing over one or two data points or factors. An excellent illustration of this point are two recent studies noting the impact of irrigation on rainfall and temperatures in the Midwest.
Although global temperatures have risen about 3/4 of a degree Celsius during the past century, the U.S. Midwest has experienced a noticeable decrease in summer temperatures in recent decades. For example, between 2000 and 2009, in Chicago, only two years tallied more than 24 days hotter than 90 degrees F, the lowest decadel total in 80 years. Rather than being an anomoly, researchers found comparable declines in extremely hot days at 13 other sites from western Iowa to eastern Indiana.
Precipitation also changed. Average rainfall in July and August from the 1970's through 2009 was more than 0.33" higher each month than it was in the precedent decades. The researchers believe that the trends are linked because humid air warms more slowly than dry air does.
One likely source of the extra moisture is agriculture, both close by and farther afield in the Great Plains. A rapid rise in irrigation in the Great Plains has boosted precipitation downwind in the Midwest, as shown by a study of more than 300 weather stations from Wisconsin and Michigan to Arkansas and Tennessee; the study found that rainfall had increased with irrigation. At sites along this track, precipitation during a "typical" July late in the 20th century was between 25 and 50 percent higher than it was early in the century.
The two studies can be found at http://ams.confex.com/ams/90annual/techprogram/paper_158547.htm and http://ams.confex.com/ams/90annual/techprogram/paper_160413.htm.