First, a science shocker: Droughts are caused by a lack of water. That either can be from a massive rainfall deficit, or because of unusual rates of evaporation from the soil. The warmer it is, the more water evaporates.
A fact, somewhat obscured by the jungle of vegetation that covers the Middle Atlantic region, is that we are prone to moisture shortages almost every summer. Supply adequate water to a lawn or a forest, calculate how much moisture is directly evaporated by solar heating or transpired by the plants, and you will discover, in an average summer, about 17 inches of water will disappear. Normal summer rainfall is about 12 inches, so in a typical year, we wind up about 5 inches short.
That would be no big deal, except that most of our summer rain comes from scattered thunderstorms that usually produce a nothing-hither-and-a-deluge-yon pattern. Because the rains are so scattered, in an average summer there is a 70 percent chance that a significant (multicounty) portion of the Middle Atlantic will experience a prolonged period without much rain and a lot of evaporation. That’s why there’s scarcely a year here in which some county does not apply for federal agricultural disaster relief.
How does that factor into warming? U.S. surface temperatures have risen a mere 0.4ºC in the last 100 years. To view that data, go to the Web site of the National Climatic Data Center, a federal facility, at lwf.ncdc.noaa.gov/oa/climate/research/2001/ann/us-summary.html#Atemp.
Are we getting drier? The answer is no. U.S. precipitation has increased about 10 percent over the 20th century, an increase of around 3 inches in the last 100 years. The precipitation data can be found at lwf.ncdc.noaa.gov/oa/climate/research/2001/ann/us-summary.html#Apcp.
Many scientists, including myself, can construct arguments relating this rainfall increase to temperature changes. That’s because surface temperatures have risen a bit in recent decades, while temperatures in the upper atmosphere, as measured both by satellites and weather balloons, have not. If the surface warms while the air above does not, the surface air is more buoyant, and that will increase precipitation intensity. You can view the satellite data at www.uah.edu/News/climate, and a comparison of the satellite and the weather balloon data at the Web site for the Journal of Climate (you may have to pay for this), ams.allenpress.–com/amsonline/?request=get-abstract&issn=1520-0442&volume=015&issue=17&page=2412.We can measure changes in wetness and dryness in a number of ways. One popular tool is the Palmer Drought Severity Index. This history begins in 1895 and can be found at lwf.ncdc.noaa.gov/oa/climate/research/2002/jul/drought-national-overview.html. A mathematical analysis of the data reveals no increase in dryness and a slight, but statistically significant, increase in wetness.
So, since precipitation is increasing, if global warming causes droughts, then a big increase in temperature must be driving a huge evaporation change. But when we plot temperatures observed in the Middle Atlantic over the last 100 years, changes are even smaller than the minuscule values observed nationally. Even in the hottest years, evaporation is only 1.5 inches above average.
Do the math. Precipitation has increased by 3 inches. Overall, evaporation hasn’t changed much at all. Even in the worst years, it has increased by 1.5 inches. So, if global warming causes climate changes, the result nationwide is an increase (not a decrease) of about 1.5 inches of available water in a bad year and 3 inches in an average year. If anything, global warming is making us wetter.
These calculations are not difficult, and a course in applied climatology wouldn’t hurt those who glibly blame this year’s drought on global warming. If you’re interested in taking that course, I know a professor you can find at www.virginia.edu.