Decreasing US aridity in a warming climate

The mean North American and world climates have warmed since the 19th century following the anthropogenic emission of large quantities of greenhouse gases. It has been suggested that this warming may increase the frequency or severity of droughts. We define a quantitative and objective aridity index that describes the precipitation forcing function of drought. Using the extensive historical database of precipitation records, we evaluate changes in the aridity in the 48 contiguous United States. The area-averaged mean fractional rate of change of aridity of 1218 sites in the period 1893-2013 was (-6.6 +/- 0.4)x10(-4) per year; the 48 contiguous United States became less arid. The rate of decrease of aridity was roughly consistent with expectations from the Clausius-Clapeyron relation and the rate of warming. The fractional rate of change of aridity was nearly uncorrelated with the aridity itself, but there were regional differences: many Western and coastal Southeastern sites showed increasing aridity, but regions of rapidly decreasing aridity were found in a band 85-100W and the Northeast.