Decreasing World Aridity in a Warming Climate

Abstract: The mean world climate has warmed since the 19th Century as the anthropogenic emission of greenhouse gases has increased the atmospheric opacity to thermal infrared radiation. Has this warming increased the frequency or severity of droughts? We define an objective aridity index that quantifies the precipitation forcing function of drought. Using the GHCN daily database from {\it c.}~1900 to the present and averaging over hundreds or thousands of sites in each of eight continental or semi-continental regions, we evaluate trends in aridity. Seven of these regions have sufficient data for significant conclusions. In them the mean aridity decreased at logarithmic rates from $(-2.2 \pm 0.6) \times 10^{-4}$ to $(-5.1 \pm 0.2) \times 10^{-3}$ per year, three or more times their nominal ($1\sigma$) uncertainties.