|% global land area with drought, p. 224 pdf|
"Monthly precipitation (P) and potential evapotranspiration (PET) from the CRUTS3.1 data set are used to compute monthly P minus PET (PMPE) for the land areas of the globe. The percent of the global land area with annual sums of PMPE less than zero are used as an index of global drought (% drought) for 1901 through 2009. Results indicate that for the past century % drought has not changed, even though global PET and temperature (T) have increased. Although annual global PET and T have increased, annual global P also has increased and has mitigated the effects of increased PET on % drought."
Caption for above graph: "Figure 1. Time series of the percent of global land area with drought conditions (i.e., annual sums of monthly precipitation minus potential evapotranspiration less than zero.") pdf, page 224
pdf: 6/27/15, "Variability and trends in global drought," Gregory J. McCabe1and David M.Wolock2, U.S. Geological Survey, Denver, Colorado, USA, 2U.S. Geological Survey, Lawrence, Kansas. via Hockey Schtick