Although most previous studies of trends in global drought have discussed the effects of temperature and precipitation on drought, none have specifically looked at the relative magnitudes of the effects of T (and/or PET) and P on drought occurrence. The objectives of this paper are to answer two basic questions regarding global drought occurrence: (1) are interannual changes in drought occurrence controlled mostly by changes in PET, P , or both equally? and (2) given global warming and associated increases in PET, has the occurrence of drought increased during the past century?

In another study of global drought Sheffield et al . [ 2012 ] examined changes in global drought using the Palmer Drought Severity Index (PDSI). In their study they show that use of the standard PDSI may overestimate drought occurrence because of the use of the Thornthwaite potential evapotranspiration (PET) equation. They show when using the standard PDSI computed using the Thornthwaite PET equation that there is a discernable increase in global drought. However, when the Penman‐Montieth PET equation is used, there is no trend in global drought.

Given global warming, there is concern that increased temperature ( T ) and associated evapotranspiration may lead to increased drought occurrence and severity [ Rind et al ., 1990 ; Wang , 2005 ; Seager et al ., 2007 ; Sheffield and Wood , 2008 ; Dai , 2011a , 2011b , 2013 ]. Using a soil moisture‐based drought index, Sheffield and Wood [ 2008 ] examined global and regional trends in drought for the 1950–2000 period. Sheffield and Wood [ 2008 ] reported an overall slight wetting trend in global soil moisture that has been driven by increases in precipitation ( P ). They also reported substantial regional variations in soil moisture trends with notable drying over West Africa, which has been forced by decreasing Sahel P . Sheffield and Wood [ 2008 ] also reported that for most regions, trends in drought duration, intensity, and severity were negative for the study period. Additionally, although there has been an overall wetting trend, Sheffield and Wood [ 2008 ] indicate that there has been a change since the 1970s to a drying trend, globally and in many regions, especially in high northern latitudes. The apparent shift to a drying trend is related to increases in T . Sheffield and Wood [ 2008 ] suggest that even though drought is forced mostly by P variability, continued global warming may result in increased drought occurrence. In addition, Dai [ 2013 ] reports that global aridity has increased and that the drying is consistent with climate model predictions. Dai [ 2013 ] also suggested that severe and widespread droughts may occur over the next 30–90 years over many land areas due to decreased P and/or increased evaporation.

The monthly CRUTS P and PET data were used to compute monthly time series of P minus PET (PMPE) for each of the CRUTS grid cells. Positive values of monthly PMPE indicate a surplus of water, and negative values indicate a deficit of water. The monthly values of PMPE were subsequently summed to produce time series of annual PMPE for each grid cell. For this study, annual PMPE values below zero were considered to be indicative of drought conditions. Using this threshold for drought, the percent (%) of the global land area represented by the CRUTS grid cells with annual PMPE values below zero was computed for each year (using % land area provides an area‐weighted value of land area in drought). The time series of the percent land area of the globe in drought (%drought) then was examined for trends.

The version of the CRUTS data set we used (CRUTS3.10) included revised and updated P data (CRUTS3.10.01 precipitation). The PET values were computed by the CRU using a variant of the Penman‐Montieth equation [ Monteith , 1964 ; Shuttleworth , 1993 ; Allen et al ., 1994 ; Ekstrom et al ., 2007 ; Sheffield et al ., 2012 ]. Specifically, this PET equation used by the CRU is the grass reference evapotranspiration, developed by the Food and Agricultural Organization [ Allen et al ., 1994 ]; this defines PET from a clipped grass‐surface 0.12 meters (m) in height and an assumed surface albedo of 0.23 [ Ekstrom et al ., 2007 ]. This model computes PET based on monthly mean T , monthly mean minimum T , monthly mean maximum T , monthly mean vapor pressure, and monthly mean cloud cover. Wind speed and T are direct outputs from the CRU model and net radiation; vapor pressure deficit and the slope of the vapor pressure curve are computed using model output. Additionally, total cloud cover is estimated from long‐wave radiation computed by the HadRM3H model and is used to estimate the relative fraction of sunshine (i.e., sunshine hours per day divided by total day length) [ Allen et al ., 1994 ]. Surface T (mean, minimum, and maximum) and relative humidity are used to compute the vapor pressure deficit and the slope of the vapor pressure/temperature curve [ Ekstrom et al ., 2007 ]. The CRUTS data are available for download at http://iridl.ldeo.columbia.edu/SOURCES/.UEA/.CRU/.TS3p1/.monthly/ , and documentation for the data set is available at http://iridl.ldeo.columbia.edu/SOURCES/.UEA/.CRU/.TS3p1/.dataset_documentation.html .

Monthly P and PET data for the globe were obtained from the Climate Research Unit (CRU) at East Anglia, United Kingdom data set (the CRUTS3.10 data set). The spatial resolution of this data set, which covers the land areas of the globe, is 0.5 degree (°) by 0.5° and spans the period 1901 through 2009. Data poleward of 70° north latitude and poleward of 60° south latitude were not used because many of the monthly P and PET values for these grid cells were generated from sparse meteorological observations. Focusing on the land area between 60°S and 70°N provided 57,087 grid cells for analysis.

3 Results and Discussion

The annual time series of globally averaged %drought indicates a mean value of 66%, a range of about 4%, and no long‐term trend (−0.2% per 100 years, nonstatistically significant) (Figure 1). Given noted increases in global T and PET during the past century, it is surprising that drought occurrence has not increased. To determine the explanation for the lack of a long‐term trend in global drought occurrence, we performed an experiment where PMPE was computed in two ways. In the first case, month‐by‐month PMPE was computed using month‐by‐month PET and long‐term mean monthly P (i.e., monthly P climatology); thus, temporal variability in month‐by‐month PMPE reflected temporal variability in PET and nonvariable P. In the second case, month‐by‐month PMPE was computed using month‐by‐month P and long‐term mean monthly PET (monthly PET climatology). The first method was termed the PETvar method and the second method was termed the Pvar method. Using these two methods we could separate the individual effects of variability in PET and P on global drought occurrence. The month‐by‐month PMPE values computed for each site using the PETvar and Pvar methods were summed to produce time series of annual PMPE. The annual PMPE time series for each method were then used to compute %drought.

Figure 1 Open in figure viewer PowerPoint 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).

The percent of the globe in drought determined using the PETvar method indicates a range of only about 2% among all years, but a statistically significant (at a 95% confidence level (p < 0.05)) positive trend in drought (1.1% per 100 years). In contrast, %drought computed using the Pvar method indicates a range of about 6% among all years and a long‐term negative trend in drought (−2.2% per 100 years, p < 0.05). Taken together, the two models indicate that the positive effects of PET on %drought are mitigated by the negative effects of P on %drought. Additionally, the magnitude of the negative trend in %drought computed using the Pvar method is twice as large as the magnitude of the trend in %drought computed using the PETvar method. Thus, increases in global P have substantially reduced the effects of increased PET (resulting from increased T) on %drought.

Although the effects of PET (and T) on %drought are smaller than are the effects of P, increases in global PET have had a positive effect on %drought as indicated by the upward trending slope for the PETvar model (Figure 2). The stronger effects of P on drought are clear, however, particularly for the post‐1990 period.

Figure 2 Open in figure viewer PowerPoint 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) computed using variable PET and variable P (complete model), variable PET and P climatology (PETvar model), and PET climatology and variable P (Pvar model).

An analysis of departures of %drought computed using the PETvar model (Figure 3) indicates that the most positive effects of PET on %drought occurred after about 1987. Because the post‐1987 period indicates a substantial effect of PET (and T) on %drought, this period (1988 through 2009) is the focus of additional analyses.

Figure 3 Open in figure viewer PowerPoint Departures from the long‐term mean (in percent (%)) of the global land area in drought conditions (i.e., annual sums of monthly precipitation minus monthly potential evapotranspiration less than zero) computed using variable PET and P climatology (PETvar model).

Using the PMPE values from the complete model (i.e., variable PET and variable P), the PETvar model, and the Pvar model, we examined mean departures in %drought by 5° latitudinal bands (Figure 4). These comparisons indicate that departures in %drought for the Pvar model closely match those of the complete model except for the high northern latitudes. In the high northern latitudes the Pvar model indicates decreased %drought (due to increased P); however, the complete model indicates increased %drought. The expectation is that increased P would result in decreased %drought (as indicated by the Pvar model), but the complete model indicates contradictory results.

Figure 4 Open in figure viewer PowerPoint Mean departures (in percent (%)) of the percentage of global land area in drought conditions (i.e., annual sums of monthly precipitation minus monthly potential evapotranspiration less than zero) for the period 1988 through 2009 by 5° latitudinal bands computed using variable PET and variable P (complete model), variable PET and P climatology (PETvar model), and PET climatology and variable P (Pvar model).

To determine the cause of the differences between the complete model and the Pvar model for the high northern latitudes, mean departures of monthly %drought were computed for the 1988–2009 period and for 5° latitudinal bands using the complete, PETvar, and Pvar models (Figure 5). The mean departures of monthly %drought for the complete model (Figure 5a) indicate positive departures of %drought in the high northern latitudes for the months of March and April. These positive departures likely contribute to the positive departures in the annual %drought for the high northern latitudes computed using the complete model (Figure 4).

Figure 5 Open in figure viewer PowerPoint Mean monthly departures (in percent (%)) of the percent of global land area with drought conditions (i.e., annual sums of monthly precipitation minus potential evapotranspiration less than zero) for the period 1988 through 2009 computed using (a) variable PET and variable P (complete model), (b) variable PET and P climatology (PETvar model), and (c) PET climatology and variable P (Pvar model).

An examination of mean monthly departures of %drought for the PETvar model (Figure 5b) indicates mostly positive de‐partures for the northern high latitudes during most months with the most positive departures for the months of March and April. The mostly positive departures in mean monthly %drought for the high northern latitudes correspond to the positive de‐partures in annual %drought for the high northern latitudes (Figure 4). In contrast, mean monthly departures of %drought for the Pvar model (Figure 5c) indicate negative departures for the high northern latitudes during most months with the most negative departures during March. The negative monthly departures for the high northern latitudes computed using the Pvar model result in the negative annual departures in the high latitudes illustrated in Figure 4.

The positive departures in mean monthly %drought during the late winter/early spring for the high northern latitudes computed using the PETvar model are larger in magnitude than are the mean monthly negative departures in %drought computed using the Pvar model, thus resulting in annual departures for the complete model (Figure 4) that are positive for the high northern latitudes. The positive departures in %drought during the late winter/early spring computed using the complete and PETvar models appear to be related to positive departures of PET and T during the winter/early spring seasons (Figure 6). These positive departures in PET, T, and %drought may be related to amplified warming observed in high northern latitudes, particularly the Arctic [Screen and Simmonds, 2010].

Figure 6 Open in figure viewer PowerPoint Mean monthly departures of (a) potential evapotranspiration (PET) and (b) temperature (T) for global land areas averaged for 1988 through 2009.

The spatial patterns of annual departures in PMPE (complete model) for the 1988 through 2009 period indicate wetter conditions in the eastern United States, northern and central South America, and parts of northern Europe and northern Asia (Figure 7a). In contrast, annual PMPE decreased (drier conditions) for parts of northern North America, western Europe, northern Asia, and central Africa (Figure 7a). The decreases in PMPE are largely related to increases in PET (Figure 7b), whereas the increases in annual PMPE appear to be mostly driven by increases in P (Figure 7c), except for the increases in PMPE in eastern North America which also appear to be related to decreases in PET (Figure 7b). The increases in PMPE for northern North America and northern Asia are consistent with the increases in %drought for the high northern latitudes (Figure 4).

The increases in annual PMPE in eastern North America (i.e., the eastern conterminous United States) likely are related to the “warming hole” in this region and associated decreased PET. The term warming hole refers to regions that have experienced cooling or have not warmed as fast as other regions, given global warming. Several possible explanations for the warming hole in the eastern conterminous United States have been proposed, including hydrologic feedbacks due to increasing soil moisture and aerosols, and internal Pacific Ocean decadal variability [Kunkel et al., 2006; Meehl et al., 2012; Kumar et al., 2013].

To evaluate changes in PMPE on a subannual scale, we also examined changes in mean October through March and mean April through September PMPE for the 1988 through 2009 period. For the October through March complete model (Figure 7d), increases in PMPE are found for northern Europe and northern Asia, and parts of North and South America. In contrast, decreases in October through March PMPE are found for a large part of Africa and central Asia (Figure 7d). The increases in October through March PMPE are mostly related to increases in P (Figure 7f). Increases in October through March PMPE in central North America also appear to be related to decreases in PET (likely associated with the warming hole) (Figure 7e).

Changes in April through September PMPE for the complete model (Figure 7g) appear to be of comparable spatial extent and magnitude as the changes in October through March PMPE (Figure 7d). Additionally, the changes in April through September PMPE appear to correspond most closely with changes in PET (Figure 7h), rather than changes in P (Figure 7i), except for decreases in PMPE in southwestern Africa (Figure 7g), which are related to decreases in P (Figure 7i) rather than increases in PET (Figure 7h).