Little change in global drought over the past 60 years

Abstract

Drought is expected to increase in frequency and severity in the future as a result of climate change, mainly as a consequence of decreases in regional precipitation but also because of increasing evaporation driven by global warming. Previous assessments of historic changes in drought over the late twentieth and early twenty-first centuries indicate that this may already be happening globally. In particular, calculations of the Palmer Drought Severity Index (PDSI) show a decrease in moisture globally since the 1970s with a commensurate increase in the area in drought that is attributed, in part, to global warming. The simplicity of the PDSI, which is calculated from a simple water-balance model forced by monthly precipitation and temperature data, makes it an attractive tool in large-scale drought assessments, but may give biased results in the context of climate change. Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades. More realistic calculations, based on the underlying physical principles that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years. The results have implications for how we interpret the impact of global warming on the hydrological cycle and its extremes, and may help to explain why palaeoclimate drought reconstructions based on tree-ring data diverge from the PDSI-based drought record in recent years.

Future discharge drought across climate regions around the world modelled with a synthetic hydrological modelling approach forced by three general circulation models

“…Dai (2013) has investigated global soil moisture drought up to 2010 and states that the PDSI changes derived from observed weather records are consistent with model predictions, which would indicate severe and extended global droughts in the 21st century resulting from either decreased precipitation and/or increased evaporation. Sheffield et al (2012) argue that the increase in global soil moisture drought since the 1980s is overestimated because the PDSI was computed with a too simple evapotranspiration model, which has consequences of how to interpret the impact global warming on global drought changes. Orlowsky and Seneviratne (2013) use meteorological drought (SPI) and soil moisture drought (anomaly) to illustrate that there will be both wetting regions in the 21st century (e.g.…”

“…Global soil moisture droughts have been often examined (e.g. Dai et al, 2004;Dai, 2011;Sheffield et al, 2012) with the Palmer Drought Severity Index (PDSI Palmer, 1965), which is calculated from a simple soil water balance, with the threshold method in combination with a more comprehensive model (e.g. Sheffield and Wood, 2007;Sheffield et al, 2009) or through anomalies (e.g.…”

Mapping (dis)agreement in hydrologic projections

“…The effect of model conceptualization on projected trends was underscored by a study on global trends in drought (Sheffield et al, 2012). Sheffield et al (2012) compared the estimate of areas in agricultural drought obtained using the temperature-based Thornthwaite equation to estimate potential evaporation, with the results obtained with the more physically founded PenmanMonteith equation. A much stronger (weaker) increase in global areas in drought was found when using the Thornthwaite (Penman-Monteith) formulation.…”

Exacerbation of the 2013–2016 Pan‐Caribbean Drought by Anthropogenic Warming

“…Since 1950, the Caribbean has seen a gradual drying trend (e.g., −0.09 self‐calibrating Palmer Drought Severity Index [scPDSI] units per decade; Dai, 2011; Herrera & Ault, 2017; Neelin et al, 2006; Sheffield et al, 2012) with several multiyear droughts, the most severe and widespread of which occurred between 2013 and 2016 (Herrera & Ault, 2017). Given its extensive spatial scale, we refer to this event as the Pan‐Caribbean Drought (Figure 1).…”