Global warming and changes in drought

Abstract

How is drought changing as the climate changes? Several recent papers in the scientific literature have focused on this question but the answer remains unclear. Here we attempt to understand this socially and ecologically relevant topic. We discuss what the expectations for changes in drought should be, and thus the prospects for the future, and we provide some recommendations for resolving outstanding issues. What is drought? As mentioned in the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC), “in general terms, drought is a ‘prolonged absence or marked deficiency of pre cipitation’, a ‘deficiency of precipitation that results in water shortage for some activity or for some group’ or a ‘period of abnormally dry weather sufficiently prolonged for the lack of precipitation to cause a serious hydrological imbalance’. ” The report 1 goes on to note that “drought has been defined in a number of ways. ‘Agricultural drought’ relates to moisture deficits in the topmost one metre or so of soil (the root zone) that impacts crops, ‘meteorological drought’ is mainly a prolonged deficit of precipitation, and ‘hydrologic drought’ is related to below-normal streamflow, lake and ground water levels.” These differences emphasize the relative roles of pre cipitation, evapotranspiration (ET) and runoff in drought caused by climatic factors. More generally, water availability is a societal and environmental concern, which also brings in the demand side, and thus there are other possible definitions related to water scarcity. The IPCC SREX report 2 includes a valuable discussion of drought or ‘dryness’, drought drivers and drought indices that complements that given here. Drought can be quantified and described in absolute terms (such as the amount of soil moisture or lake levels) or through relative measures (for instance, PDSI in various forms), and these can be compared 3,4 . Because drought is defined by one tail of the probability distribution function of a drought measure, such as soil moisture content or stream flow, a small reduction in the mean (for example, −5%) will translate into a much larger increase in drought frequency based on other drought definitions 5 . Consequently, this difference has caused some confusion regarding the magnitude