World drought frequency, duration, and severity for 1951-2010

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Abstract

In the context of climate change characterized by rising temperature and more extreme precipitation regimes, drought is one of the most relevant natural disasters. This paper presents maps of global drought frequency, duration, and severity for the periods 1951–1970, 1971–1990, and 1991–2010, to give an overview of the respective drought hot spots. Drought frequency is defined as the number of drought events occurred, drought duration as the number of months in drought conditions, and drought severity as the sum of the integral area below zero of each event. Because drought is mainly driven by rainfall deficits, we chose the Standardized Precipitation Index (SPI) as the base indicator to derive drought-related quantities. SPI-12 has been calculated on a monthly basis using a Gamma distribution fitted to a 60-year baseline period (1951–2010). Global grids (0.5°×0.5°) of the Full Data Reanalysis Version 6.0 dataset provided by the Global Precipitation Climatology Centre (GPCC) have been used as precipitation data input. The regions most exposed to prolonged and severe droughts during 1951–1970 were the Central United States, the Argentinian Pampas, Russia, and Central Australia; during 1971–1990 they were Southern Chile, the Sahel, and Siberia; during 1991–2010 they were the Amazon Forest, the Congo River Basin, Mongolia, North Eastern China, and Borneo. A linear trend analysis between 1951 and 2010 shows a small global increase in each drought component, but drought frequency decreased in the Northern Hemisphere. The increase in drought frequency, duration, and severity is found to be significant in Africa, Eastern Asia, Mediterranean region, and Southern Australia, while the Americas and Russia show a decrease in each drought component.

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Little change in global drought over the past 60 years.

Justin Sheffield, Eric F. Wood, Michael L. Roderick (2012)

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.