Little change in global drought over the past 60 years

• Published in: Nature (London) 2012-11, Vol.491 (7424), p.435-438
• Main Authors: Sheffield, Justin, Wood, Eric F., Roderick, Michael L.
• Format: Article
• Language: English
• Subjects: 704/106/242; 704/106/694; Australia; Climate change; Climatic changes; Climatology. Bioclimatology. Climate change; Drought; Droughts; Droughts - statistics & numerical data; Earth, ocean, space; Environmental aspects; Evaporation; Exact sciences and technology; External geophysics; Global Warming; Humanities and Social Sciences; Hydrologic cycle; letter; Meteorology; Models, Theoretical; multidisciplinary; Observations; Paleoclimate; Remote sensing; Science; Studies; Temperature; Time Factors; Trends; United States; Water in the atmosphere (humidity, clouds, evaporation, precipitation); Wind speed
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature11575

A physically based approach to drought modelling shows that there has been little change in drought from 1950 to 2008, contradicting previous work that suggested an increase in recent years. New figures for global drought trend Published assessments of historic changes in drought during recent decades have suggested that the frequency and area of droughts have been increasing. Here Justin Sheffield et al . show that this prior work was flawed, because of an inappropriate calculation of drought metrics. Using a more physically based approach, the team shows that there has in fact been little change in drought during the period 1950 to 2008. 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 1 , 2 , 3 . 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 4 , 5 . 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 6 . Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation 7 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 8 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 9 , 10 .