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No Change in Global Drought in the Past 60 Years

Sheffield, J., Wood, E.F. and Roderick, M.L. 2012. Little change in global drought over the past 60 years. Nature 491: 435-438.
A moderate to severe drought can have devastating impacts on regional agriculture, water resources and the environment. In recent years, many climate scientists and agriculturists have expressed growing concern about world-wide drying of land areas and increasing evaporation due to Global Warming (GW). Also, recent peer-reviewed studies (e.g., Briffa et al., 2009; Cai et al., 2009) seem to suggest severity and length of droughts increasing over various regions due to GW. Against this backdrop, it is indeed a relief and good news to note the conclusion of this latest short study in which Sheffield et al. (2012) utilize global datasets on the temperature, precipitation and surface energy parameters (wind, specific humidity etc) to calculate the well-known Palmer Drought Severity Index (PDSI) using two different equations.

According to the authors, the standard PDSI calculation uses a simplified model of potential evaporation (PE), which responds only to changes in temperature and this leads to an overestimation of the drought index because of warming of the Earth's surface in recent years. Therefore, the authors used a new approach in which additional meteorological data (e.g., temperature, radiation, wind speed, etc.) were used to calculate PE, after which they compared the two approaches, identified as PDSI-TH (Thornthwaite 1948, calculation of PE) and PDSI-PM (Penman 1948 and Montieth 1964 calculation), for the past 60 years (1950-2010).

Using the conventional index (PDSI-TH), drought areas show an increase in recent years. On the other hand, the new PDSI-PM index revealed smaller and (statistically) insignificant increases in drought areas. Also the severity of the drought index was reduced in recent years when the PDSI-PM was used compared with the conventional index. Such results suggest that previous calculations of global drought are over-estimated.

The authors realize that there are uncertainties in precipitation, temperature and humidity data in various regions of the world that may affect the calculations. Nevertheless, the regions of decreasing trends in PE estimated using the new index (PDSI-PM) are in general agreement with recent observations of decreasing PE due to global dimming, decreasing wind speed, and other surface parameters. The authors further conclude that the results of this study have implications for understanding changes in the terrestrial hydrological cycle and future impacts of global warming on agriculture and water resources in various regions.

In summary, the results of this study appear to contradict the IPCC assessment that 'mid-continental droughts will be exacerbated in future due to Global Warming!" Another study (Sheffield et al., 2009) reported here (NIPPC post October 5 2011) arrives at similar conclusions.

Additional References
Briffa, K.R., van der Schrier, G. and Jones, P.D. 2009. Wet and dry summers in Europe since 1750: evidence of increasing drought. International Journal of Climatology 29: 1894-1905.

Cai, W., Cowan, T., Briggs, P. and Raupach, M. 2009. Rising temperature depletes soil moisture and exacerbates severe drought conditions across southeast Australia. Geophysical Research Letters 36: L21709.

Sheffield, J., Andreadis, K.M., Wood, E.F. and Lettenmaier, D.P. 2009. Global and Continental Drought in the Second Half of the Twentieth Century: Severity-Area-Duration Analysis and Temporal Variability of Large-Scale Events. Journal of Climate 22: 1962-1981.

Archived 13 March 2013