The trend of droughts

IPCC SREX using the PDSI as an index to analyze the condition of observed global droughts and found there are still large uncertainties in  global scale trends. Dai 2012 argued that the prediction from model has consensus with the change of observed global dryness, which illustrate widespread and severe droughts over many regions in following 30-90 years due to declined precipitation and/or increased evaporation.

Figure1 Trend maps for precipitation and sc_PDSI_pm and time series of percentage dry regions. Long term trends during 1950-2010 in annual mean a. observed precipitation b. calculated sc_PDSI_pm using observation-based forcing c. smoothed time series of the drought regions

From Figure2, we can see that the PDSI_Th has a declining trend since 1970s, however this is not same as the PDSI_PM. As for the global area in drought during 1980-2008, the PDSI_Th has a significant increasing trend, although the trend of the PDSI_PM is not very obvious, it still has an increasing trend which is seven times smaller than the PDSI_Th (Sheffield et al. 2012).

Figure 2 Global average time series of the PDSI and area in drought. a, PDSI_Th (blue line) and PDSI_PM (red line). b, Area in drought (PDSI<-3.0) fro the PDSI_Th (blue line) and PDSI_PM (red line). Th: Thornthwaite algorithms. PM: Penman-Monteith algorithms.

However, Sheffield et al. 2012 argued that the PDSI derives from a simple water-balance model which is popular in large scale drought assessment but bring an overestimated result of global droughts and the trend of droughts has just changed a little over the past 60 years according to more further analysis. From paleoclimatic data, it can be found that recent droughts are not unparalleled because several severe megadroughts can be found in the paleoclimatic record (IPCC SREX).

Even regional droughts may bring some global impact, for example, the largest wheat producer and consumer in the world----China has to import grain due to a persistent drought in the northern growing area and world's largest wheat importer Egypt is having some economic and political problems resulting in dramatic increasing trend of food costs (Sternberg 2011).