The journal Nature published a study, which strongly criticizes the Palmer Drought Severity Index (PDSI), used as the basis by IPCC in their 4th Assessment Report. In this document it was stated that more intense and longer lasting droughts have been observed since the 1970s.
Scientists from Princeton University, led by Justin Sheffield, suggest that there is very little evidence that the method used in a landmark UN report is badly flowed, despite what IPCC suggested in 2007. The team states that PDSI should not be used as a measure. It has been developed in the 1960s to help allocating aid for farmers.
PDSI does not take into account other factor besides rising temperatures to estimate drought. According to the team, it is understandable that the simple model would respond to recent rise in temperatures. However, this overestimates the drought predictions and hence climate change. If more realistic calculations are performed, taking into account changes in available energy, humidity and wind speed, the change in drought over the past 60 years.
This year, IPCC published a special report entitled SREX, in which they step back from the 2007 drought assessment. There they state that there are many uncertainties associated with global-scale trends in droughts.
The 4th Assessment Report was also criticized for inaccuracies in one of their sections. In 2010, a five-month independent probe into “Climategate” urged IPCC to make sure there are no similar slips in their 5th assessment report due in 2014. The main findings, however, were not challenged, since they were approved by the majority of the climatologists.