Reference
Trenberth, K.E., Dai, A., van der Schrier, G., Jones, P.D., Barichivich, J., Briffa, K.R. and Sheffield, J. 2014. Global warming and changes in drought. Nature Climate Change 4: 17-22.
Background
The authors report that "several recently published studies have produced apparently conflicting results of how drought is changing under climate change," and they say that "the reason is thought to lie in the formulation of the Palmer Drought Severity Index (PDSI) and the data sets used to determine the evapotranspiration component."
What was done
In the words of Trenberth et al., the international research team made "an assessment of the issues with the PDSI." But they say that in doing so, they also discovered "several other sources of discrepancy," including "how precipitation has changed and is analyzed."
What was learned
The seven scientists - hailing from France, Saudi Arabia, the Netherlands, the United States and the United Kingdom - discovered that (1) "uncertainties have not always been adequately appreciated," that (2) "the PDSI model itself contains uncertainties," that (3) "there remain substantial issues on how to best deal with changes in evapotranspiration," that (4) "what is more surprising, and disappointing, are the disparities between precipitation data sets," that (5) "the general availability of precipitation data and differences in the primary precipitation data sets continue to be a concern," and that (6) "the other major issue is the role of natural variability, especially ENSO, which biases the land precipitation towards wetter conditions, and with less drought globally under La Niña conditions."
What it means
In light of these several observations, Trenberth et al. ultimately conclude that "it is probably not possible to determine reliable decadal and longer-term trends in drought due to climate change without first accounting for the effects of ENSO and the Pacific Decadal Oscillation," the former of which phenomena they describe as "the most common source of episodic droughts around the world." Thus, it would appear that there are still numerous significant problems that need to be resolved before the desired results can be obtained. And until this occurs, drought predictions should be looked at with a very jaundiced eye.