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Modelling Drought Throughout the World

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.
According to Trenberth et al. (2014), "several recently published studies have produced apparently conflicting results of how drought is changing under climate change," and they say "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." Against this backdrop, this international research team set out to make "an assessment of the issues with the PDSI." But in doing so, they also discovered "several other sources of discrepancy," including "how precipitation has changed and is analyzed." More specifically, 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," (2) "the PDSI model itself contains uncertainties," (3) "there remain substantial issues on how to best deal with changes in evapotranspiration," (4) "what is more surprising, and disappointing, are the disparities between precipitation data sets," (5) "the general availability of precipitation data and differences in the primary precipitation data sets continue to be a concern," and (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."

In light of these several observations, Trenberth et al. ultimately conclude "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 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.

Archived 1 July 2014