26 March 2013

While much of the extreme weather we are currently experiencing is transitory, drought is more of a gradual phenomenon, slowly taking hold of an area, tightening its grip with time. In severe cases, drought can last for many years and have a devastating effect on local communities and agriculture.

In the recent paper Little change in drought over the past 60 years, researchers introduced additional physical factors into the index equation such as sunlight, humidity, wind–speed and temperature in order to measure the effects of drought more accurately. The results showed that statistically there has not been any significant increase in global drought for over 60 years, contradicting statistics published over the last decade that reported an increase in global drought since the 1950s. What the research has revealed is that droughts, by nature, are a complex phenomenon and a diverse range of factors influence where and how they occur.

“Many climate change researchers still rely on the Palmer Drought Severity Index (PDSI) to calculate evaporation as a sole function of temperature,” said Professor Michael Roderick, a Chief Investigator at the ARC Centre of Excellence for Climate System Science. Dr Roderick is a co-author of the paper along with Princeton University researchers, Dr Justin Sheffield and Professor Eric Wood.

“In the recent study, we used a good theoretical basis and much of this theoretical basis is the foundation for  how we represent droughts in the climate models used for projecting the Earth’s climate. It is reasonable to think that the global climate models have the appropriate mechanisms built in, but this is something that needs further examination,” said Professor Roderick.

“Until we establish a more reliable method to measure present-day conditions, the link between climate changes, global warming and global drought cannot be determined with any precision. It is difficult to predict what is going to happen to global drought in the warming world, for example, currently Darwin has a warmer annual average temperature than Alice Springs but the evaporative demand is higher in Alice Springs because of the low humidity, higher winds, and stronger sunlight.”

“When there is drought in one part of the world, there is rain in the other, drought works on the ebb and flow principle, the rainfall shifts from one global region to another.”

Professor Roderick has now demonstrated the basic flaws in the PDSI and offered valuable perspective on how to move forward—namely the use of other methods to measure changes in conditions (e.g. ocean variability) and the coordination of several research programs within the Centre.

“It is time to stop using simplified drought indices in research because they are biased and this has had implications for how extremes and changes in the hydrological cycles over land have been interpreted. It is important that we gain our perspective of the impacts of climate change based on the best physical understanding of processes available,” explains Dr Roderick.

“We are trying to examine this in more detail at a process level, in other words, we are not asking, are droughts changing, but what are the mechanisms that drive changes in the distribution, duration, and intensity of drought?”

For more information please visit the ARC Centre of Excellence for Climate System Science website.