Researchers have found global drought levels have not significantly increased over the past 60 years. This has led to questioning of the belief that climate change causes drought, writes ANU’s Simon Copland.
New research has questioned the consensus among many scientists that climate change is leading to a global increase in drought levels. The research has wide implications for how we predict the impacts of, and adapt to, climate change, concluding we need to take a much more regional approach to the issue.
The paper, Little change in global drought over the past 60 years, was completed by Dr Michael Roderick from the Australian National University, and Dr Justin Sheffield and Professor Eric Wood from Princeton University. Conducting an extensive review of data, the team found that global drought levels have not significantly increased over the past 60 years.
Roderick says the trend can be seen both globally, and within Australia. ”Our new model shows that over the past 60 years, we’ve got a slight increase in drought in eastern part of Australia and a slight increase in drought in the south-west. However, we also have a very large decrease in drought over much of South Australia, Northern Territory and the top half of Western Australia,” he said.
“Under previous research methods, some places in Australia increased and some places decreased, but there were a lot more increases than decreases. Under our method, they balance out.”
The research comes to this conclusion through questioning a drought measure used by many scientists called the Palmer Drought Severity Index (PDSI). Named after Wayne Palmer, PDSI was developed as an agricultural monitoring tool in the 1960s to help the United States government allocate aid to drought stricken farmers. PDSI looks at the moisture of the soil (and hence agricultural drought), through calculating a simple balance between precipitation, evaporation and ground run-off.
As global climate change has accelerated, this method has been used by many scientists to calculate the effects it will have on drought levels. However, it was never intended to be used for changes over time. Despite that, it has been the preferred option by many as the data on rainfall and temperature were readily available. Yet, Roderick says that as new meteorological data has become available, PDSI has proven to be inaccurate. He says that it is too simple, in particular in calculating evaporation levels.
“The use of the PDSI has led to a bias in results that indicated an increase in the area of global drought where none has actually occurred,” Roderick said. “What they’ve done that’s incorrect is that they’ve assumed that water would evaporate faster if the temperature went up.”
Roderick says there are far more important impacts on evaporation levels than increasing temperature. Their new model takes into account the underlying physical principles of evaporation. In particular, it looks at the impact of sunlight, humidity and wind, which Roderick says are generally more important for evaporation than temperature.
By including these measures, the team was able to conclude that PDSI dramatically overestimated global drought levels, and these levels have in fact not increased in a statistically important manner over the past 60 years.
This doesn’t mean however we’re out of the woods when it comes to the impact climate change will have on drought. In using their new method, what Roderick and his team found was that changes in drought levels are far more regionalised. This means that in some areas we may see a significant increase in drought, whilst in others we will see the opposite occur, with significant increases in moisture level. This matches the growing consensus in the climate community that while climate change will present a global trend of increased temperature, the drought impacts of this may well be highly regionalised.
For example, in a paper released earlier this year, NASA scientist James Hansen stated that we could already see the impacts climate change was having on what is known natural variability (the localised variations in weather on a day-to-day or month-to-month basis). Hansen pointed towards the European heat wave of 2003, the Russian heat wave of 2010 and the droughts in Texas and Oklahoma last year as examples of the impacts of climate change that we can see right now.
Hansen’s conclusion was that while we were seeing a trend of a global increase in temperature, at a regional level this translated into more extremes of both heat, and cold. The Princeton-ANU paper shows that we can see a similar trend when it comes to drought, with little global trend, but significant regional variations.
This makes for a much more complex picture for climate scientists and the community. Roderick concludes this is a challenge we will need to meet in order to get a better understand of climate processes. ”It is important that we gain our perspective of the impacts of climate change based on the best physical understanding of processes possible,” he said.