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Nov 16, 2012

Climate change may not cause drought: new research

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.

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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.

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8 thoughts on “Climate change may not cause drought: new research

  1. Bill Williams

    Surprise surprise. Australia’s policy makers at both national and state level were quick to assume that the warmer equals drier hypothesis was fact, and proved the wisdom of occam’s razor with many expensive investments based purely on the warmer = drier hypothesis. These mistakes included:

    1. The Wonthaggi desalination plant
    2. The Victorian North South Pipeline (and the abandoning of plans to increase Melbourne’s existing rainwater catchments)
    3. The New Murray Darling Basin plan with its complete and utter lack of respect for the wisdom that underpinned the existing water sharing plan between NSW, Victoria and South Australia).
    4. The Koondrook-Perricoota Forest Flooding Plan which was intended to save gum trees that were dying because of “natural drought” rather than “unnatural drought” and which was gazetted when the drought had broken and the forest was flooded. [And which now seems to have resulted in the creation of an artificial forest lake which may drown more vegetation than it saves…not to mention creating artificial blackwater events].
    5. Canberra’s Murrumbidgee to Googong Water Transfer Project
    6. Sydney’s Warragamba Dam Deep Water Recovery Project.

    Amazingly enough, not one of these projects were operational before the ending of the drought rendered them unnecessary.

    But even more amazing is how out of step Australia was with other countries’ response to global warming. A significant component of the South Korean response, for example, has been the assumption that global warming will result in higher levels of evaporation and a generally more active water cycle. The South Koreans expect much more rain…..and are busy ensuring their dams can cope with much higher inflows.

    Tim Flannery warned Australians that Lake Hume would “never fill again”. He can tell that to the thousands of trees that were planted above the 75% capacity water mark……almost all of which are currently underwater and drowned.

  2. Moloch

    So in Australia the deserts will get a bit wetter while the food growing areas dry out a bit slower than expected…

    Like Stevo said, phew!

    This ‘balancing’ commentary is just nonsense – as the last two paragraphs of the article make plain. I’d expect that kind of silly and misleading commentary from Andrew Bolt or IPA stooge Bob Carter I’m surprised to see it here.

    It’s rather like saying there’s nothing to worry about as the northern icecap melts because the Antarctic sea ice is expanding.

    We don’t grow food in any quantity in the north of WA, NT or most of SA, we grow it in the east and the south of WA…

    This article would have been profoundly better if it also mentioned what’s been happening in some of the critical areas for food production around the world rather than concentrating on the abstract point about slower drought.

    And despite what the boosters try to tell us Australia is the tiniest of tiny minnows when it comes to feeding the world. We couldn’t even keep our neighbours in Java fed if their agriculture failed – and they manage near self-sufficiency on an island not much bigger than half the size of Victoria!

    Dragging our attention to the places where the vast majority of food is grown, the northern hemisphere – and specifically the world’s greatest food basket, the USA – we’re seeing an ongoing problem.

    That regionalisation of drought mentioned in the article is biting very hard into a food producing region that was still expecting a reasonable crop when this paper was being finalised. That hope died about the same time as this paper was submitted, back in July.

    The new research that has shown the effect of the melting northern Ice Cap, and the resulting ‘blocking’ weather patterns, that have increased rain and bitter cold in Northern Europe, winter heatwaves in Southern Europe and the worst one year drought in US history, needs to be brought into an article like this which purports to report on ‘new research’.

    Otherwise you end up with an inaccurate, and frankly misleading, headline like “Climate change may not cause drought”.

    I’m assuming that the author understands that the loss of 70-odd percent of the northern ice volume,and the resulting drought that we’re seeing now across the Great Plains of the US is due to Climate Change…

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