New study questions link between warming and past droughts

Jan 04, 2013 by John Sullivan
Link between warming and past droughts questioned
A new analysis of drought conditions over the past 50 years has yielded a nuanced view of global trends. Red areas have experienced increasing levels of drought while blue areas have become less prone to dry conditions. Overall, there has been less of a trend toward drought globally than previously thought, Princeton researchers have found. Credit: Justin Sheffield

(Phys.org)—A series of recent droughts from Australia to the United States has led some scientists to warn that global warming has already begun to increase worldwide drought. But new research from Princeton and the Australian National University in Canberra has found that this might not be the case.

The theory that increased temperature would lead to more rapid evaporation and increase the frequency and severity of droughts seems logical. As the Intergovernmental Panel on Climate Change (IPCC) reported in 2007, "more intense and longer droughts have been observed over wider areas since the 1970s."

But the new research indicates that the development of is much more complex than previously thought and that the reports of increasing drought were caused by weaknesses in the mathematical model used to simulate drought rather than any real drying trend.

"The overall view has been that as temperature increases drought is going to increase," said Justin Sheffield, a research scholar in Princeton's Department of Civil and Environmental Engineering. "But it is not that simple."

In an article published in November in the scientific journal Nature, the researchers reported that errors resulting from a model commonly used to assess drought, the Palmer Drought Severity Index, had led to overestimates of the severity of drought worldwide. Besides Sheffield, the paper's authors were Eric Wood, the Susan Dod Brown Professor of Civil and Environmental Engineering at Princeton, and Michael Roderick, a professor in the Research School of Earth Sciences and the Research School of Biology at Australian National University.

Wood said that better models are available for climate scientists to use, but the Palmer Index is frequently used because of its simplicity. That simplicity, however, can allow errors to creep in.

Originally developed in the 1960s as a way to determine levels of federal aid for U.S. farms damaged by drought, the Palmer Index relies heavily on temperature data to estimate the extent of drought. In part, that approach reflects what was available to hydrologists at the time. Temperature data was relatively accurate and easy to obtain. But because the Palmer Index is primarily driven by temperature, it is extremely sensitive to warming. 

This heavy reliance on temperature, Sheffield said, is "where things start to go wrong."

Another problem is that the Palmer Index was never intended to work in a changing climate. It was designed to be used as a local tool to predict the impact of drought in defined regions in which climate variables stay relatively constant. As climate in general changes, however, other factors may counterbalance the impact that rising temperatures might have on water supplies.

"Many other things are changing—wind speed, solar and infrared radiation, humidity of the air," Sheffield said. The Palmer Index, he said, "is too simple a model to take these changes into account."

Wood's group at Princeton created a more sophisticated model that includes inputs for data such as solar radiation, humidity and wind. Wood and Sheffield used a similar model to make calculations for their African Drought Monitor, an international effort that provides current assessments of drought in Africa over the Internet. Because the model accounts for a wider array of variables, it gives a more complete and accurate picture of conditions on the ground.

"You can go a long way by using a more physically realistic approach, a more appropriate approach, to make these estimates," Wood said. 

The greater detail of the Princeton model does mean it is more difficult to use and requires a far greater amount of data than other estimates. In fact, the researchers said the data requirements precluded its widespread use by climate scientists until relatively recently, when better satellite coverage and improvements in global data from ground weather stations provided more extensive and reliable estimates of meteorological variables such as precipitation, humidity and wind speed. 

Wood said that improved data analysis and models already had led to some reassessment of the record of drought and climate change in recent years. In a special report in March, the IPCC found there were large uncertainties in drought trends, Wood said. This is a change from the panel's 2007 report, which directly linked drought and climate change. 

Although the work questions whether has already led to increasing drought, Sheffield emphasized that it does not mean that drought will not increase in the future.

"It certainly does not mean that things are not going to be bad in the future," he said. "It does not negate the idea that we are having an impact on the atmosphere and that this will have an impact on drought."

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More information: www.nature.com/nature/journal/… ull/nature11575.html

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User comments : 18

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rod_russell_9
2.5 / 5 (16) Jan 04, 2013
What heresy! Every idiot knows that whatever bad happens weather-wise is due to man-induced global warming, and the only solution is to give control of all fossil fuels to the United Nations!
rod_russell_9
1.9 / 5 (9) Jan 04, 2013
Even this enlightened study does not appear to have included cloud-cover as a variable.
mrtea
2.8 / 5 (10) Jan 04, 2013
@rod_russel_9 - your sarcasm is duly noted. Also note the concluding paragraphs:

"Although the work questions whether climate change has already led to increasing drought, Sheffield emphasized that it does not mean that drought will not increase in the future.

"It certainly does not mean that things are not going to be bad in the future," he said. "It does not negate the idea that we are having an impact on the atmosphere and that this will have an impact on drought."
rubberman
2.5 / 5 (13) Jan 04, 2013
Even this enlightened study does not appear to have included cloud-cover as a variable.


Cloud cover as a variable in what equation?

What heresy! Every idiot knows that whatever bad happens weather-wise is due to man-induced global warming, and the only solution is to give control of all fossil fuels to the United Nations!


You might want to google the definitions of "climate" and "weather".
tadchem
3.7 / 5 (3) Jan 04, 2013
I'll accept a comprehensive survey over anecdotal data any day.
RealityCheck
2.7 / 5 (12) Jan 04, 2013
Hi rod_russell_9.
What heresy! Every idiot knows that whatever bad happens weather-wise is due to man-induced global warming, and the only solution is to give control of all fossil fuels to the United Nations!


Effectively what they are saying is that the patterns/variables are MORE complex because of the increased warming (energy) in the planetary climate system dynamics.

It IS warming and it WILL create more EXTREME maxima/minima of existing phenomena like droughts and floods and cold storms and heatwaves etc etc.

It's just that NOW the 'prediction' of climate 'locally (weather) is recognized as becoming even MORE complex than thought because global warming is NOT just about the 'warming', it's about what changes that warming will bring as the system tries to maintain balance and 'dump' energy wherever it can during 'transition' to new (extreme) weather/climate/pattern 'norms'. That's all. :)

Cheers!

Maggnus
2.8 / 5 (9) Jan 04, 2013
What heresy! Every idiot knows that whatever bad happens weather-wise is due to man-induced global warming, and the only solution is to give control of all fossil fuels to the United Nations!


What an idiot.
Maggnus
2.8 / 5 (9) Jan 04, 2013
Effectively what they are saying is that the patterns/variables are MORE complex because of the increased warming (energy) in the planetary climate system dynamics.

It IS warming and it WILL create more EXTREME maxima/minima of existing phenomena like droughts and floods and cold storms and heatwaves etc etc.

It's just that NOW the 'prediction' of climate 'locally (weather) is recognized as becoming even MORE complex than thought because global warming is NOT just about the 'warming', it's about what changes that warming will bring as the system tries to maintain balance and 'dump' energy wherever it can during 'transition' to new (extreme) weather/climate/pattern 'norms'. That's all. :)

Cheers!


Very well explained!
Howhot
3.7 / 5 (6) Jan 05, 2013
the only solution is to give control of all fossil fuels to the United Nations!

Hear Hear! Sounds like a great idea. I applaud your deep thought processes in coming up with that solution. (sarcasm!)
Howhot
3 / 5 (4) Jan 05, 2013
At issue with predicting drought conditions;
The Palmer Index is too simple a model to take these changes into account.


To sum it up drought modeling;
"It certainly does not mean that things are not going to be bad in the future," he said. "It does not negate the idea that we are having an impact on the atmosphere and that this will have an impact on drought."


Bottom line is the Paulmer model for drought prediction is not reliable, but it does not reflect on the obvious fact that drought is directly correlated to climate change.
The Alchemist
1.6 / 5 (7) Jan 05, 2013
Bottom line is the Paulmer model for drought prediction is not reliable, but it does not reflect on the obvious fact that drought is directly correlated to climate change.

Sometimes the obvious is brilliant! Very well said.
The problem with all this biased studies and counter studies are they don't bother to look at the dependant variables, but look at confusing ones. For example, until VERY recently CO2 was not increasing. Therefore, CO2 was not causing warming. Nobody was looking at the fact we were releasing many nuclear-bomb equivalents of waste heat everyday. So the real question is, what effects, if any, does the release of waste energy from fossil fuels, etc., have on the environment. After 30 years of study, I can comfortably say two things: 1. Change is happening, and 2. You're soaking in it.
ValeriaT
1 / 5 (5) Jan 05, 2013
IMO the main portion of global warming is not of antropocentric, but geothermal origin. The Earth crust and oceans are heated with neutrino accelerated decay of radioactive elements, the potassium in particular. On the other hand, the global temperature increase has a low chance to change the global droughts, because the hotter the Earth is, the higher is the speed of water evaporation. What will be changed is the distribution of precipitation, because most of water will condense in coastal areas due the accelerated vertical circulation of the atmosphere.
The Alchemist
1 / 5 (4) Jan 06, 2013
Valeria-In 30 years I have never heard Neutrinos enter into the argument. 10 out of 10 for creativity. However, though novel, I expect no matter what the effect is/are, it has been in equilibrium with the Earth's heat cycle, just like other forms of Sun's energy. So without it, we may notice something, but with it-it is situation normal. I say again without sarcasm, that it is a novel approach, keep up that kind of thinking! Of course if you're thinking something else, I'll be just as estatic to be wrong. Best regards!
Mandan
3 / 5 (2) Jan 06, 2013
In central North America, it is estimated that nearly 800 trillion gallons of water remain in the Ogalalla aquifer. For decades this has been pumped out in unknown quantities. The level of the aquifer has dropped over 100 feet in many places during that time.

This water is transpired through the crops and/or evaporates, massively bolstering evapotransport over the region. There is no doubt such groundwater is being similarly used all over the world in arid regions to augment rainfall. I think I saw an article here estimating that one eighth inch of sea level rise is due to this modern exploitation of deep fossil water.

The rate of most depletion cannot be offset by natural recharge, and these irrigation sources are running out at an alarming rate. Without this augmentation, we may very well see an equal and opposite reaction in terms of rainfall/drought that will more than make up for the "transfusions" we have enjoyed, and then we may find ourselves becoming very "dry" indeed.
The Alchemist
1 / 5 (5) Jan 06, 2013
It's just that NOW the 'prediction' of climate 'locally (weather) is recognized as becoming even MORE complex than thought because global warming is NOT just about the 'warming', it's about what changes that warming will bring as the system tries to maintain balance and 'dump' energy wherever it can during 'transition' to new (extreme) weather/climate/pattern 'norms'. That's all. :)

Cheers!


Not for nothing, but I've had a predictive model for years, that woks to any degree of fidelity to the limit of your personal patience. The zero-level is very simple: The Earth-Sun are at near equilibrium, there is very little waste heat or heat deficit from the Sun. What's left? Waste heat generated by people. What happens as a result of waste heat? Warming? Not much, the Earth taps its own resouces (glacier melting) to compensate.
To complete the model, you yourself look at your geography, prevailing weather patterns, and ask, if the above is true, what happens next and SHAZAM!
ScooterG
1.6 / 5 (8) Jan 07, 2013
"But new research from Princeton and the Australian National University in Canberra has found that this might not be the case."

Amazing! Physorg seldom publishes anything that does not edify the global warming furor. Their (physorg's) lead climate change editor must've been on vacation.

The Alchemist
1 / 5 (5) Jan 08, 2013
Scooter-Can I suggest a different perspective? I think six and more years ago Phys.org would be going against GW. It's just that evidence is coming into its own.
Don't worry though, in addition to predictive models of for GW, I predict the media (probably along with the declining sun-spot cycle) will present overwhelming proof GW is declining.
They won't be wrong, (or right) just confusing people, as par, they're measuring the wrong variable.
Let me ask it this way, instead of calling it "warming," what if we just ask the question: What if we were to increase the Enthalpy of the planet Earth, in addition to the Sun's energy, by, say, burning fossil fuels?
cantdrive85
1 / 5 (4) Jan 09, 2013
Here is an article that discusses the variability of the Sun and the climate, this is not properly accounted for in the "models".

http://science.na...climate/

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