Drought may threaten much of globe within decades

Oct 19, 2010
Widespread drought in 2099, based on current projections of greenhouse gas emissions. Credit: UCAR

The United States and many other heavily populated countries face a growing threat of severe and prolonged drought in coming decades, according to a new study by National Center for Atmospheric Research (NCAR) scientist Aiguo Dai. The analysis concludes that warming temperatures associated with climate change will likely create increasingly dry conditions across much of the globe in the next 30 years, possibly reaching a scale in some regions by the end of the century that has rarely, if ever, been observed in modern times.

Using an ensemble of 22 computer climate models and a comprehensive index of conditions, as well as analyses of previously published studies, the paper finds that most of the Western Hemisphere, along with large parts of Eurasia, Africa, and Australia, will be at risk of extreme drought this century.

In contrast, higher-latitude regions from Alaska to Scandinavia are likely to become more moist.

Dai cautioned that the findings are based on the best current projections of . What actually happens in coming decades will depend on many factors, including actual future emissions of greenhouse gases as well as natural climate cycles such as El Niño.

The new findings appear this week as part of a longer review article in Wiley Interdisciplinary Reviews: Climate Change. The study was supported by the National Science Foundation, NCAR's sponsor.

"We are facing the possibility of widespread drought in the coming decades, but this has yet to be fully recognized by both the public and the climate change research community," Dai says. "If the projections in this study come even close to being realized, the consequences for society worldwide will be enormous."

Drought in the year 2069, as determined in a new analysis by scientists at NCAR. Credit: UCAR

While regional climate projections are less certain than those for the globe as a whole, Dai's study indicates that most of the western two-thirds of the United States will be significantly drier by the 2030s. Large parts of the nation may face an increasing risk of extreme drought during the century.

Other countries and continents that could face significant drying include:

  • Much of Latin America, including large sections of Mexico and Brazil
  • Regions bordering the Mediterranean Sea, which could become especially dry
  • Large parts of Southwest Asia
  • Most of Africa and Australia, with particularly dry conditions in regions of Africa
  • Southeast Asia, including parts of China and neighboring countries
The study also finds that drought risk can be expected to decrease this century across much of Northern Europe, Russia, Canada, and Alaska, as well as some areas in the Southern Hemisphere. However, the globe's land areas should be drier overall.

"The increased wetness over the northern, sparsely populated high latitudes can't match the drying over the more densely populated temperate and tropical areas," Dai says.

A climate change expert not associated with the study, Richard Seager of Columbia University's Lamont Doherty Earth Observatory, adds:

"As Dai emphasizes here, vast swaths of the subtropics and the midlatitude continents face a future with drier soils and less surface water as a result of reducing rainfall and increasing evaporation driven by a warming atmosphere. The term 'global warming' does not do justice to the climatic changes the world will experience in coming decades. Some of the worst disruptions we face will involve water, not just temperature."

A portrait of worsening drought

Previous climate studies have indicated that global warming will probably alter precipitation patterns as the subtropics expand. The 2007 assessment by the Intergovernmental Panel on Climate Change (IPCC) concluded that subtropical areas will likely have precipitation declines, with high-latitude areas getting more precipitation.

In addition, previous studies by Dai have indicated that may already be having a drying effect on parts of the world. In a much-cited 2004 study, he and colleagues found that the percentage of Earth's land area stricken by serious drought more than doubled from the 1970s to the early 2000s. Last year, he headed up a research team that found that some of the world's major rivers are losing water.

In his new study, Dai turned from rain and snow amounts to drought itself, and posed a basic question: how will affect future droughts? If rainfall runs short by a given amount, it may or may not produce drought conditions, depending on how warm it is, how quickly the moisture evaporates, and other factors.

Droughts are complex events that can be associated with significantly reduced precipitation, dry soils that fail to sustain crops, and reduced levels in reservoirs and other bodies of water that can imperil drinking supplies. A common measure called the Palmer Drought Severity Index classifies the strength of a drought by tracking precipitation and evaporation over time and comparing them to the usual variability one would expect at a given location.

Dai turned to results from the 22 computer models used by the IPCC in its 2007 report to gather projections about temperature, precipitation, humidity, wind speed, and Earth's radiative balance, based on current projections of greenhouse gas emissions. He then fed the information into the Palmer model to calculate the PDSI number. A reading of +0.5 to -0.5 on the index indicates normal conditions, while a reading at or below -4 indicates extreme drought.

By the 2030s, the results indicated that some regions in the United States and overseas could experience particularly severe conditions, with average readings over the course of a decade potentially dropping to -4 to -6 in much of the central and western United States as well as several regions overseas, and -8 or lower in parts of the Mediterranean. By the end of the century, many populated areas, including parts of the United States, could face readings in the range of -8 to -10, and much of the Mediterranean could fall to -15 to -20. Such readings would be almost unprecedented.

Dai cautions that global climate models remain inconsistent in capturing precipitation changes and other atmospheric factors, especially at the regional scale. However, the 2007 IPCC models were in stronger agreement on high- and low-latitude precipitation than those used in previous reports, says Dai.

There are also uncertainties in how well the Palmer index captures the range of conditions that future climate may produce. The index could be overestimating drought intensity in the more extreme cases, says Dai. On the other hand, the index may be underestimating the loss of soil moisture should rain and snow fall in shorter, heavier bursts and run off more quickly. Such precipitation trends have already been diagnosed in the and several other areas over recent years, says Dai.

"The fact that the current drought index may not work for the 21st century climate is itself a troubling sign," Dai says.

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Provided by National Center for Atmospheric Research

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User comments : 19

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2.3 / 5 (9) Oct 19, 2010
Drought may threaten much of globe within decades

Or not.
1.8 / 5 (10) Oct 19, 2010
Australia and South Africa had very good rains last season, the Murrey is in flood.

Another worthless computer modelling exercise.

When will go back to proper evidence based science and stop playing computer games?
not rated yet Oct 19, 2010
2.8 / 5 (4) Oct 19, 2010
In the past, the only reliable guide we have, colder climates have been drier and warmer climates wetter. Why should the current warming be different?
3.7 / 5 (6) Oct 19, 2010
Quibbling over the validity of experimental methods isn't going to be any help when drinking water isn't available for a couple billion people, much less the rain needed to grow their food.

The authors did state that there would be some inaccuracy in the model's predictions, but it should still be apparent that dire -if not catastrophic- straits lie ahead.

As far as everyone's belief that response to AGW is a government conspiracy to tax the hell out of good, god-fearing citizens is concerned, I have a 100% GUARANTEED prediction for you: in an ongoing basis, from this day forward into the future, the wealthy will continue to get wealthier, and everyone else will see net reduction in income and quality of existence.

If you happen to be a person not a part of the life-support system of the Plutocracy, your chances of having a substantially better life than you enjoy now are mainly nil, as the competition for ALL available positions is quickly eroding the valuation of EACH.

3 / 5 (4) Oct 19, 2010
In the past, the only reliable guide we have, colder climates have been drier and warmer climates wetter. Why should the current warming be different?

CS, that statement is only conditionally true, though. Yes, a warmer climate is wetter, but only to a point, and the same is true of a colder climate.

All of it is dependent on where it is that the conditions exist, to what degree, and over what period of time. The atmosphere is just as much a fluid as the hydrosphere, and local, temp-driven evapotranspiration and the precipitation side of the cycle are generally expanded by increasing temp, assuming that there aren't any geomorphological barriers that would otherwise foreshorten the process.

Since the bulk of solar irradiation occurs at the equator, those heat driven atmospheric convection cells of evaporatranspiration expand further and further N/S in latitude as more heat is added to the system. This will ultimately produce much dryer conditions at mid-latitudes.
5 / 5 (4) Oct 19, 2010
Drought and heavily populated countries is a problem now and will get worse as population increases.

We already (should) plan for the 1 in a 100 year flood when developing urban development. With a global economy we have 1 country in drought and the other countries provide aid. Thinking ahead there is bound to be times when several countries that currently provide much of the world food excess will be in drought at the same time.

When this happens there will be severe economic shock to go with the starvation and civil unrest.
4 / 5 (3) Oct 19, 2010
What is the problem with computer simulations? We have crude methods and equations that can predict future events to some degree of accuracy and we use computers to make the calculations less of a burden to ourselves. They are not computer games. Why can't we do science the old way with proper evidence based science? We do, but evidence is hard to come by when trying to predict the future since all the evidence is..........ready? IN THE FUTURE!!. Tell ya what, when the future comes, I'm sure they'll use that evidence to gauge their accuracy and adjust their computer games. Why would earth not follow the same colder/drier, warmer/wetter pattern? I don't know, I'm not an expert in this subject. But my guess is that earth doesn't like choosing between two option all the time. Things change, we never have the same initial conditions and our observations are simply a sliver in the vastness of time. We may have just linearized a clearly non-linear function.
1 / 5 (3) Oct 19, 2010
What invalidates these simulations from very onset is that they assume everything else being constant, which in dynamic systems never is.

Feedback loops of mankind adapting and competing for resources in response to change is likely to make the ultimate outcome either significantly worse or better.

Yet another study demonstrating that scientist time is generally better spent looking for solutions rather than predictions.

5 / 5 (3) Oct 19, 2010
they assume everything else being constant
They do no such thing. They account for dynamism in atmospheric and oceanic circulation. They account for dynamism in vegetation and land use, as well as aquifers, rivers, lakes, the tundra/permafrost, ice and glacier floes and cleaving, and so forth. And they account for dynamism in emissions control.

When it comes to land use, emissions control, population growth and things like that, they obviously have no choice but to rely on projections. However, those aren't some arbitrary numbers plucked from a random number generator: they're based on real economic and demographic forecasts. In fact, they're based on overly cautious and CONSERVATIVE forecasts -- for which the IPCC has been criticized quite strongly.

One thing is certain: the current level of greenhouse gas emissions is quite unlikely to reverse its rate of change any time soon.
1.4 / 5 (5) Oct 19, 2010
The Saudis are growing wheat and raising dairy cattle.
Maybe the ROW may have to learn how to stop wasting water and learn how to recycle it.
2.3 / 5 (3) Oct 20, 2010
It's a yet another pre-Cancun propaganda shot by climate models hong-wei-bings. Numerical climate models are pseudo-science. They're not based on sound physics, because they can't be: there are severe limitations to what can be done this way. They are physically incomplete for starters. Whatever physics is implemented is simplified through parameterized fits, slope corrections, and similar kludgery. They are of limited resolution: incapable of correct reproduction of turbulence cascades, boundary interactions, etc. Mickey Mouse cartoons all.
2.3 / 5 (4) Oct 20, 2010
Look at the "Related stories" box at top right. Predictions are all over the lot from rainy to dry. In the meantime, temperatures are about the same as for the last 10,000 years, with a few degrees of variability. Where is the missing heat? Where are any predictions which have come true? Crying wolf is wearing thin.
4.5 / 5 (2) Oct 20, 2010
Predictions are all over the lot from rainy to dry.
Looking at the map above, that shouldn't be surprising. Some areas get drier, others get wetter. On the whole, looks like the world's landmass is getting drier, though.
Where is the missing heat?
Right here:

Crying wolf is wearing thin.
Playing Polyanna isn't?
5 / 5 (2) Oct 20, 2010
Numerical climate models are pseudo-science.
Just climate models? All other types of numerical models are OK, then?
They're not based on sound physics, because they can't be: there are severe limitations to what can be done this way.
Oh really? Does this mean "sound physics" cannot in principle describe large-scale aggregate behavior accurately, and must always analyze every system at the Planck scale? Gee whiz; there went Cosmology, Geology, Anthropology, Biology, Linguistics, Ecology, Paleontology, Neuroscience, etc. and so on and so forth, not to mention pretty much all of Physics and Chemistry with the solitary exception of Quantum Mechanics...
They are physically incomplete for starters.
Unless you have discovered the Unified Theory of Everything, then so is every other field of science.
Whatever physics is implemented is simplified through parameterized fits, slope corrections, and similar kludgery.
Approximations can't possibly be good enough?
2.2 / 5 (6) Oct 24, 2010
Quibbling over the validity of experimental methods isn't going to be any help when drinking water isn't available for a couple billion people, much less the rain needed to grow their food.

100 1GW fission plants, with attendant desalination, will take care of that. We need the cheap power anyway.

Warmer climates mean longer growing seasons. If it does warm to the same level as the Medieval Climate Optimum; bully for us.
4 / 5 (2) Oct 24, 2010
Warmer climates mean longer growing seasons.
If the results reported above are anywhere near reality, then 100 1GW desalination plants will be a tiny drop in the bucket. Longer growing seasons aren't worth crap when you've got no water to grow things with.
1 / 5 (1) Oct 24, 2010
Warmer climates mean longer growing seasons.
If the results reported above are anywhere near reality, then 100 1GW desalination plants will be a tiny drop in the bucket. Longer growing seasons aren't worth crap when you've got no water to grow things with.

It doesn't take that much. Especially when it is recycled.
not rated yet Oct 26, 2010
Of course, one instance disproves the entire model, and every possible similar outcome.

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