Worldwide Drought: Current Conditions and Future Predictions

From rising temperatures to rising seas, the consequences of global warming are here now, and predictions for the future are dire. In his review “Drought under global warming,” UCAR author Aiguo Dai indicates that yet another consequence is already affecting our planet and, over time, will almost certainly become more severe: worldwide drought.

Wait, drought is already a problem?
Research by Dai and other scientists shows that drought has increased worldwide over the last century. In one 2004 study, Dai showed that between 1870 and 2002, the percentage of land around the globe affected by serious drought more than doubled. In a later study, he examined 925 of the earth’s largest rivers and found a 30% decrease in water levels over the last 50 years.

So what does the future look like?
In his most recent study, Dai used projections about temperature, precipitation, humidity, wind speed, and Earth’s radiative balance to predict future drought locations and severities. In order to classify drought severity, Dai used the Palmer Drought Severity Index (PDSI): an index that tracks precipitation and evaporation over time and compares these measures to what would normally be expected in a particular region. A reading of +0.5 to -0.5 indicates normal conditions; a reading at or below -4 indicates extreme drought. By the 2030s, Dai’s projections show PDSI readings dropping to -4 to -6 over most of the western two-thirds of the United States, and -8 or lower in regions bordering the Mediterranean Sea. Large parts of Latin America, southern Asia, Africa, and Australia face significant drought risks, as well.

What can we do about it?
Geoengineering offers a number of large-scale solutions to global warming (and, consequently, the drought caused by it). These solutions fall into two main categories:

  1. CO2 Capture: removing carbon dioxide from the atmosphere. Methods include planting forests, using biofuels, and building carbon-scrubber towers to capture CO2 from the air. These are generally viewed as safe and technically feasible; however, they are also criticized as being too slow to take effect.
  2. Solar Radiation Management : reflecting sunlight back into the sky, which would lower the Earth’s temperature. Methods include using space mirrors; scattering fine white reflective particles of sulfate high into the stratosphere; or using ships to spray water and generate reflective low-altitude clouds. These can be implemented quickly; however, they run higher risks of disrupting the Earth’s ecosystems and causing other unforeseeable damage to the planet.

On a smaller scale, individuals can combat global warming with their own actions. The Union of Concerned Scientists (UCS) makes a number of suggestions, including using low- or no-emission tools, fertilizer, and pest control; planting shrubs and trees; composting organic waste; and watering plants during the coolest part of the day.

Read Dai’s most recent study, “Drought under global warming: a review,” freely accessible on WIREs.

Learn more about climate change and global warming at the University Corporation for Atmospheric Research (UCAR) website.

Find more suggestions from the UCS in their guide The Climate Friendly Gardener.

Check out Hack the Planet: Science’s Best Hope – or Worst Nightmare – for Averting Climate Catastrophe

by Eli Kintisch to learn more about geoengineering techniques for worst-case climate scenarios.

Look for The Warming Papers

edited by David Archer and Ray Pierrehumbert, coming in January 2011.



1. Dai, A., Trenberth, K., & Qian, T. (2004). A Global Dataset of Palmer Drought Severity Index for 1870–2002: Relationship with Soil Moisture and Effects of Surface Warming Journal of Hydrometeorology, 5 (6) DOI: 10.1175/JHM-386.1

2. Dai, A., Qian, T., Trenberth, K., & Milliman, J. (2009). Changes in Continental Freshwater Discharge from 1948 to 2004 Journal of Climate, 22 (10), 2773-2792 DOI: 10.1175/2008JCLI2592.1

3. Dai, A. (2010). Drought under global warming: a review Wiley Interdisciplinary Reviews: Climate Change DOI: 10.1002/wcc.81

Fitter Kids Means Bigger Brains

Parents take note: if you want your kids to grow bigger brains, think twice about letting schools cut recess or skimp on physical education.

Animal and human studies have long shown that exercise increases neurogenesis, especially in memory- and learning-related areas of the brain.1, 2 More recently, research on human adolescents has not only confirmed these findings, but highlighted the importance of physical activity for children.

Art Kramer and several colleagues at the University of Illinois published two studies this year that demonstrated correlations not only between exercise and improved cognitive abilities, but exercise and actual brain growth, as well. In the first study3, scientists recruited 9-and-10 year-old children who were either very physically fit or not fit at all. They then asked both groups to complete computer tasks measuring how well the children could filter out extraneous information and attend to relevant cues. Finally, they used MRI to measure the volume of certain structures in the children’s brains. What they found was that not only did the more physically fit children score better on the computer tasks, but their basal ganglia—a brain structure responsible for maintaining attention and coordinating actions and thoughts with precision—was also significantly larger.

In the second study4, another group of high- and low- fitness 9- and 10-year olds was recruited. This time the children were tested on their complex memory, for which brain activity has been linked the hippocampus. As may have been predicted from the results of the first experiment, MRI brain scans showed that the fitter children possessed larger hippocampi.

So if exercise promotes neurogenesis, what else might help grow your brain?

Alcohol. In moderation, alcohol may actually increase brain cells. Mice that consumed moderate amounts of ethanol experienced cell proliferation in their dentate gyrus, a part of the hippocampus.5

Chocolate. Cocoa, the major ingredient in chocolate, contains a antioxidant called epicatechin, which has been shown to improve spatial memory in mice.6

Marijuana. A controversial study found that stimulating rats’ brain receptors for marijuana increased neurogenesis.7 Gary Wenk at Ohio State University has continued in this line of research, exploring medicinal use of THC (the main psychoactive substance in marijuana) to prevent Alzheimer’s disease. However, the subject remains controversial, as a recent review in Drug and Alcohol Review will attest.8

1. van Praag, H. (1999). Running enhances neurogenesis, learning, and long-term potentiation in mice Proceedings of the National Academy of Sciences, 96 (23), 13427-13431 DOI: 10.1073/pnas.96.23.13427

2. Praag, H. (2008). Neurogenesis and Exercise: Past and Future Directions NeuroMolecular Medicine, 10 (2), 128-140 DOI: 10.1007/s12017-008-8028-z

3. Chaddock, L., Erickson, K., Prakash, R., VanPatter, M., Voss, M., Pontifex, M., Raine, L., Hillman, C., & Kramer, A. (2010). Basal Ganglia Volume Is Associated with Aerobic Fitness in Preadolescent Children Developmental Neuroscience, 32 (3), 249-256 DOI: 10.1159/000316648

4. Chaddock, L., Erickson, K., Prakash, R., Kim, J., Voss, M., VanPatter, M., Pontifex, M., Raine, L., Konkel, A., & Hillman, C. (2010). A neuroimaging investigation of the association between aerobic fitness, hippocampal volume, and memory performance in preadolescent children Brain Research, 1358, 172-183 DOI: 10.1016/j.brainres.2010.08.049

5. Åberg, E., Hofstetter, C., Olson, L., & Brené, S. (2005). Moderate ethanol consumption increases hippocampal cell proliferation and neurogenesis in the adult mouse The International Journal of Neuropsychopharmacology, 8 (04) DOI: 10.1017/S1461145705005286

6. van Praag, H., Lucero, M., Yeo, G., Stecker, K., Heivand, N., Zhao, C., Yip, E., Afanador, M., Schroeter, H., Hammerstone, J., & Gage, F. (2007). Plant-Derived Flavanol (-)Epicatechin Enhances Angiogenesis and Retention of Spatial Memory in Mice Journal of Neuroscience, 27 (22), 5869-5878 DOI: 10.1523/JNEUROSCI.0914-07.2007

7. Jiang, W. (2005). Cannabinoids promote embryonic and adult hippocampus neurogenesis and produce anxiolytic- and antidepressant-like effects Journal of Clinical Investigation, 115 (11), 3104-3116 DOI: 10.1172/JCI25509

8. DOWNER, E., & CAMPBELL, V. (2009). Phytocannabinoids, CNS cells and development: A dead issue? Drug and Alcohol Review, 29 (1), 91-98 DOI: 10.1111/j.1465-3362.2009.00102.x