Perspective

Dry and drier

Short term and long, the U.S. outlook is parched

Bob Henson • August 6, 2012 | Whether you’re looking at the next few weeks or the next few decades, many parts of the United States are likely to face the silent but devastating impacts of drought. New research, and corrections to an earlier study, help bring this point home.

In 2010, NCAR scientist Aiguo Dai (who joins the faculty of the University of Albany, State University of New York, this fall) reviewed the state of knowledge on how drought is expected to evolve as Earth’s climate warms. His 2010 paper in Wiley Interdisciplinary Reviews: Climate Change (WIRES) included a global portrait of 20th century drought, based on observations, and a projection for the 21st century, based on 22 climate models. The resulting maps show much of the globe at an increasing risk of extreme drought conditions over the coming century, if we continue to emit heat-trapping greenhouse gases.

Revised drought maps for globe
Projections averaged across a suite of climate models show a progressively increasing drought risk across much of the globe, as measured by the Palmer Drought Severity Index (PDSI). These maps have been revised from earlier versions. For details and a larger-format version, see the associated news release.

This outlook is based on decadal averages of soil moisture content and the Palmer Drought Severity Index (PDSI), a widely accepted measure of drought severity. The PDSI is calculated from precipitation, temperature, radiation, and other factors that influence soil moisture.

For 2030–39, Dai’s maps (see right) show much of the central U.S. experiencing drier soils than in 2000–09. The drought risk expands further in the 2060s, and by the 2090s, most of southern Europe and about half of the United States is gripped by extreme drought. These projections are not meant to be literal forecasts for each decade, since there is no way to know exactly how natural variability will play out from decade to decade or how quickly greenhouse gas emissions will increase.

New models, similar picture

Dai has now strengthened the case by showing that climate models successfully replicate the effects of El Niño and La Niña on global wetness and drought, as well as a worldwide tendency toward drier soils over the last several decades. Dai’s new study, which appeared on August 5 in the online version of Nature Climate Change, also includes fresh projections for the 21st century that draw on new climate runs and scenarios developed for the next major assessment from the Intergovernmental Panel on Climate Change (IPCC).

Natural variations will shape the course of drying, Dai points out. Models don’t yet capture the important swings of the Interdecadal Pacific Oscillation (IPO), which affects ocean temperatures as well as North American climate. The oscillation, whose causes are poorly understood, has been in a cold (negative) phase since around 1999. As Dai outlined in another recent paper, this tends to foster drought across the western and central United States.

Each phase of the IPO typically lasts 20–30 years, which suggests that the dry conditions that have prevailed across much of the western United States for much of the last decade may last for another 10 to 20 years. After that, one might look for some long-term drought relief, as the IPO could swing back to a warm phase, during which western and central U.S. states typically see above-normal precipitation. However, Dai’s new work stresses that the drying effect of human-produced greenhouse gases should overwhelm natural variability by later this century.

“The U.S. may never again return to the relatively wet conditions experienced from 1977 to 1999,” he says.

How quickly will we dry out?

As noted in an update to this 2010 news release, Dai recently discovered an error in the way that he had extended 21st century projections from the year 2000 onward. He has since recalculated the projections and produced new maps, and a formal correction has been submitted to WIRES.

Quantitatively, the error in Dai’s original study is significant, overstating the severity of drought in some regions by a factor of two. However, the corrected maps displayed here continue to show that large parts of the world will continue to move toward increased drought risk, albeit at a slower pace—with the intensity in many locations roughly half as much as indicated in the original maps.

The drought projections for the 2090s generated for Dai’s newest paper with the latest IPCC models also show a similar spatial pattern with a reduced intensity. Dai emphasizes that the ultimate magnitude of the drying depends on the rate of future greenhouse gas emissions, while the spatial patterns would not be as directly affected by the rate. For both of his recent papers, Dai employed relatively moderate emission scenarios.

While the strength of expected drying is less than in earlier projections, the overall message from Dai’s work is unchanged: drought is likely to become an increasingly widespread and serious threat as this century unfolds. That general conclusion is shared by other recent studies, including a special report on climate extremes issued in late 2011 by the Intergovernmental Panel on Climate Change.

Wildfire smoke visible just west of NCAR's Mesa Lab on June 28, 2012
In this aerial photo, looking south, smoke from the Flagstaff Fire is visible about two miles (threee kilometers) southwest of the NCAR Mesa Laboratory on June 28, 2012. The building was evacuated for more than a day as near-record heat and tinder-dry conditions fueled the fire. (Photo © Marijke Unger, NCAR Computational & Information Systems Laboratory.)

Scientists are also going a step further, pairing global models with fire models to directly evaluate fire risk. A paper published early this year by scientists at the Max Planck Institute for Meteorology; Cornell; University of California, Irvine; and NCAR found that global fire emissions will rise substantially by 2100, with North America showing a strong trend toward increased emissions. And a major study involving 16 global models, published this month in the journal Ecosphere, found that “substantial and rapid shifts are projected for future fire activity across vast portions of the globe.”

Drought and heat

More drought and fire don’t necessarily imply less rain and snow. Some regions are projected to see drought risk going up even without any drop in overall precipitation.

Why the mismatch? It’s largely because the key drought-boosting variable looming in the future isn’t less rain and snow—it’s warmer air. Moisture evaporates from the ground more readily when the atmosphere is warmer, and temperatures rise more quickly when soils are dry. These processes work together to help intensify drought in the absence of rain-making weather features.

This dance of drought is playing out right now across much of the United States. A generally dry spring left soils parched, and a blistering summer has intensified the drying across much of the central United States. The weekly U.S. Drought Monitor issued on August 2 (see map below) shows severe, extreme, or exceptional drought now covering most of the area from Nevada to Illinois and from Nebraska to Oklahoma, as well as central Georgia.

Some hope for this autumn is evident in NOAA’s most recent seasonal drought outlook, issued on July 19. While drought is projected to persist across the Midwest and Great Plains, improvements are foreseen for the Southwest due to continued periods of showers and storms already being produced by an active North American Monsoon pattern. And most of the world’s longer-range computer models are pointing to the likelihood of an El Niño developing by year’s end. A strong El Niño would boost the odds of above-average precipitation during the winter months across many of the areas now experiencing drought.

One important caveat: El Niño doesn’t guarantee that moist conditions will stage a comeback—it simply hikes the odds of that outcome. In the meantime, the nation will likely continue to experience a taste of the longer-term drought conditions that may become increasingly common later this century.

U.S. Drought Monitor for June 28, 2012
The U.S. Drought Monitor issued August 2, 2012, for the week ending July 31 shows a patchwork of intensifying drought over much of the nation. (Image courtesy U.S. Drought Monitor.)