All Dried Up: Drought Variability Modeling and the Megadroughts to Come

The Dust Bowl doesn’t currently get much attention. But that eight-year onslaught of severe dust storms in the 1930s wreaked havoc over prairie ecosystems and puts California’s current four-year drought into perspective. However, even these events pale in comparison to history’s periodic multi-decade “megadroughts.” For the first time, climate scientists are taking a look at past megadroughts to glean information about risks of future droughts brought on by climate change. Through the use of robust models and metrics, scientists at Columbia University’s Lamont-Doherty Earth Observatory are exploring what the future holds for the U.S. Southwest and Great Plains. Some of their research, recently published in Science Advances, predicts that in the latter half of the 21^st century these regions will experience chronic drought far worse than anything in the physical record. This prediction is bleak when reflecting on the devastation that arose from similar periods in the past. One megadrought in the late 13^th century is believed to have caused the decline of the Ancient Pueblo Peoples in the Colorado Plateau—a civilization that was far less populous and resource-intensive than our own.

To overcome the limitations of individual models and metrics typically used to study drought, the researchers combined a variety of approaches. They used 17 state-of-the-art general circulation models (GCMs) to collect large-scale soil moisture measurements. The Palmer Drought Severity Index (PDSI), which calculates soil moisture availability by analyzing the balance between precipitation and evapotranspiration, was modified to address previous critiques and limitations. And finally, the researchers combined these tools with data from the North American Drought Atlas (NADA). NADA, a tree-ring-based reconstruction of the PDSI, provides data on drought variability over the last millennium. This data shows that both the Southwest and Central Plains experienced more severe megadroughts during the Medieval periods (1100–1300) than during any other period on record.

The researchers ran multi-model analyses for two different scenarios: one “business-as-usual” scenario where greenhouse gas emissions would continue increasing in a fashion similar to recent years, and one moderate emissions scenario where emissions would be curbed in some fashion. Of the 17 models tested for both the Central Plains and the Southwest regions, only two predict wetter conditions in the Central Plains, and one predicts wetter conditions in the Southwest. The business-as-usual model reveals that in both the Southwest and Central Plains regions, drying trends in the later 21^st century will primarily result from responses to increased greenhouse gas concentrations.

Responses in the Southwest will manifest through a reduction in precipitation during cold seasons, as well as an increase in evaporative processes due to the warming climate. Though models suggest an increase in precipitation over certain parts of California, these trends are overwhelmed by the increase in evaporation, producing an overall decrease in soil moisture that could lead to drying periods lasting from 10–20 years. Precipitation responses in the Central Plains are less consistent, but there is an increased evaporative trend in many regions. In the moderate emissions scenario, significant drying also occurs but at a reduced level.

Risk calculations for the occurrence of decadal or multi-decadal drying were calculated for two periods. From 1950 to 2000, the risk of a megadrought in both regions is less than 12 percent. For the period from 2050 to 2099, most projections predict an astonishing 80 percent risk of multi-decadal megadrought.

Cook and his colleagues from Columbia University believe there will be an “unprecedented climate shift” in the latter part of the century, bringing about dryness “exceeding even the persistent megadroughts that characterized the Medieval era.” Note that the certainty of these projections seems to stand in stark contrast to the more ambiguous tone of the most recent assessment report from the Intergovernmental Panel on Climate Change. However, consistent results from almost all 17 general circulation models as well as the PDSI measuring precipitation and evaporation point to the same looming reality spurred by climate change: In the Central Plains and Southwest regions, 35-year (or longer) droughts of unparalleled intensity may become commonplace. Given the increasing number of people moving to these regions, as well as their high demands for water resources, it may be wise to treat this possibility with urgency.

 

Article Source: “Unprecedented 21^st century drought risk in the American Southwest and Central Plains” Cook, Ault, & Smerdon. Science Advances, 2015.

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