In the August issue of Physics Today, climate scientists Toby Ault and Scott St. George share a pair of startling research findings. Between roughly 800 and 1500 CE, the American West suffered a succession of decades-long droughts, much longer than anything we’ve endured in modern history. And statistical models suggest that, as the climate warms, such megadroughts are increasingly likely to return.
How can scientists be so sure about the duration and extent of droughts that happened long before the era of instrument-based precipitation records? As Ault and St. George explain, the annual growth rings of ancient trees contain a rich paleoclimatic record of precipitation and soil moisture patterns. The width of a tree ring gives clues as to how well nourished the tree was in a given year. The map shows four western US megadroughts predicted from tree-ring data.
Ring-width analyses provide the most complete set of data on past moisture levels. But researchers have other ways of determining those conditions. Here are four of them:
Underwater tree stumps
Roughly the area of 35 000 football fields, Mono Lake, nestled in the eastern Sierra Nevada, is California’s fourth-largest inland body of water. Before 1940 it was even larger. That year, the city of Los Angeles began diverting water from the lake’s influent streams to provide municipal water. The receding shoreline of Mono Lake exposed two generations of low-lying tree stumps that had been hidden under the surface for centuries.
In 1994, California State University geographer Scott Stine used carbon dating to determine that the stump populations were the remains of trees that had died out around 1100 and 1350, respectively. A simple ring count suggested that at least some of the trees had lived for half a century before drowning under rising lake levels. Together, those pieces of information suggested that twice—once during the late 11th century and again during the early 13th century—Mono Lake dropped to exceptionally low levels for periods of 50 years or more. Stine’s study also yielded evidence of contemporaneous water-level drops in the Osgood Swamp and West Walker River, near Lake Tahoe, and in Lake Tenaya in Yosemite National Park. (S. Stine, Nature 369, 546, 1994.)
During protracted water shortages, societies often have no choice but to alter behavioral habits. During the California drought that lasted from 2011 to 2017, residents who watered their lawns, washed their cars with garden hoses, or committed other unnecessary water-related deeds risked being fined. But archaeological data suggest that those inconveniences pale in comparison with the disruptions suffered by pre-15th-century Native American cultures. Archaeologists can deduce those groups’ population shifts and migration patterns by carbon dating ceramic wares and other artifacts.
A 1979 study by a team of anthropologists, geologists, and botanists suggested that migration patterns of ancestral Puebloans, who populated the Four Corners region of the American Southwest, were closely linked to the availability of water. During drought periods, Puebloans left the warm, dry highlands for the cooler, wetter areas near riverbanks, which were easier to irrigate. The two largest migration events were the abandonment of the Grand Canyon and other uplands in 1150 and the abandonment of the Mesa Verde cliff dwellings in 1300. (R. C. Euler et al., Science 205, 1089, 1979.)
Follow the South Platte River northeast from Denver as it snakes toward Nebraska, and you’ll pass vast rolling hills covered in sagebrush. Those hills, like many others across the Great Plains region, contain a geomorphological record of ancient moisture conditions. During past extended dry spells that killed off the sagebrush, the hills came alive as aeolian sand dunes, shaped and fed by the wind. As moisture levels recovered, the dunes were stabilized by returning vegetation cover and coated with nutrient-rich soil. As wet and dry spells alternated, the dunes became stratified with alternating layers of soil and sand.
Twenty years ago, scientists from the US Geological Survey and the University of Colorado extracted meters-deep cores from northeastern Colorado’s dune fields and used radiocarbon dating to obtain a moisture history that went back more than 20 000 years. The team’s analysis suggested that sand dunes were last active in the region around the 10th century, around the same time that tree-ring data indicate a regional megadrought. (D. R. Muhs et al., Geomorphology 17, 129, 1996.)
Last year an estimated 27 million trees in California died due to the lingering effects of drought. Indeed, extended dry spells can remake entire ecosystems, with moisture-dependent flora dying off as drought-adapted species proliferate. As a result, ancient moisture conditions can often be estimated from changes in vegetation patterns, which in turn can be inferred from the composition of pollen deposits in sediment cores.
In 2015, a team of geologists applied that technique to a core extracted from the Santa Barbara Basin, just off the coast of Southern California. Because the basin is fed by rivers and runoff from inland mountains and foothills, the pollen grains deposited there closely reflect the composition of the inland vegetation. From the ratio of pollen from oaks, which thrive in cool woodland environments, to pollen from chaparral, which thrives under arid conditions, the researchers were able to compute a moisture index that dated back more than 1200 years. The index suggested that California suffered deep dry spells during the periods 800–1100 and 1200–1300, time spans for which tree-ring data suggest the region was struck repeatedly by megadroughts. (L. E. Heusser, I. L. Hendy, J. A. Barron, Quaternary Int. 387, 23, 2015.)