Sunday, April 19, 2020

U.S. Droughts Then And Now

The recent drought in the American Southwest was one of the three most severe in tree ring recorded history.
The drought in southwestern North America that lasted from 2000 to 2018 is among the most severe to strike the region in the last 1,200 years, a new study finds. Tree ring–based reconstructions of past climate reveal just one drier 19-year period: a powerful “megadrought” in the late 16th century. The recent drought, researchers say, was made 47 percent more severe by human-caused climate change
Tree rings are yearly growth bands of variable width, depending upon the ready availability of water. Using tree ring records from 1,586 sites across the western United States and northwestern Mexico — amounting to thousands of trees — hydroclimatologist Park Williams of Columbia University and colleagues created a climate history for the region going back to about the year 800. Between about 850 and 1600, several decades-long, intense “megadroughts” struck the region, on a scale not seen again until the present day, the researchers report in the April 17 Science. 
A particularly devastating drought that lasted from about 1575 to 1593 is recounted in historical records and tree ring reconstructions alike, Williams says. “That was a really impressive event, and kind of the last gasp of the megadrought era,” he says. The drought may have contributed to the abandonment of New Mexico pueblos and the devastating spread of disease brought by Spanish conquistadors among Native Americans. 
One of the biggest factors controlling precipitation in southwestern North America is the El Niño-Southern Oscillation, a natural cycle in which changes in tropical Pacific Ocean temperatures can alter regional weather patterns. During “La Niña” episodes of this pattern, colder Pacific sea surface temperatures create atmospheric waves that block Pacific storms from reaching southwestern North America, reducing rainfall. The 16th century megadrought, for example, coincided with a powerful La Niña event
From Science News, citing the following papers:
Severe and persistent 21st-century drought in southwestern North America (SWNA) motivates comparisons to medieval megadroughts and questions about the role of anthropogenic climate change. We use hydrological modeling and new 1200-year tree-ring reconstructions of summer soil moisture to demonstrate that the 2000–2018 SWNA drought was the second driest 19-year period since 800 CE, exceeded only by a late-1500s megadrought. The megadrought-like trajectory of 2000–2018 soil moisture was driven by natural variability superimposed on drying due to anthropogenic warming. Anthropogenic trends in temperature, relative humidity, and precipitation estimated from 31 climate models account for 47% (model interquartiles of 35 to 105%) of the 2000–2018 drought severity, pushing an otherwise moderate drought onto a trajectory comparable to the worst SWNA megadroughts since 800 CE.
A.P. Williams et al., "Large contribution from anthropogenic warming to an emerging North American megadrought."368 Science 314 (April 17, 2020). doi: 10.1126/science.aaz9600.
Historical documents from the Spanish Entrada on the northern frontier of New Spain (now the U.S. Southwest) include anecdotal evidence for unusual aridity in the late 16th century (1). However, a quantitative record of the 16th-century megadrought has only recently been obtained from hundreds of exactly dated and moisture-sensitive tree-ring chronologies developed across Canada, the United States, and Mexico. On page 314 of this issue, Williams et al. (2) provide a new assessment of proxy climate data from the U.S. Southwest. They determine that the 16th-century megadrought was the worst multidecadal drought episode in the Southwest over the past 1200 years, and that the second-worst event occurred from 2000 to 2018 over southwestern North America (SWNA) and may be ongoing. The study also pinpoints substantial anthropogenic (human) contribution to the severity of the current drought.
D.W. Stahle, "Anthropogenic megadrought." 368 Science 238 (April 17, 2020). doi: 10.1126/science.abb6902.

Another megadrought in the American Southwest that left archaeological traces despite actually being less severe than the one in 21st century, came in the late 1200s CE, when the Ancient Puebloans a.k.a. Anasazi precipitously left their complex of villages in Southwest Colorado in the vicinity of Mesa Verde.

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