Past Droughts Have Been Much More Severe

Scientists continue to publish new drought reconstructions indicating there were far more frequent and severe drought periods in the past several thousand years than anything observed in the modern period. ¹ California baked in two serious droughts about 1,100 years ago. The first lasting 220 years and the second 140 years. Each was much more intense than the dry spells Californians are experiencing at present. It's important to note that this was long before there were SUV's or coal fired power plants.

The water levels dropped during these periods, as much as 50 feet in some cases. The droughts were not only much longer, they were more severe than the drought of 1928 to 1934, California's worst in modern times, or the more recent severe dry spell of 1987 to 1992. Since 2000, the longest duration of drought in California lasted 376 weeks beginning on December 27, 2011 and ending on March 5, 2019. Note that all of the droughts since 1928 have been considerably less than the really long droughts from earlier times. ² The findings suggest that relatively wet periods like the 20th century have been the exception rather than the rule in California for at least the last 3,500 years, and that mega-droughts are likely to occur. All of this in an area that is 25% desert and these days has a population of 40 million. Mega-droughts, by definition, are occasional events of unusual severity lasting for at least 20 years. Looking at the data since 2000, it looks like we are currently in a mega-drought trajectory. And while 20 years is a long time to live with drought, the mega-droughts recorded in the paleorecord lasted far longer. ³ Kenneth Richard presents a recent review of a number of studies reporting that drought is now less common and severe than centuries ago. The following are from his article. ¹ A recent study indicates there has long been 'qualitatively incorrect' estimates of the impact modern and future warming has on drought trends and dryland expansion because atmospheric aridity is 'not an accurate proxy of the future extent of drylands.' Consequently, the authors suggest there will be 'no global drylands expansion under greenhouse warming, contrary to previous claims based on atmospheric aridity.' ⁴

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Instead of being driven by increasing greenhouse gas concentrations and warming, droughts along the western coast of the United States are 'predominantly (84%) driven by internal atmospheric variability' and cooling sea surface temperatures. ⁵ Many new drought reconstructions even indicate the modern period has recorded no increasing drought trends relative to the last few thousand years. The periods 1931-1938 and 1909-1917 were the longest and most severe wet and dry periods. ⁶ Despite its significant recent impacts, the 2000s drought is ranked as only the sixth driest run in the CHU reconstruction, is only half or less the duration of the 1700s, early 1900s, and 1950s droughts, and has a magnitude 50-75% less severe than these others. The magnitude of the 1950s drought in reconstruction is only 67% of the magnitude of the highest ranking droughts in the same record, 1818-1834 and 1728-1744. The 20th century has less frequent and long dry or wet periods. The findings are also consistent with studies that document pre-instrumental droughts that have been more intense or longer lasting than dry periods of the 20th century. ⁷ There are also a number of other references in Richard's article. ¹ An interesting tidbit- The IPCC's, AR4, report chapter 10, page 750 says: Mean precipitation-globally averaged mean water vapor, evaporation and precipitation are projected to increase. Hardly a recipe for drought.

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References

1. Kenneth Richard, “Several more new studies show drought is now less common and severe than centuries, millennia ago,” notrickszone.com, June 14, 2021
2. William K. Stevens, “Severe ancient droughts: a warning to California,” The New York times, July 19, 1994
3. Brian Handwerk, “The American West may be entering a mega-drought worse than any in historical record,” smthsonianmag.com, April 16, 2020
4. Alexis Berg and Kaighin A. McColl, “No projected global drylands expansion under greenhouse warming,” Nature Climate Change, 11, 331, 2021
5. Seung H. Baek et al., “USA Pacific coastal droughts are predominantly driven by internal atmospheric variability,” American Meteorological Society, 34, Issue 5, March 1, 2021
6. Liangjun Zhu et al., “Influence of the Atlantic Multidecadal Oscillation on drought in northern Daxing'an Mountains, Northeast China,” CATENA, 198, March 2021
7. Becky Brice et al., “Comparing tree-ring based reconstructions of snowpack variability at different scales for the Navajo Nation,” Climate Becky Brice et al., “Comparing tree-ring based reconstructions of snowpack variability at different scales for the Navajo Nation,” Climate Services, 22, April 2021