Nine Western states are currently in the midst of a historic “megadrought”, according to scientists at Columbia University who just published their findings in the journal Science. They say it’s the worst one in 500 years and has likely been ongoing since 2000.
The researchers were studying the occurrence of megadroughts in the Western U.S., relying heavily on tree ring data to measure the amount of rainfall and soil moisture over the centuries. A megadrought doesn’t have a strict scientific definition but they are broadly recognized as severe and extended periods of drought typically lasting a couple of decades, at least. The study shows the years 2000 to 2018 rank as the second-driest 19-year period in the past 1,200 years, exceeded only by the last megadrought event that happened between 1576 and 1603.
The current megadrought is gripping a wide swath of the southwestern United States and northern Mexico. Park Williams, a bioclimatologist at Columbia University and lead author of the study says the severity varies throughout the region, “But when you look at the region as a whole, this is really truly a monumental event.”
Over the past 20 years, California has had three stretches of short-term drought, Williams said. They were 2000-2003, 2007-2009 and 2012 to 2016. “In California it looks more like three individual droughts, but when we look at the larger scale, we can see it’s really one,” Williams said.
The most severe conditions over the past two decades have come in Arizona and Southern California, the research shows. Overall, a record number of wildfires, hundreds of millions of dead trees in Western forests, declining groundwater levels and drying soil moisture levels are all evidence of the current megadrought, the researchers said.
California suffered through a five-year drought from 2012 to 2016. Former Gov. Jerry Brown declared it over when reservoirs filled after huge storms in 2017. But Bill Patzert, a retired oceanographer and research scientist for 35 years at NASA’s Jet Propulsion Laboratory, says what he’s learned about droughts in the West is that they are large and they are long. “They don’t go for a few months. They wax and they wane. They are on-again off-again. Everybody is too quick to terminate droughts.”
Patzert and others say the best way to measure whether a drought is over in the West is to look at the level of Lake Mead, the vast reservoir behind Hoover Dam on the Colorado River, which flows through seven Western states. The lake, a critical water source for the West, is currently just 44% full.
One of the more disturbing finds in the study is that the 20th century was the wettest century in the entire 1,200 years the researchers looked at. That means that despite record levels of rainfall, the Western U.S. is still in a historic drought.
Natural climate cycles play a big role in the current drought but the study concludes climate change is also a driving force. The models they used to investigate its influence on the current situation suggest that the warmer and drier conditions brought by climate change account for nearly half the severity of the current drought.
The study’s authors say the findings should serve as a reminder that previous droughts in western North America have been just as severe and even longer than the current event—with only natural climate cycles to blame. Prior research looking thousands of years into the past has suggested that some previous dry periods in the western United States have lasted for hundreds of years at a time. The study can be found HERE. I also included a couple of drought maps below. It’s crazy to look at the rest of the country and how little drought there is compared to the some of the previous years. (Sources: Scientific American, Mercury News)