Megadroughts are defined as periods of drought that exceed two decades. According to a team of researchers led by the University of California, Los Angeles (UCLA), the drought that has enveloped the southwestern part of North America (from southern Montana to northern Mexico, and from the Pacific Ocean to the Rocky Mountains) in the past 22 years is the region’s driest megadrought since at least the year 800.
The high temperatures and low precipitation levels this area faced in 2020 and 2021 has caused the current drought to exceed in severity a late-1500s megadrought which had previously been identified as the driest in the past 1,200 years.
The scientists calculated the intensity of droughts by analyzing tree ring patterns, which can provide accurate information about annual soil moisture levels during extended time periods. They found that, since 2000, the average soil moisture deficit (a metric describing how little moisture the soil contains compared to normal levels of saturation), was twice as severe as during any other drought of the 20th century, and higher than it was over even the driest periods of the past 12 centuries.
Although the current megadrought would have been quite severe even without global warming, researchers argue that anthropogenic climate change may be responsible for approximately 42 percent of the soil moisture deficit since 2000. Over the past two decades, temperatures in the region were 0.91 degrees Celsius higher than the average from the second half of the 20th century, significantly increasing evaporation, a process that dries out soil and vegetation.
“Without climate change, the past 22 years would have probably still been the driest period in 300 years,” said study lead author Park Williams, an associate professor of Geography and Bioclimatology at UCLA. “But it wouldn’t be holding a candle to the megadroughts of the 1500s, 1200s, or 1100s.”
According to Professor Williams and his colleagues, this megadrought is causing significant water shortages in the area, with two of the largest water reservoirs in North America (Lake Mead and Lake Powell), reaching their lowest recorded levels.
The water conservation measures implemented by local regulators in response to low water levels were helpful in the short term. However, the scientists argue that water conservation efforts extending beyond times of drought are necessary to ensure that people have sufficient water as climate change continues to cause increasingly more frequent and severe droughts.
The study is published in the journal Nature Climate Change.