The U.S. Southwest has been enduring two decades of severe drought, and while March brought some rain to parts of the region, warm and dry conditions in April and May have led to fast melting of the winter snowpack in the Colorado Rockies. In fact, the persistent heat and dryness have led some researchers to think the region is now in the middle of a megadrought, informs the New York Times.
While we expect droughts to end, and a wet spring and good snowpack in the fall and winter of 2019 were reason for optimism, stream flows peaked this spring as snow melted but decreased quickly due to warm spring temperatures. In addition, runoff was not as high as usual because the dry vegetation and soil soaked up the moisture.
Snowpack is a crucial source of water for the western United Sates, accounting for as much as 75% of the freshwater. It is used to predict droughts in the region. But droughts are complex processes, where factors like temperature, precipitation, humidity, wind and cloud cover come into play. Researchers are studying their interactions and relative contribution, relying to a large extent on computer models, as in some cases little data is available.
Hot and dry weather has continued in much of the Southwest in June. According to the United States Drought Monitor, moderate to extreme drought affects now parts of Colorado, New Mexico, Arizona and all of Utah.
With no precise definition, a megadrought would be a dry period that lasts several decades, and they have occurred several times in the Southwest in the past 1,200 years. By studying tree growth as an indicator of soil moisture, scientists have determined there have been four megadroughts longer than 20 years each since 800 A.D. The study, led by Park Williams, bioclimatologist at Lamont-Doherty Earth Observatory at Columbia University, was published in Science last April. A megadrought in the 13th century lasted more than 90 years, while the most recent one occurred in the 16th century. Different factors affect dry conditions in the Southwest, including the El Niño-Southern Oscillation (ENSO) cycle, with global warming playing a significant role since the 20th century.
Dr Williams hypothesises that maybe the last 400 years were wetter than the long-term average ─ for reasons yet unknown ─ and we could just be returning to that average. In that sense, the 1900s’ development and overallocation of water resources came at a bad time. In fact, scientists are starting to use “aridification” to refer to the transition to a increasingly hot and dry climate in the Colorado basin, rather than just a temporary drought, as Dr Brad Udall noted in his blog post last week.