Southwestern North American megadrought

The southwestern region of North America (SWNA) is defined as the areas between 30 and 40 degrees North and 105- and 125-degrees West, comprising areas within Northern Mexico and the Southwestern United States. This area is roughly bounded by central Chihuahua to the southeast, the northwest coast of Baja California to the southwest, the Northern California coast to the northwest, and northcentral Colorado to the northeast. This area includes much of the Basin and Range Province, which contains the four deserts on the continent: the Chihuahuan Desert, the Sonoran Desert, the Mojave Desert, and the Great Basin Desert, as well as the Colorado Plateau, which is largely high desert.

Megadroughts, a term used to describe periods of multidecadal drought, are a recurring feature of the North American Southwest over the past millennium.^[3] For example, droughts lasting at least a decade occurred in Texas in each century of the past millennium.^[4] Researchers used tree ring chronologies to reconstruct summer soil moisture and snow water equivalents back to 800 CE. This allowed the identification of 40 SWNA drought events of at least 19-years duration. Of these, four megadroughts were 0.25 standard deviations drier than any experienced in the 20th century: 863–884, 1130–1151, 1276–1297, and 1571–1592.^[5] The droughts of the twelfth and thirteenth centuries bracketed the Ancestral Puebloans' Pueblo III Period, with the thirteenth century drought coinciding with the abandonment of Mesa Verde, Chaco Canyon, and other settlements in the Four Corners region.^[6][7] The sixteenth century megadrought may be associated with the 1576 cocoliztli epidemic in New Spain.^[8]

The megadrought that began in 2000 was preceded by the wettest period in at least 1200 years. From 1980 to 1998, climate models begin projecting increased decadal precipitation swings in the SWNA starting in the latter half of the twentieth century as a result of climate change, but with an overall drying trend as a result of warming. Models indicate that the years 2000–2018 would have trended towards megadrought conditions regardless of climate change, with a predicted severity of the 11th worst period of drought since 800. However, anthropogenic warming pushed conditions into a severe megadrought.^[5] From 2000 to 2021, mean annual precipitation in the region was 8.3% below the 1950–1999 average and the temperature was 0.91 °C above average. The megadrought that began in 2000 was the driest 22-year period since at least 800 and, if it persists through 2022, will match the duration of the severe late-1500s megadrought. Both 2002 and 2021 were drier than any of the previous nearly 300 years and were, respectively, the 11th and 12th driest years between 800 and 2021.^[9]

The drought is largely driven by temperature, which increases the rate of evaporation, with some contribution from the lack of precipitation. The several wet years since 2000 were not sufficient to end the drought. Researchers calculated that without climate change-induced evaporation, the precipitation in 2005 would have broken the drought. While monsoon rains in the desert Southwest in mid-2021 and heavy rain and snow in California in late 2021 had raised hope of ending the drought, January 2022 was characterized by record dry conditions across much of the West.^[10] Researchers noted that even in wet years in the Colorado River watershed, water from melting snows is soaked up by dry soils before it can reach the river.^[11]

Throughout history, California has experienced many droughts, such as 1841, 1864, 1924, 1928–1935, 1947–1950, 1959–1960, 1976–1977, 1986–1992, 2006–2010, 2011–2017, 2018, and 2020-^[12][13] 2021. Precipitation in California is limited to a single, fairly short wet season, with the vast majority of rain and snowfall occurring in the winter months across the state. This delicate balance means that a dry rainy season can have lasting consequences. California is the most populous state and largest agricultural producer in the United States, and as such, drought in California can have a severe economic as well as environmental impact. The historical and ongoing droughts in California are caused by lack of rainfall (or snowfall), higher average temperatures, and drier air masses in the atmosphere. This leads to less water availability in the natural environment and in snowpack, rivers, and reservoirs for human use; these water shortages can have major impacts on agriculture and other water-intensive land uses.

The 2017 Fourth National Climate Assessment (NCA4) noted that, under the worst-case scenario of RCP8.5, the annual average temperature of the Southwestern United States was projected to increase 8.6 °F (4.8 °C) by 2100. The southern Southwest could receive 45 additional days per year above 90 °F (32 °C). NCA4 noted that higher temperatures increased the probability of both droughts and megadroughts in the region.^[14]

In 2013 a group of scientists from the Princeton University published a study suggesting that total deforestation of the Amazon rainforest can strongly exacerbate drought conditions in the western states of the USA. According to the study "an Amazon stripped bare could mean 20 percent less rain for the coastal Northwest and a 50 percent reduction in the Sierra Nevada snowpack, a crucial source of water for cities and farms in California." Generally, the air in western USA from December to February will be drier.^[15][16]

Later studies found that "destruction of tropical forests is disrupting the movement of water in the atmosphere, causing major shifts in precipitation that could lead to drought in key agricultural areas in China, India, and the U.S. Midwest. ” Those include the 3 main tropical rainforest: Amazon, Congo, South-East Asia. 50% of the rain in the Midwest come from water evaporating from the land and the Amazon Rainforest probably providing part of it.^[17]