The southwestern United States has not experienced such a drought in 1,200 years

Southwestern North America has been experiencing a mega-drought since the year 2000. A recent study shows that 42% of this is due to man-made warming and that such extreme situations will only increase as the century progresses.

The southwestern part of North America has been experiencing low precipitation and aridity since 2000. These particularly dry conditions over the past 22 years are unprecedented since at least 1901. Those years were punctuated by an extreme drought between the summer of 2020 and the summer of 2021, forcing unprecedented restrictions on the use of the Colorado River. A new study, published in Nature on Monday, February 14, by research climate scientist A. Park Williams and colleagues, found that 42% of this mega-drought in southwestern North America is induced by anthropogenic climate change.

A dendrological approach to measuring soil moisture levels

For this study, teams of climate scientists from UCLA, Nasa, and Columbia University examined soil moisture levels through analysis of tree ring model data. Soil moisture is a particularly important factor as it impacts agricultural productivity and irrigation quantity, ecosystem productivity and well-being, but also forest fire activity and heat wave intensity.

With this dendrological approach, scientists were able to observe that tree rings have a weak growth pattern, typical of dry periods. These observations are in agreement with the average precipitation between 2000 and 2021, which is 8.3% lower than the average between 1950 and 1999, contributing to a decrease in soil moisture and thus to an increase in drought.

Furthermore, when comparing with the infamous mega-droughts that occurred several times between the 800s and 1600s, scientists found that the one in our millennium was much more severe, ranking the last 22 years as the driest period in at least 1,200 years.

Drier decades ahead

Although there have been breaks in these episodes of abnormally dry conditions during these 22 years, aridity continues to dominate. The paper’s lead author, A. Park Williams, had conducted a similar study in 2018 and assumed that this long drought event might have ended in 2019 due to high precipitation in that year. However, the drought had resumed in 2020 and exceptionally intensified in 2021, indicating to climate scientists that it was far from over and that even drier decades would come. Indeed, the team of researchers showed that rising temperatures will increasingly increase the risk of long, severe conditions like this. In fact, they even show that, without anthropogenic climate change, the period 2000-2021 would not have seen even a single prolonged drought event like now.

In addition to the fact that they will increase during this century, these droughts are likely to spread geographically and appear on other continents. However, one of the authors, Jason E. Smerdon, remains optimistic because our knowledge of what is happening and what will happen gives us the ability to take action and anticipate these extreme periods so that they are less difficult to live through.