What are the historically longest droughts in California

USA: West suffers from "mega-drought"

Fatal climate change: The western United States is currently experiencing the second worst dry spell in the last 1,200 years, as tree ring data show. Of the last four “mega-droughts”, only one was more severe at the end of the 16th century. While dry periods are quite normal for this region, it was only climate change that turned the current drought into a “mega-drought”, as researchers report in the specialist journal “Science”.

The southwest of the USA has suffered from chronic water shortages since 2000: In winter there is too little rain and snow, the rivers have low water levels and the summers are hotter and drier than usual in the 20th century. In principle, such drought periods are not unusual for the arid western United States: "Paleoclimatic data show that the southwest of North America has experienced many strong changes in the hydroclimate before," explain Park Williams of Columbia University and his colleagues.

Four historic mega-droughts in the past 1,200 years

But how “normal is the current dry period? And what role does climate change play? Williams and his team have now examined these questions by looking back at the past. For their study, they evaluated more than 1,500 tree ring data in order to reconstruct the climate and, above all, moisture history over the last 1,200 years from Oregon and Montana in the north to California to northern Mexico.

The result: Since the year 800 there have been many shorter dry periods in the southwest of North America, but only four phases classified as "mega-droughts". Such periods are periods of severe drought that last for more than around two decades. Such mega-droughts occurred in the late ninth, mid-12th, 13th, and late 16th centuries.

Current drought is second worst - yet

But the current dry period is already more serious than three of the four previous mega-droughts: "The period from 2000 to 2018 is the second driest 19-year period since the year 800," Williams and his team report. “We have enough data to say that we are on the same course as the worst droughts in the past.” Only the mega-drought from 1575 to 1603 surpasses the current one.

However: the difference between the mega-drought of the 16th century and today's is no longer great. The water deficit was around 0.8 standard deviations below the long-term average; today it is 0.74, as the researchers report. On the other hand, the current drought affects a larger area than that of the Middle Ages - and it is still going on.

Exacerbated by climate change

But what is the reason? It is clear that the southwest of North America naturally shows strong climatic fluctuations, as the scientists explain. But as they determined with the help of a climate simulation, this natural variability is increasingly being influenced by climate change. According to their calculations, it is responsible for around 47 percent of the current dry period.

"Even without the anthropogenic climate trends, the period from 200 to 2018 would be classified among the eleven longest droughts in the reconstruction," explains Williams and his team. “But anthropogenic warming was decisive in bringing the current drought on a course that would raise it to the level of the most severe mega-droughts in the past.” The higher air temperatures of the present increase evaporation from the soil and thus exacerbate the water shortage.

"We will need more luck"

"Because it is getting warmer overall, the dice are increasingly tipped towards longer and more severe dry spells," says Williams. If we're lucky, the natural variability will soon bring us more rain again. But the further this development goes, the more luck we will need to break out of the drought. "

If the drought continues in the southwestern United States, the researchers do not rule out that the current mega-drought turns out to be the worst in 1,200 years. (Science, 2020; doi: 10.1126 / science.aaz9600)

Source: Earth Institute at Columbia University

April 20, 2020

- Nadja Podbregar