Source: Shutterstock

Ralph Alexander
Date: 7 August 2026

The next figure depicts observational data showing the percentage of the contiguous U.S. in drought from 1895 up until 2015. There’s clearly no trend over this period, apart from pronounced drought during the record heat waves of the 1930s.

North America

A 2007 U.S. study was able to reconstruct the drought pattern in western North America over the last 1,200 years, using tree rings as a proxy. The width and colour of the rings consti­tute a record of past climate, including droughts. The reconstruc­tion, illustrated in the following figure, reveals that several unprecedently long and severe “megadroughts” have occurred in North America since the year 800, droughts that the study authors re­mark have never been experienced in the modern era.

Comparison of the previous figure with the yearly data in this figure (indicated in gray; the thick black line is a 60-year mean), shows that the long-term pattern of overall drought in North America is featureless, despite global warming during both the Medieval Warm Period and today. A similar conclusion was reached by a 2021 study that compared the duration and sever­ity of U.S. hydrological droughts between 1475 and 1899 to those from 1900 to 2014. A hydrological drought refers to drought-induced decreases in streamflow, reservoir levels and groundwa­ter.

Medieval

That some histori­cal droughts were even longer and more severe than those of today can be seen in another tree-ring reconstruction, of drought in California where I live. The next figure shows the pattern of dry and wet periods in my drought-prone state over the past 1,200 years. Although the third-driest period in the 1100s and the fifth driest period in the 1200s both occurred during the Medieval Warm Period, the driest (1500s) and fourth driest (800s) periods of drought occurred during rela­tively cool epochs.

Just like North America, Europe has also experienced mega­droughts over the past millennium, although in different peri­ods. The figure below illustrates a 2021 European reconstruction, again from tree ring proxies, of the drought pattern in central Europe from 1000 to 2012, with observational data from 1901 to 2018 ­superimposed. Black in the figure depicts the PDSI or Palmer Drought Severity Index that measures both dryness (negative values) and wetness (positive values); red denotes the so-called self-calibrated PDSI (scPDSI); and the blue line is the 31-year mean.

The authors of the study point out that droughts from 1400 to 1480 and from 1770 to 1840 were much longer and more severe than those of the 21st century. Their conclusions are reinforced by the results of another recent study, which failed to find any statistically significant trend in meteorological droughts in western Europe during the last 170 years. A meteorological drought describes one arising from a precipitation deficit alone.

What stands out in all these studies is the lack of any long-term trend in drought worldwide over at least a millennium. There is no evidence of the warming that began in the late 19th century, after the Little Ice Age ended, having played any role so far.

Antarctica

Indeed, ice cores from Antarctica demonstrate that much more dust – a sign of a dry climate – was deposited during the ice ages than during warmer interglacial periods. Even the UNDRR (UN Office for Disaster Risk Reduction), in its Special Report on Drought 2021 which drew a link between drought and global warming, acknowledged that one of the main sources of episodic droughts globally is the natural El Niño Southern Oscillation.

There are regional variations, however. A 2021 research pa­per found that, from 1901 to 2017, the risk of meteorological droughts increased in the southwestern and southeastern U.S., while it decreased in northern states. Indeed, the UNDRR report found that such regional differences in drought are not restricted to the U.S. but occur worldwide.

Although generally caused by a severe fall-off in precipitation, droughts can be aggravated by other factors such as elevated temperatures, soil erosion and overuse of available groundwater. The consequences of drought, which can be catastrophic for human and animal life, include crop failure, starvation and mass migration. A major exodus of early humans out of Af­rica about 135,000 years ago is thought to have been driven by drought.

This article was published first on Science Under Attack.com on 3 August 2026

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