Since the turn of the 21st century, researchers probing evidence locked in tree rings and other clues to past climate conditions have been building an increasingly unnerving picture of southwestern North America as prone to deep, prolonged droughts.
Megadrought is the emerging term for the worst of these extreme dry spells — those lasting two decades or more. What these scientists couldn’t have known until recently (that’s how long droughts work) is that while they were studying the drought history of this region, the area was sliding into a potent new megadrought — and a unique one because it wouldn’t exist without the growing boost from human-driven global warming.
A paper published on Monday in Nature Climate Change — co-authored by Columbia Climate School’s Jason Smerdon and Benjamin Cook — appropriately got a lot of news coverage for advancing understanding three ways:
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The researchers said the off-the-charts extreme dryness of 2020 and 2021 made this the worst megadrought in the region in at least 1,200 years.
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They found a firm signature of human-driven global warming, accounting for more than 40 percent of the severity of soil dryness. “In fact, without [human-driven climate change], 2000–2021 would not even be classified as a single extended drought event,” the authors wrote. (In the art above, figure b shows the differential depth of the drought with the amped-up greenhouse effect compared to what models projected if it was excluded.)
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They said it was likely that more years of extreme dryness and all that comes with it are still ahead before the drought breaks — as all droughts do. The lead author, A. Park Williams of the University of California, Los Angeles, stressed that climate change is not stopping: “Climate change is changing the baseline conditions toward a gradually drier state in the West and that means the worst-case scenario keeps getting worse.”
As you’ve seen if you tracked news coverage last summer of the unprecedented low reservoir levels behind the Hoover and Glen Canyon dams along the Colorado River, the West is finally recognizing that it faces a sustained and intensifying water crisis.
The crisis is driven only partly by the mix of natural climate variability and human-caused global warming. Decades of water-dependent population growth and economic development have created the classic “expanding bull’s eye” pattern that turns a natural hazard (in this case exacerbated by greenhouse gases) into an unnatural disaster.
At the grandest time scale, there’s another reason the West finds itself in such a profound hydrological predicament: a protracted megadrought gap. While tree rings show a series of megadroughts since the year 800 (those vertical pink bands in the graph at the top of this post and below), have a look at the yawning blank zone from roughly 1700 until 1900:
That was also a span when the colonial presence into the West was in its earliest stages. But if there had been a megadrought, there almost surely would have been some history, including through Indigenous history, to weigh as the 20th century dawned and plans were drawn up for the future of water.
That, of course, didn’t happen.
