Climatologists have been predicting megadrought in the western United States and northern Mexico but it seems to have already begun, scientists suggest in Science on Thursday. While the combination of natural circumstances could have caused a megadrought at this time anyway, this one – exacerbated by global warming – could be the worst in thousands of years, the team warns.
The Mediterranean region, eastern North Africa, the Middle East and Chile are also undergoing accelerated desertification, the recent wet winter in Israel notwithstanding. Natural climatic cycles caused megadroughts in the Mediterranean region 120,000 and 10,000 years ago, each lasting thousands of years.
Over in the western United States and northern Mexico, drought clusters began in around 2000 and scientists have been cautioning that climate change might be pushing the region toward extremes. Now they aren’t warning any more. It’s apparently here.
The study by Park Williams, Edward Cook, Jason Smerdon, Kasey Bolles and Seung Hun Baek, of Lamont-Doherty Earth Observatory; John Abatzoglou of the University of Idaho; and Andrew Badger and Ben Livneh of the University of Colorado, Boulder, is based on weather observations, tree-ring data going back 1,200 years and dozens of climate models.
“Earlier studies were largely model projections of the future,” Williams says. “We’re no longer looking at projections, but at where we are now. We now have enough observations of current drought and tree-ring records of past drought to say that we’re on the same trajectory as the worst prehistoric droughts.”
Tale of a tree
As many trees grow in girth, rings form under the bark. Dendrochronology is the art of counting rings to estimate a tree’s age, and the form of the rings speaks volumes about the environmental conditions. Trees typically grow better in nice, warm wet weather, resulting in a wider ring. In a drought, the ring will be thin.
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Written history goes back about 5,000 years but, unlike scribes, trees are an objective source of data. Tree-ring data has been central to the theory that the Toltec and Aztec civilizations were brought low by megadrought, to name just one example. In our parts, the great Assyrian empire may also have been fatally weakened by drought.
The new study is based on dendrochronological data from thousands of trees in nine U.S. states, from Oregon and Montana down through California and New Mexico, and part of northern Mexico, starting 1,200 years ago. The researchers detected the four megadroughts lasting decades: in the late ninth century, 12th century, 13th century and 16th century.
As for the drought conditions today, the team didn’t need tree rings: we have records going back to 1901. They calculated drought severity from 2000 to 2018 using a soil-moisture model.
“Pretend that the ground is covered by buckets. We use precipitation data to fill the buckets up, and we use temperature, humidity, wind and sunlight to drive evaporation,” Williams explains. Then they filtered human-caused climate trends from the climate data and reran the bucket simulations. Thus, they estimate that the intensity of the 2000 to 2018 drought would have been reduced by about half.
The current drought is already outdoing three prior ones from the ninth, 12th and 13th centuries, according to their calculations. The fourth historic one, which lasted from 1575 to 1603, may have been worse, but the process today is still gaining momentum. Today’s drought is also afflicting a wider area than the preceding ones: this is a fingerprint of global warming, the researchers say.
“As of 2018, climate models suggest that the extra heat in our study region due to global warming was 1.5-1.6 degrees Celsius,” Williams tells Haaretz. (The equivalent in Fahrenheit would be about 2.4 to 2.6 degrees.)
If hotter air holds more moisture and the western United States is drying, where is that rain going to fall? Climate change models predict more precipitation across most of the eastern side of the country.
Hand of La Niña and of humans
Archaeologists have demonstrated that people began destroying their local environs from the time they settled down thousands of years ago – for instance, by deforestation and resource depletion. The first settlers in the ancient northern Israeli city of Acre devastated the indigenous Mediterranean forest and fauna, depleted the water supply and drove a local temperature increase: the stone construction retains more heat than natural vegetation.
To be clear, regional megadrought is driven by nature, and that’s still the case. A study published last year, led by Lamont’s Nathan Steiger, correlated the historic megadroughts – and apparently this one – with the weather phenomenon La Niña, which pushes storm tracks northward, depriving the Midwest of rain.
If La Niña happens every few years, how could it explain a megadrought every few centuries, let alone one lasting almost 100 years?
La Niña represents a pattern of Pacific sea-surface temperatures and is said to happen every three to seven years. But it isn’t actually cyclic, Williams explains: that’s an average. (El Niño isn’t cyclic either). “So you can have decades or centuries with an anomalously high frequency of La Niña years. Further, La Niña and El Niño are the extremes, but between those extremes you can have strings of decades when the sea-surface patterns are somewhere between average, but tilted more toward La Niña or El Niño,” he says: During megadroughts, it appears there are some major La Niña events that were superimposed on background conditions conducive to drought.
Now, because of global warming, there is more evaporation and the hotter air tends to hold more moisture, which intensifies the desiccation of soils already starved of precipitation.
The drought isn’t theoretical. The California wildfires were fueled by aridity. Lake Mead and Lake Powell along the Colorado River have shrunk dramatically. Fluctuations if one thing but global warming clearly makes the situation worse.
Worse yet: Given the trajectory of emissions and global warming, the drought in the western U.S. has no natural end in sight – the same as in the Middle East. It could last centuries, Williams says.
“Because the background is getting warmer, the dice are increasingly loaded toward longer and more severe droughts,” he adds.
The suffering won’t be limited to American farmers. Separate research has shown that a redux of the 1930s Dust Bowl-type catastrophe would cause a global food shock cascade.
Could the megadrought gaining momentum in the region it be irreversible? “Even if we stop emitting greenhouse gases tomorrow, the globe will continue to warm for decades as the ocean continues to warm due to the gases already in the atmosphere,” Willliams answers. “So the odds that this particular drought ends in the next several years would not be significantly affected.”
Even if humanity is collectively behaving like a deer in the headlights (vis-à-vis climate change, not the coronavirus), maybe nature will help. “It’s still possible for natural climate variability to end the drought on its own,” Williams qualifies. “In the longer term, we’re still just in the beginning of the human-caused warming trend, assuming we don’t majorly curb greenhouse gases immediately. That means that with continued emissions, human-caused warming will make it increasingly difficult for natural climate swings to swing western North America out of drought.”
And like in Israel, the year 2019 was a relatively wet one for the Midwest; 2020 isn’t shaping up that way.