Climate crisis: Historic megadrought driven by global warming ‘already impacting western US and Mexico’, study warns

After unusually wet 20th Century, current dry spell on track to match most arid periods since 800AD, scientists say

Harry Cockburn
Friday 17 April 2020 14:39 BST
The climate crisis is exacerbating naturally occurring periods of aridity in the south west US and northern Mexico, scientists say
The climate crisis is exacerbating naturally occurring periods of aridity in the south west US and northern Mexico, scientists say (Getty )

A megadrought worse than any in “known prehistory” may already be underway in the western United States and northern Mexico, researchers have suggested.

After experiencing a wetter-than-average 20th century, the region has been suffering an ever-lengthening string of dry years starting in 2000, and scientists have now warned the worsening climate crisis is partly responsible for pushing the region towards extreme long-term drought.

The study is based on modern weather observations, along with 1,200 years of tree-ring data and dozens of climate models.

“Earlier studies were largely model projections of the future,” said lead author Park Williams, a bioclimatologist at Columbia University‘s Lamont-Doherty Earth Observatory.

“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.”

Reliable modern observations date only to about 1900, but tree rings have allowed scientists to infer yearly soil moisture for centuries before humans began influencing the climate.

The study has rung alarm bells, as previous research has linked the catastrophic ancient droughts recorded in tree rings to concurrent upheavals among indigenous medieval-era civilisations in the southwest US.

The new study is the most up-to-date and comprehensive long-term analysis, the researchers have said. It covers an area stretching across nine US states from Oregon and Montana down through California and New Mexico, and part of northern Mexico.

Using rings from thousands of trees, the researchers charted dozens of droughts across the region, starting in 800 AD.

Four stand out as so-called megadroughts, with extreme aridity lasting decades: the late 800s, mid-1100s, the 1200s, and the late 1500s. After 1600, there were other droughts, but none on such a large scale.

The team then compared the ancient megadroughts to soil moisture records calculated from observed weather in the years from 2000 to 2018.

They found the current drought is already outdoing the three earliest ones. The fourth, which spanned 1575 to 1603, may have been the worst of all — but the difference is slight enough to be within the range of uncertainty.

Furthermore, the current drought is affecting wider areas more consistently than any of the earlier ones — this is the “fingerprint” of climate change and associated temperatures, the researchers said.

All of the ancient droughts lasted longer than 19 years — the one that started in the 1200s ran for nearly a century — but all began by following a similar path to what is happening now, they said.

The ancient droughts weren’t driven by manmade climate change, and nature is still playing a strong role today.

A study last year led by Nathan Steiger at the Lamont-Doherty Earth Observatory of Columbia University showed among other things, that unusually cool periodic conditions over the tropical Pacific Ocean (commonly called La Niña) during the previous megadroughts pushed storm tracks further north, and starved the region of precipitation.

These conditions, alongside other natural factors, appear to have also cut precipitation in recent years. However, with the addition of global warming, the authors say average temperatures since 2000 have been pushed 1.2C above what they would have been otherwise.

This has a considerable impact on soils as hotter air tends to hold more moisture and that moisture is being pulled from the ground. This has intensified drying of soils already starved of precipitation.

In all, the study indicates rising temperatures may be responsible for about half the pace and severity of the current drought.

If this overall warming were subtracted from the equation, the current drought would rank as only the 11th worst detected — “bad, but nowhere near what it has developed into”, the authors said.

“It doesn’t matter if this is exactly the worst drought ever,” said coauthor Benjamin Cook, who is affiliated with Lamont and the Goddard Institute for Space Studies.

“What matters is that it has been made much worse than it would have been because of climate change.”

Since temperatures are projected to keep rising, and with the world currently on course to see 3-4C higher temperatures by the end of the century unless dramatic action is taken to limit fossil fuel emissions, it is likely the drought will continue for the foreseeable future, or fade briefly only to return, the researchers say.

“Because the background is getting warmer, the dice are increasingly loaded toward longer and more severe droughts,” said Professor Williams.

“We may get lucky, and natural variability will bring more precipitation for a while. But going forward, we’ll need more and more good luck to break out of drought, and less and less bad luck to go back into drought.”

He said it is conceivable the region could now remain arid for centuries. “That’s not my prediction right now, but it’s possible,” he said.

Lamont climatologist Richard Seager was one of the first to predict, in a 2007 paper, that climate change might eventually push the region into a more arid climate during the 21st century.

He speculated at the time that the process might already be underway. By 2015, when 11 of the past 14 years had seen drought, Benjamin Cook led a followup study projecting that warming climate would cause the catastrophic natural droughts of prehistory to be repeated by the latter 21st century.

A 2016 study co-authored by several Lamont scientist reinforced those findings. Now, says Dr Cook, it looks like they may have underestimated. “It’s already happening,” he said.

The research team said the effects of the current drought are already palpable.

“The mighty reservoirs of Lake Mead and Lake Powell along the Colorado River, which supply agriculture around the region, have shrunk dramatically. Insect outbreaks are ravaging dried-out forests. Wildfires in California and across wider areas of the US West are growing in area. While 2019 was a relatively wet year, leading to hope that things might be easing up, early indications show that 2020 is already on a track for resumed aridity,” the team said.

Angeline Pendergrass, a staff scientist at the US National Centre for Atmospheric Research, said that she believes it is too early to say whether the region is at the cusp of a true megadrought, because the study confirms that natural weather swings are still playing a strong role.

But she warned: “Even though natural variability will always play a large role in drought, climate change makes it worse.”

The research also revealed that the 20th century was the wettest century in the entire 1200-year record. It was also during this time the population boomed, and that has continued.

“The 20th century gave us an overly optimistic view of how much water is potentially available,” said Dr Cook.

“It goes to show that studies like this are not just about ancient history. They’re about problems that are already here.”

The research is published in the journal Science.

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