Climate change fueling more intense megadroughts in U.S. West
Climate change is pushing the western United States and northern Mexico toward an extreme long-term drought that could be worse than any in recorded history, scientists revealed in a new study published in the journal Science Thursday.
Using 1,200 years of tree-ring data, modern weather observations and dozens of climate models, researchers identified four historic megadroughts dating back to 800 A.D. Characterized by severe arid conditions that last decades, the four megadroughts occurred in the late 800s, mid-1100s, 1200s and late 1500s.
Because trees grow more in wet years and less in dry years, scientists can examine tree rings to determine periods of extreme dryness. In this study, researchers analyzed 1,586 tree rings to chart drought history for an entire region dating back 1,200 years.
By comparing ancient megadroughts to soil moisture records from 2000 to 2018, researchers found the most recent drought surpassed one of the worst periods of severe dryness that started in the 1200s and ran nearly a century.
“We can compare the soil moisture anomalies that we experienced during 2000-2018 to all other 19-year periods in the entire record,” said lead author Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory. “We see these past 19 years go toe to toe with the worst 19-year periods since 1200 A.D.”
Previous studies have linked naturally occurring megadroughts recorded in tree rings to upheavals among indigenous medieval-era civilizations in the Southwest. Archeologists found that ancestral Puebloans, a group of Native American cliff dwellers, abandoned Mesa Verde in present-day southwestern Colorado in the 1200s during a time of severe drought.
“Societal unrest can occur for many reasons, but at the very least the occurrence of a severe megadrought at that time probably wouldn’t have helped,” Williams said.
Another megadrought that dried up the West from 1575 to 1603 was identified as most severe, but the difference between that and the 2000-2019 drought is slight and within a range of uncertainty, meaning the modern drought may be as bad or worse.
Both of those droughts have similarities in that they came on quickly and a La Niña weather pattern was a contributing factor. La Niña is an atmospheric phenomenon characterized by the cooling of surface waters in the Pacific Ocean that can push storms north and starve the region of precipitation.
La Niña is a natural phenomenon that existed before humans started altering the climate by burning fossil fuels, but “the jury is still out” on whether human-caused climate change affects weather patterns such as La Niña and El Niño, Williams said.
One effect that scientists know is caused by human activities is warming temperatures, and that has a major impact on drought severity, according to Benjamin Cook, co-author of the study and a climate scientist with Columbia University’s Lamont-Doherty Earth Observatory and the Goddard Institute for Space Studies.
The research team found that without warming temperatures, the 2000-2019 drought would have been the 11th worst in history, instead of the second or potentially worst drought in 1200 years.
“If you take the climate change component out of the story, there still would have been a significant drought, but it would have ranked more as a kind of normal type of drought that we’ve see in more recent decades,” Cook said.
Wetter winters in 2016-2017 and 2018-2019 “could mean we are slowly drifting out of this period of bad luck,” Williams added, but humans can’t reliably predict drought events more than six months in advance.
Regardless of when the current drought ends, the study indicates that climate change will continue to make droughts more frequent and severe in southwestern North America.
“We are pretty confident that the likelihood of these events is increasing with warming,” Cook said.
Megadroughts also carry major consequences for water supply. Key reservoirs, including Lake Mead and Lake Powell, have seen water levels drop dramatically. The drought has also reduced the flow of water in the Colorado River, which supplies water to more than 40 million Americans.
The arid conditions also make it harder for healthy trees to produce sap, making them more vulnerable to bark beetles. By drying out vegetation, drought can also make wildfires more frequent and severe.
Williams said the findings of this study should be viewed as wake-up call about the dangers of failing to curb carbon emissions. A warming climate and more severe, prolonged droughts will shrink the availability of a valuable natural resource: water.
“We are hurting ourselves by not pushing for renewable energy resources,” Williams said. “Hopefully this study causes us to think about our greenhouse gas emissions.”
He further warned of the need for more sustainable water supply strategies in the western part of the country. States are currently guaranteed more water from the Colorado River than is available in an average year, he said.
“Any long-term plans need to incorporate information on how climate change is going to affect these resources,” Cook added. “Otherwise you’re simply not going to be prepared.”
To follow up on these findings, Cook said he hopes to look at how climate-driven drought affects stream flow, groundwater and vegetation health.
Williams said he wants to study how climate-driven drought affects other regions and the connection to global issues, such as international trade. He would also like to use the 1,200 years of soil moisture records to examine how vegetation has changed over time and the impact on carbon storage by landscapes.
“These reconstructions can give us a better sense of variability — how much carbon is stored in lands, and how sensitive atmospheric CO2 will be to changes in carbon stored on land,” Williams said.