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Washington Post, May 12, 2020
America’s longest river was recently drier than during the Dust Bowl.
And it’s bound to happen again.
By Darryl Fears
For the first decade of the century, the Upper Missouri River Basin was
the driest it’s been in 1,200 years, even more parched than during the
disastrous Dust Bowl of the 1930s, a new study says.
The drop in water level at the mouth of the Missouri — the country’s
longest river — was due to rising temperatures linked to climate change
that reduced the amount of snowfall in the Rocky Mountains in Montana
and North Dakota, scientists found.
The basin has continued to experience droughts this decade — in 2012,
2013 and 2017 — but their severity in comparison with historic drought
is unknown. The “Turn-Of-The-Century Drought” study, published Monday in
the Proceedings of the National Academy of Sciences, focused only on the
10 years after 2000.
“In terms of the most severe flow deficits, the driest years of the
Turn-Of-The-Century-Drought in the [Upper Missouri River Basin] appear
unmatched over the last 1,200 years,” the study said. “Only a single
event in the late 13th century rivaled the greatest deficits of this
most recent event.”
Researchers familiar with drought of this magnitude in the dry Southwest
were surprised to find it in the Midwest. The Missouri River winds 2,300
miles from western Montana to St. Louis, where it joins the Mississippi
River. It runs through the Dakotas and past Nebraska, Iowa, Kansas and
Missouri, where it supports farming in several states and provides fresh
water for dozens of municipalities.
“We don’t tend to think of the upper Missouri region as being as
drought-threatened as a region like the Southwest United States,” said
Erika Wise, an associate professor in the geography department at the
University of North Carolina at Chapel Hill and study co-author.
“These findings show that the upper Missouri Basin is reflecting some of
the same changes that we see elsewhere across North America, including
the increased occurrence of hot drought" that’s more severe than usual,
Wise said.
A 'megadrought' will grip the U.S. in the coming decades, NASA
scientists say
The study is the latest to show how human-influenced climate change
threatens to reshape the landscape by making naturally occurring drought
far more severe.
A study published last month in the journal Science found that a vast
region of the western United States — California, Arizona and New Mexico
north to Oregon and Idaho — is already in the grips of the first climate
change-induced megadrought.
Scientists at NASA and Columbia University had predicted such a drought
would start sometime around 2030, but last month’s study showed that the
phenomenon is already here. “The megadrought era seems to be reemerging,
but for a different reason than the [past] megadroughts,” said Park
Williams, the study’s lead author and a researcher at the Lamont-Doherty
Earth Observatory at Columbia University.
Prolonged drought can disrupt agriculture and hurt economies, the
researchers said. It affects dams that manage water resources and slows
commercial traffic in rivers. On top of that, it harms marine life that
must cope with lower water levels and animals such as waterfowl that
rely on fish to survive. Runoff from the mountains to the Upper Missouri
started decreasing in the 1950s, then dramatically declined between 1980
and 2000, the study said.
“Future warming is anticipated to cause increasingly severe droughts by
enhancing water deficits that could prove challenging for water
management,” the Missouri River study said. Historically, the Upper
Missouri accounts for about 30 percent of the total flow at the mouth of
the Missouri and will diminish over time, the authors said.
“The watershed is a critical source of water for the region — supporting
megafarms, hydropower, tourism, and healthy ecosystems,” one author
said. Declines in stream flow will be felt downstream on farms from Iowa
to Missouri, especially if the southern part of the Missouri River
experiences drought when the northern portion is dry.
“In the UMRB, what we’re really worried about is a future of snow
droughts,” Wise said. “Snowpack in the Rocky Mountains is very sensitive
to warming temperatures. Snow provides the water for stream flow to the
Upper Missouri, and we’ve designed our agriculture and infrastructure
around expectations that this water will be provided at a certain pace
over a certain part of the year.”
Wise said a team of nearly 20 government and academic researchers knew
the Upper Missouri was unusually dry between 2000 and 2010, but they set
out to determine whether that level of drought was normal for the basin
of creeks, streams and other waters that feed the main river.
Instrumental records could tell them what happened only in the past 100
years, so the researchers relied heavily on tree ring data, where rings
are wide during periods of normal to high moisture and narrow when it’s
low. Connie Woodhouse, a University of Arizona professor and study
co-author, oversaw that area of research.
“These data provided us a detailed record of stream flow for every year
since 800 A.D.,” said Woodhouse, who sampled trees throughout the
watershed, many of them far from the river, to determine how they were
affected by precipitation over time. Records allow scientists to examine
the rings of trees both living and dead.
“I didn’t set out expecting to find that warming temperatures are
impacting stream flow in the UMRB in the same way that they are
impacting the Upper Colorado River Basin,” said Woodhouse, who has also
studied that river basin. For one thing, the Upper Missouri has a more
complex topography than the Upper Colorado, “so I didn’t necessarily
expect to find this same result, but we did!”
Writing in a commentary that accompanied the study, Jonathan Overpeck, a
climate scientists at the University of Michigan, and Bradley Udall, a
scientist at Colorado State University, said drought in the Upper
Missouri “mirrors the change occurring in the Southwest, where rivers
provide the only large sustainable water supply to the region and over
40 million water users, yet flows have already declined significantly
since just the late 20th century.”
In the West, rising temperatures brought on by heat-trapping emissions
from human activity “is also contributing to drier soil, widespread tree
death, and more severe wildfires,” as recent events in California show,
the two wrote. “Greater aridity is redefining the West in many ways, and
the costs to human and natural systems will only increase as we let the
warming continue.”
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