Washington, D.C. – Droughts in the U.S. Southwest
and Central Plains during the last half of this century could be drier
and longer than drought conditions seen in those regions in the last
1,000 years, according to a new NASA study.
The study, published Thursday in the journal Science Advances, is
based on projections from several climate models, including one
sponsored by NASA. The research found continued increases in
human-produced greenhouse gas emissions drives up the risk of severe
droughts in these regions.
“Natural droughts like the 1930s Dust Bowl and the current drought in
the Southwest have historically lasted maybe a decade or a little less,”
said Ben Cook, climate scientist at NASA’s Goddard Institute for Space
Studies and the Lamont-Doherty Earth Observatory at Columbia University
in New York City, and lead author of the study. “What these results are
saying is we’re going to get a drought similar to those events, but it
is probably going to last at least 30 to 35 years.”
According to Cook, the current likelihood of a megadrought, a drought
lasting more than three decades, is 12 percent. If greenhouse gas
emissions stop increasing in the mid-21st century, Cook and his
colleagues project the likelihood of megadrought to reach more than 60
percent.
However, if greenhouse gas emissions continue to increase along
current trajectories throughout the 21st century, there is an 80 percent
likelihood of a decades-long megadrought in the Southwest and Central
Plains between the years 2050 and 2099.
The scientists analyzed a drought severity index and two soil
moisture data sets from 17 climate models that were run for both
emissions scenarios. The high emissions scenario projects the equivalent
of an atmospheric carbon dioxide concentration of 1,370 parts per
million (ppm) by 2100, while the moderate emissions scenario projects
the equivalent of 650 ppm by 2100. Currently, the atmosphere contains
400 ppm of CO2.
In the Southwest, climate change would likely cause reduced rainfall
and increased temperatures that will evaporate more water from the soil.
In the Central Plains, drying would largely be caused by the same
temperature-driven increase in evaporation.
The Fifth Assessment Report, issued by the United Nations
Intergovernmental Panel on Climate Change (IPCC) in 2013, synthesized
the available scientific studies and reported that increases in
evaporation over arid lands are likely throughout the 21st century. But
the IPCC report had low confidence in projected changes to soil
moisture, one of the main indicators of drought.
Until this study, much of the previous research included analysis of
only one drought indicator and results from fewer climate models, Cook
said, making this a more robust drought projection than any previously
published.
“What I think really stands out in the paper is the consistency between
different metrics of soil moisture and the findings across all the
different climate models,” said Kevin Anchukaitis, a climate scientist
at the Woods Hole Oceanographic Institution in Woods Hole,
Massachusetts, who was not involved in the study. “It is rare to see all
signs pointing so unwaveringly toward the same result, in this case a
highly elevated risk of future megadroughts in the United States.”
This study also is the first to compare future drought projections directly to drought records from the last 1,000 years.
“We can’t really understand the full variability and the full
dynamics of drought over western North America by focusing only on the
last century or so,” Cook said. “We have to go to the paleoclimate
record, looking at these much longer timescales, when much more extreme
and extensive drought events happened, to really come up with an
appreciation for the full potential drought dynamics in the system.”
Modern measurements of drought indicators go back about 150 years.
Cook and his colleagues used a well-established tree-ring database to
study older droughts. Centuries-old trees allow a look back into the
distant past. Tree species like oak and bristle cone pines grow more in
wet years, leaving wider rings, and vice versa for drought years.
By comparing the modern drought measurements to tree rings in the
20th century for a baseline, the tree rings can be used to establish
moisture conditions over the past 1,000 years.
The scientists were interested in megadroughts that took place
between 1100 and 1300 in North America. These medieval-period droughts,
on a year-to-year basis, were no worse than droughts seen in the recent
past. But they lasted, in some cases, 30 to 50 years.
When these past megadroughts are compared side-by-side with computer
model projections of the 21st century, both the moderate and
business-as-usual emissions scenarios are drier, and the risk of
droughts lasting 30 years or longer increases significantly.
Connecting the past, present and future in this way shows that 21st
century droughts in the region are likely to be even worse than those
seen in medieval times, according to Anchukaitis.
“Those droughts had profound ramifications for societies living in
North America at the time. These findings require us to think about how
we would adapt if even more severe droughts lasting over a decade were
to occur in our future,” Anchukaitis said.
NASA monitors Earth’s vital signs from land, air and space with a
fleet of satellites and ambitious airborne and ground-based observation
campaigns. NASA develops new ways to observe and study Earth’s
interconnected natural systems with long-term data records and computer
analysis tools to better see how our planet is changing.
The agency shares this unique knowledge with the global community and
works with institutions in the United States and around the world that
contribute to understanding and protecting our home planet.
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