Climate scientists reconsider the meaning and implications of drought in
light of a changing world
Date:
March 14, 2022
Source:
University of California - Santa Barbara
Summary:
Maps of the American West have featured ever darker shades of red
over the past two decades. The colors illustrate the unprecedented
drought blighting the region. In some areas, conditions have blown
past severe and extreme drought into exceptional drought. But
rather than add more superlatives to our descriptions, one group
of scientists believes it's time to reconsider the very definition
of drought.
FULL STORY ==========================================================================
Maps of the American West have featured ever darker shades of red over
the past two decades. The colors illustrate the unprecedented drought
blighting the region. In some areas, conditions have blown past severe
and extreme drought into exceptional drought. But rather than add more superlatives to our descriptions, one group of scientists believes it's
time to reconsider the very definition of drought.
========================================================================== Researchers from half a dozen universities investigated what the future
might hold in terms of rainfall and soil moisture, two measurements
of drought. The team, led by UC Santa Barbara's Samantha Stevenson,
found that many regions of the world will enter permanent dry or wet
conditions in the coming decades, under modern definitions. The findings, published in the Proceedings of the National Academy of Sciences, reveal
the importance of rethinking how we classify these events as well as
how we respond to them.
"Essentially, we need to stop thinking about returning to normal as a
thing that is possible," said Stevenson, an assistant professor in the
Bren School of Environmental Science & Management. This idea affects
both how we define drought and pluvial (abnormally wet) events and how
we adapt to a changing environment.
A drought is when conditions are drier than expected. But this concept
becomes vague when the baseline itself is in flux. Stevenson suggests
that, for some applications, it's more productive to frame drought
relative to this changing background state, rather than a region's
historical range of water availability.
To predict future precipitation and soil moisture levels, Stevenson
and her colleagues turned to a new collection of climate models from
different research institutions. Researchers had run each model many
times with slightly different initial conditions, in what scientists
call an "ensemble." Since the climate is an inherently chaotic system, researchers use ensembles to account for some of this unpredictability.
The results show a world where certain regions are in permanent
drought while others experience perennial pluvial for the rest of
the 21st century. The team calculated the year in which average soil
moisture will exceed the threshold that defines either a megadrought
or a megapluvial. "In other words, at what point do average conditions
exceed what we would consider a megadrought if it happened now, [and
never return to 'normal']" Stevenson said.
==========================================================================
The western United States has already crossed this benchmark, and
there are other places headed that way as well, including Australia,
southern Africa and western Europe. "But, again, that's if we use today's definition of a drought," Stevenson said.
The authors argue that we need to move away from fixed definitions toward
a more nuanced account of drought and pluvial. "Our idea of normal
is, in a sense, meaningless when 'normal' is continuously changing,"
Stevenson added.
Climate models indicate that average soil moisture in many regions will continue to drop. That said, the team's ensembles suggests that soil
moisture will continue to experience drought-related variation similar
to today, relative to the ever-drier baseline.
The fluctuation highlights the need to consider both long term changes
and the usual ups and downs associated with historic droughts and
pluvials. "The most important management challenge will be to adjust
for the relentless declines in water availability, as this exceeds the
expected impact of future megadroughts," said co-author Julia Cole,
a professor at the University of Michigan.
Precipitation patterns, on the other hand, will become much more
extreme. Warm air holds more moisture than cold air. So as the atmosphere
heats up, it'll be able to suck more moisture from dry areas and dump
more precipitation on wet regions.
"We wanted to consider both precipitation and soil moisture at the same
time because that can be important for water management," Stevenson
said. For instance, we will need to adapt infrastructure to more arid conditions in the American West, but that infrastructure will also need
to handle more intense rainfall.
"When we talk about being in a drought, the presumption is that eventually
the drought will end, and conditions will return to normal," Stevenson
said. "But if we're never returning to normal, then we need to adapt
all of the ways that we manage water with the expectation that normal
will continually be drier and drier every year."
========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Barbara. Original written by Harrison
Tasoff. Note: Content may be edited for style and length.
========================================================================== Journal Reference:
1. Samantha Stevenson, Sloan Coats, Danielle Touma, Julia Cole, Flavio
Lehner, John Fasullo, Bette Otto-Bliesner. Twenty-first century
hydroclimate: A continually changing baseline, with more frequent
extremes. Proceedings of the National Academy of Sciences, 2022;
119 (12) DOI: 10.1073/pnas.2108124119 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/03/220314181504.htm
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