Predicting drought in the American West just got more difficult

Date:
August 8, 2020
Source:
University of Southern California
Summary:
A new study of more than 1,000 years of North American droughts and
global conditions found that forecasting a lack of precipitation
is rarely straightforward.



FULL STORY ========================================================================== People hoping to get a handle on future droughts in the American West
are in for a disappointment, as new USC-led research spanning centuries
shows El Nin~o cycles are an unreliable predictor.


========================================================================== Instead, they found that Earth's dynamic atmosphere is a wild card that
plays a much bigger role than sea surface temperatures, yet defies predictability, in the wet and dry cycles that whipsaw the western
states. The study, published Monday in Science Advances, is a detailed assessment of long-term drought variability.

The findings are significant for water management, agriculture, urban
planning and natural resources protection. Recent droughts have claimed
many lives and caused damaging crop losses, making drought forecasting
a high priority.

Meanwhile, the West faces rapid population growth at the same time that forecasts show dry times ahead due to global climate change.

"The main finding is not terribly hopeful for short-term drought
prediction," said Julien Emile-Geay, a study author and associate
professor of Earth sciences at the USC Dornsife College of Letters,
Arts and Sciences. "We found that, historically speaking, year-to-year
droughts in the western United States were less predictable than previous studies have claimed." New study examines 1,000 years of droughts in
the West and beyond Emile-Geay and postdoctoral scholar Michael Erb,
who is lead author from USC and now at Northern Arizona University,
joined with other scientists at the University of Washington and Columbia University to produce the study.



==========================================================================
The researchers set out to answer the question: What determines droughts
in the West? They examined North American droughts and global conditions spanning more than 1,000 years. Megadroughts, which lasted decades, and
dry spells predate the Industrial Revolution, American expansion on the continent or European colonialism. For example, a megadrought in the late
13th century likely contributed to the dispersal of the Anasazi people.

The prevailing explanation is that the El Nin~o-Southern Oscillation plays
a key role in these drought episodes. The oscillation is a two-sided coin
based on water conditions in the eastern equatorial Pacific Ocean. El
Nin~o conditions occur when sea temperatures in the region are warmer
than normal and are associated with wet years in the American Southwest;
La Nin~a conditions occur when water is cooler than normal and are
associated with dry years in the Southwest.

But the scientists found that rule of thumb didn't jibe well with all
drought cycles of the past. While it's true there's a correlation between
La Nin~a and drought, these ocean water conditions accounted for only
about 13% of the variability, the study says.

"La Nin~a proved to not be the only game in town," Emile-Geay said. "La
Nin~a is part of the game, but not the biggest part." A notable
example of this phenomenon occurred in 2015-16, an El Nin~o year when
Southern California did not receive the increased precipitation that
was predicted. Instead, the relief came unexpectedly the following year,
a La Nin~a year that should have been drier than normal.



==========================================================================
What other variables can lead to drought? The scientists also examined
other drought influencers, including water temperatures in the Atlantic
Ocean and volcanic activity. While those phenomena can influence drought conditions, they are too weak or episodic to explain many droughts.

Instead, the study says droughts can originate in the atmosphere. The
air around Earth is highly dynamic and influenced by more variables than
the ocean alone.

"The atmosphere creates a lot of variations in moisture supply on its
own, and it can cook up droughts all by itself, without being told what
to do by the ocean," Emile-Geay said.

But while the past is key to the future, it does not hold all the keys. In
the future, research says carbon emissions will continue to trap heat
and warm the surface, and the West will experience increasingly dry
conditions as a result.

"Our study suggests that the atmosphere will continue to add a strongly unpredictable element to moisture conditions in the southwestern
United States, on top of drying induced by global warming," Emile-Geay
said. "That is, the Southwest is headed for a drier future overall but
with the atmosphere adding a wildcard that may, at times, make things
better or worse for the people and ecosystems that depend on that water."
The study integrates numerous sources of information spanning centuries
to support the findings. The dataset, called the Last Millennium
Reanalysis, aggregates climate models, modern temperature measurements
and nearly 3,000 climate proxies, such as tree rings, corals, and ice
cores. The reanalysis was developed by scientists at USC, the University
of Washington and the University of Colorado, with the support of the
National Oceanic and Atmospheric Administration. According to the NOAA,
the reanalysis aimed to "transform the ways in which the climate community investigates low-frequency climate."

========================================================================== Story Source: Materials provided by
University_of_Southern_California. Original written by Gary
Polakovic. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. M. P. Erb, J. Emile-Geay, G. J. Hakim, N. Steiger, E. J. Steig.

Atmospheric dynamics drive most interannual U.S. droughts over
the last millennium. Science Advances, 2020; 6 (32): eaay7268 DOI:
10.1126/ sciadv.aay7268 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200808085750.htm

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