10 3 JANUARY 2020 • VOL 367 ISSUE 6473 sciencemag.org SCIENCEPHOTO: RODRIGO GARRIDO/REUTERS/NEWSCOMthe Air Force Research Laboratory.
A team of scientists at Los Alamos and
NASA’s Goddard Space Flight Center is
spearheading a second experiment in VLF
precipitation. In April 2021, the team plans
to launch a sounding rocket carrying the
Beam Plasma Interactions Experiment, a
miniature accelerator that would create
a beam of electrons, which in turn would
generate VLF waves capable of sweeping
up particles. Reeves, who leads the ex-
periment, believes the compact electron
accelerator could ultimately be a better
broom than a gigantic VLF antenna. “If we
validate it with this experiment, we have a
lot more confidence we can scale it up to
higher power,” he says.
A third experiment would coax the at-
mosphere itself to kick up turbulent waves
that would draw down electrons. In the
summer of 2021, the Naval Research Labo-
ratory plans to launch a mission called the
Space Measurements of a Rocket-Released
Turbulence. A sounding rocket will fly
into the ionosphere—an atmospheric layer
hundreds of kilometers up that’s awash in
ions and electrons—and eject 1.5 kilograms
of barium atoms. Ionized by sunlight, the
barium would create a ring of moving
plasma that emits radio waves: essentially
a space version of a magnetron, the gadget
used in microwave ovens.
The missions should help show which
RBR system is most feasible, although an
operational system may be years off. What-
ever the technology, it could bring risks.
A full-scale space cleanup might dump as
much energy into the upper atmosphere
as the geomagnetic storms caused by the
Sun’s occasional eruptions. Like them,
it could disrupt airplane navigation and
communication. And it would spawn heaps
of nitrogen oxides and hydrogen oxides,
which could eat away at the stratospheric
ozone layer. “We don’t know how great
the effect would be,” says Allison Jaynes,
a space physicist at the University of Iowa.
Besides safeguarding against a nuclear
burst, RBR technology could have a ci-
vilian dividend, Jaynes notes. NASA and
other space agencies have long wrestled
with shielding astronauts from the Van
Allen belts and other sources of radia-
tion on their way to and from deep space.
VLF transmitters might be used to clear
out high-energy electrons just before a
spacecraft enters a danger zone. “When
we become more active space travelers,”
she says, “it could provide a safe passage
through the radiation belts.” j
Richard Stone is senior science editor at the
Howard Hughes Medical Institute’s Tangled Bank
Studios in Chevy Chase, Maryland.
NEWS | IN DEPTH
F
or 10 years, central Chile has been
gripped by unrelenting drought. With
30% less rainfall than normal, verdant
landscapes have withered, reservoirs
are low, and more than 100,000 farm
animals have died. The dry spell has
lasted so long that researchers are calling it a
“megadrought,” rivaling dry stretches centu-
ries ago. It’s not so different from the decade-
long drought that California, some 8000 kilo-
meters away, endured until last year.
By analyzing tree ring records, scientists
have now found evidence that such tandem
droughts are more than a coincidence: They
are surprisingly common over the past 1200
years, and they may often share a common
cause—an abnormally cool state of the east-
ern Pacific Ocean known as La Niña. “We
did not expect there to be as much coher-
ence as we see,” says Nathan Steiger, a paleo-
climatologist at Columbia University who
presented the work last month at a meeting
of the American Geophysical Union. “They
just happen together.” The results suggest
that, in the future, extreme aridity could
strike all along the Americas’ western coast.
Evidence for synchronous, hemisphere-
wide droughts first emerged in a 1994 study in
Nature, which documented dead tree stumps
in the middle of lakes and rivers in both Pa-
tagonia and California’s Sierra Nevada. For
trees to grow in stream- and lakebeds, the
droughts must have lasted for decades, and
at least one of these megadroughts seemed to
have hit both continents simultaneously.But pinning down the exact timing wasn’t
possible in the 1990s. Ancient drought is
typically detected in variations in the width
of tree rings, but tree ring records then were
spotty. Since, however, tree ring scientists
have put together “drought atlases” that
provide consistent records for much of the
world. “We’ll have most hemispheric land ar-
eas covered by the end of the year,” says Ed
Cook, a tree ring scientist at Columbia.
Steiger combined these records with
thousands of other proxies for dryness
and temperature from trees, corals, ocean
sediments, and ice cores, and fed them
into a global climate model. Aligning it-
self to the records, it generated a global
view of the changing climate, even in
places with sparse proxies. The model
confirmed that, from 800 to 1600 C.E.,
multiple megadroughts occurred simulta-
neously across the hemispheres. “It’s there,”
Cook says. “Without question, it’s there.”
Besides correlating the varied climate re-
cords, the model also identified the key fac-
tors driving the climate variations. Steiger
and his co-authors, including Cook, first
used the new tool to look at megadroughts
in the U.S. Southwest. Their study, published
last year in Science Advances, is “amazing,”
says David Stahle, a tree ring scientist at the
University of Arkansas in Fayetteville. “It’s a
bit like ... they took those black-and-white
films and colorized them.” They found that
megadroughts in the Southwest were in-
fluenced by three factors: an anomalously
warm North Atlantic Ocean, small global
temperature rises driven by factors such asPast megadroughts hit North
and South America in tandem
Strong La Niña conditions drove deep medieval droughts
C L I M AT EBy Paul VoosenPublished by AAAS