WEATHER MAKERS Forests supply the world with rain. A controversial Russian - theory claims they also make wind By Fred Pearce - mediaTUM
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
NE WS
FEATURES
WEATHER MAKERS
Forests supply the world with rain. A controversial Russian
theory claims they also make wind By Fred Pearce
Downloaded from http://science.sciencemag.org/ on June 18, 2020
1302
Published by AAAS
NEWS
E
very summer, as the days get long, Yet, if correct, the idea could help explain The importance of this recycled moisture
Anastassia Makarieva leaves her why, despite their distance from the oceans, for nourishing rains was largely disregarded
lab in St. Petersburg for a vacation the remote interiors of forested continents until 1979, when Brazilian meteorologist
in the vast forests of northern Rus- receive as much rain as the coasts—and why Eneas Salati reported studies of the isoto-
sia. The nuclear physicist camps on the interiors of unforested continents tend pic composition of rainwater sampled from
the shores of the White Sea, amid to be arid. It also implies that forests from the Amazon Basin. Water recycled by tran-
spruce and pine, and kayaks along the Russian taiga to the Amazon rainforest spiration contains more molecules with the
the region’s wide rivers, taking don’t just grow where the weather is right. heavy oxygen-18 isotope than water evapo-
notes on nature and the weather. They also make the weather. “All I have rated from the ocean. Salati used this fact
“The forests are a big part of my inner life,” learned so far suggests to me that the biotic to show that half of the rainfall over the
she says. In the 25 years she has made her pump is correct,” says Douglas Sheil, a for- Amazon came from the transpiration of the
annual pilgrimage north, they have become est ecologist at the Norwegian University of forest itself.
a big part of her professional life, too. Life Sciences. With the future of the world’s By this time, meteorologists were track-
For more than a decade, Makarieva forests in doubt, “Even if we thought the ing an atmospheric jet above the forest, at
has championed a theory, developed with theory had only a small chance of being a height of about 1.5 kilometers. Known
Victor Gorshkov, her mentor and colleague true, it would be profoundly important to as the South American Low-Level Jet, the
at the Petersburg Nuclear Physics Institute know one way or the other.” winds blow east to west across the Amazon,
(PNPI), on how Russia’s boreal forests, the
largest expanse of trees on Earth, regulate
the climate of northern Asia. It is simple
physics with far-reaching consequences, de-
Downloaded from http://science.sciencemag.org/ on June 18, 2020
scribing how water vapor exhaled by trees
drives winds: winds that cross the continent,
taking moist air from Europe, through Sibe-
ria, and on into Mongolia and China; winds
that deliver rains that keep the giant rivers
of eastern Siberia flowing; winds that wa-
ter China’s northern plain, the breadbasket
of the most populous nation on Earth.
With their ability to soak up carbon di-
oxide and breathe out oxygen, the world’s
great forests are often referred to as the
planet’s lungs. But Makarieva and Gorshkov,
who died last year, say they are its beat-
ing heart, too. “Forests are complex self-
sustaining rainmaking systems, and the
major driver of atmospheric circulation
on Earth,” Makarieva says. They recycle
vast amounts of moisture into the air
and, in the process, also whip up winds
that pump that water around the world.
The first part of that idea—forests as
PHOTOS: ANASTASSIA MAKARIEVA; (OPPOSITE PAGE) BRUSINI AURÉLIEN/HEMIS/ALAMY STOCK PHOTO
rainmakers—originated with other sci-
entists and is increasingly appreciated
by water resource managers in a world Anastassia Makarieva and Victor Gorshkov developed the biotic pump theory at a nuclear physics institute.
of rampant deforestation. But the second
part, a theory Makarieva calls the biotic Many meteorology textbooks still teach about as fast as a racing bike, before the
pump, is far more controversial. a caricature of the water cycle, with ocean Andes Mountains divert them south. Salati
The theoretical foundation of the work evaporation responsible for most of the and others surmised the jet carried much
has been published, albeit in lesser known atmospheric moisture that condenses in of the transpired moisture, and dubbed it
journals, and Makarieva has received sup- clouds and falls as rain. The picture ignores a “flying river.” The Amazon flying river is
port from a small coterie of colleagues. But the role of vegetation and, in particular, now reckoned to carry as much water as the
the biotic pump has faced a head wind of trees, which act like giant water fountains. giant terrestrial river below it, says Antonio
criticism, especially from climate model- Their roots capture water from the soil for Nobre, a climate researcher at Brazil’s Na-
ers, some of whom say its effects are negli- photosynthesis, and microscopic pores in tional Institute for Space Research.
gible and dismiss the idea completely. The leaves release unused water as vapor into For some years, flying rivers were
dispute has made Makarieva an outsider: a the air. The process, the arboreal equivalent thought to be limited to the Amazon. In
theoretical physicist of sweating, is known as transpiration. In the 1990s, Hubert Savenije, a hydrologist
Half of the Amazon’s in a world of mod- this way, a single mature tree can release at the Delft University of Technology, be-
rain comes from the elers, a Russian in hundreds of liters of water a day. With its gan to study moisture recycling in West Af-
forest’s own moisture. a field led by West- foliage offering abundant surface area for rica. Using a hydrological model based on
Could it also make ern scientists, and the exchange, a forest can often deliver weather data, he found that, as one moved
winds that ferry rain a woman in a field more moisture to the air than evaporation inland from the coast, the proportion of
across continents? dominated by men. from a water body of the same size. the rainfall that came from forests grew,
SCIENCE sciencemag.org 19 JUNE 2020 • VOL 368 ISSUE 6497 1303
Published by AAAS
NE WS | F E AT U R E S
reaching 90% in the interior. The finding into clouds, a gas becomes a liquid that oc- is known to influence weather in faraway
helped explain why the interior Sahel re- cupies less volume. That reduces air pres- places through “teleconnections,” so, too,
gion became dryer as coastal forests disap- sure, and draws in air horizontally from could Amazon deforestation diminish rain-
peared over the past half-century. areas with less condensation. In practice, it fall in the U.S. Midwest and snowpack in the
One of Savenije’s students, Ruud van der means condensation above coastal forests Sierra Nevada, says Roni Avissar, a climato-
Ent, took the idea further, creating a global turbocharges sea breezes, sucking moist air logist at the University of Miami who has
model of airborne moisture flow. He com- inland where it will eventually condense and modeled such teleconnections. Far-fetched?
bined observational data on rainfall, hu- fall as rain. If the forests continue inland, the “Not at all,” he says. “We know El Niño can do
midity, wind speed, and temperature with cycle can continue, maintaining moist winds this, because unlike deforestation, it recurs
theoretical estimates of evaporation and tran- for thousands of kilometers. and we can see the pattern. Both are caused
spiration to create the first model of moisture The theory inverts traditional thinking: by small changes in temperature and mois-
flow at scales larger than river basins. It is not atmospheric circulation that drives ture that project into the atmosphere.”
In 2010, van der Ent and his colleagues the hydrological cycle, but the hydrological Lan Wang-Erlandsson, who researches
reported the model’s conclusion: Globally, cycle that drives the mass circulation of air. interactions between land, water, and cli-
40% of all precipitation comes from the Sheil, who became a supporter of the the- mate at Stockholm University, says it’s time
land rather than the ocean. Often it is more. ory more than a decade ago, thinks of it as an for water resource managers to shift their
The Amazon’s flying river provides 70% of embellishment of the flying river idea. “They focus from water and land use within a
the rain falling in the Río de la Plata Basin, are not mutually exclusive,” he says. “The river basin to land-use changes occurring
which stretches across southeastern South pump offers an explanation of the power of outside it. “We need new international
America. Van der Ent was most surprised to the rivers.” He says the biotic pump could ex- hydrological agreements to maintain the
find that China gets 80% of its water from plain the “cold Amazon paradox.” From Janu- forests of source regions,” she says.
Downloaded from http://science.sciencemag.org/ on June 18, 2020
the west, mostly Atlantic moisture recycled ary to June, when the Amazon Basin is colder Two years ago, at a meeting of the United
by the boreal forests of Scandi- Nations Forum on Forests, a
navia and Russia. The journey high-level policy group on which
involves several stages—cycles “We need new international hydrological agreements all governments sit, David
of transpiration followed by
downwind rain and subse-
to maintain the forests of source regions.” Ellison, a land researcher at the
University of Bern, presented a
quent transpiration—and takes Lan Wang-Erlandsson, Stockholm University case in point: a study showing
6 months or more. “It contra- that as much as 40% of the total
dicted previous knowledge that you learn than the ocean, strong winds blow from the rainfall in the Ethiopian highlands, the main
in high school,” he says. “China is next to an Atlantic to the Amazon—the opposite of what source of the Nile, is provided by moisture
ocean, the Pacific, yet most of its rainfall is would be expected if they resulted from dif- recycled from the forests of the Congo Basin.
moisture recycled from land far to the west.” ferential heating. Nobre, another early aco- Egypt, Sudan, and Ethiopia are negotiating
lyte, enthuses: “They don’t start with data, a long-overdue deal on sharing the waters of
IF MAKARIEVA IS CORRECT, the forests supply they start with first principles.” the Nile. But such an agreement would be
not just the moisture, but the winds that Even those who doubt the theory agree worthless if deforestation in the Congo Ba-
carry it. that forest loss can have far-reaching climatic sin, far from those three nations, dries up the
For a quarter-century, she worked with consequences. Many scientists have argued moisture source, Ellison suggested. “Inter-
Gorshkov, initially as his pupil, at PNPI— that deforestation thousands of years ago actions between forests and water have been
part of Russia’s foremost civil and military was to blame for desertification in the Aus- almost entirely ignored in the management
nuclear research agency, the Kurchatov In- tralian Outback and West Africa. The fear of global freshwater resources.”
stitute. They were mavericks from the start, is that future deforestation could dry up The biotic pump would raise the stakes
studying ecology in a place full of physicists other regions, for example, tipping parts of even further, with its suggestion that forest
who use neutron beams from nuclear reac- the Amazon rainforest to savanna. Agricul- loss alters not just moisture sources, but
tors to study materials. As theorists, she says, tural regions of China, the African Sahel, and also wind patterns. The theory, if correct,
they had “exceptional freedom of research the Argentine Pampas are also at risk, says would have “crucial implications for plane-
and thought,” pursuing atmospheric physics Patrick Keys, an atmospheric chemist at Col- tary air circulation patterns,” Ellison warns,
wherever it took them. “Victor taught me: orado State University, Fort Collins. especially those that take moist air inland
Do not be afraid of anything,” she says. In 2018, Keys and his colleagues used a to continental interiors.
In 2007, in Hydrology and Earth System model, similar to van der Ent’s, to track the
Sciences, they first outlined their vision for sources of rainfall for 29 global megacities. THE THEORY’S SUPPORTERS are a minority.
the biotic pump. It was provocative from He found that 19 were highly dependent on In 2010, Makarieva, Gorshkov, Sheil, Nobre,
the outset because it contradicted a long- distant forests for much of their water sup- and Bai-Lian Li, an ecologist at the Univer-
standing tenet of meteorology: that winds ply, including Karachi, Pakistan; Wuhan and sity of California, Riverside, submitted what
are driven largely by the differential heating Shanghai, China; and New Delhi and Kol- was meant to be a landmark description of
of the atmosphere. When warm air rises, it kata, India. “Even small changes in precipi- the biotic pump to Atmospheric Chemistry
lowers the air pressure below it, in effect tation arising from upwind land-use change and Physics, a major journal with open
creating space at the surface into which air could have big impacts on the fragility of ur- peer review. Titled “Where Do Winds Come
moves. In summer, for example, land sur- ban water supplies,” he says. From?” the paper faced a barrage of criticism
faces tend to heat faster and draw in moist Some modeling even suggests that by re- online, and it took the journal many months
breezes from the cooler ocean. moving a moisture source, deforestation to find two scientists willing to review it.
Makarieva and Gorshkov argued that a could alter weather patterns beyond the paths Isaac Held, a meteorologist at Princeton
second process can sometimes dominate. of flying rivers. Just as El Niño, a shift in cur- University’s Geophysical Fluid Dynamics
When water vapor from forests condenses rents and winds in the tropical Pacific Ocean, Laboratory, finally volunteered—and rec-
1304 19 JUNE 2020 • VOL 368 ISSUE 6497 sciencemag.org SCIENCE
Published by AAAS
ommended rejection. “This is not a mysteri- was published “not as an endorsement” but For her part, Makarieva is building on
ous effect,” he says. “It is small and included “to promote continuation of the scientific the theory, arguing in a series of recent
in some atmospheric models.” Critics said dialogue on the controversial theory [that] papers that the same mechanism can af-
the expansion of air from heat released may lead to disproof or validation.” fect tropical cyclones, which are driven
when water vapor condenses counter- Since then, there has been neither vali- by the heat released when moisture con-
acts the space-creating effect of condensa- dation nor disproof, but largely a standoff. denses over the ocean. In a 2017 paper in
tion. But Makarieva says the two effects are Gavin Schmidt, a climate modeler at Colum- Atmospheric Research, she and her col-
spatially separate, with the warming effect bia University, says, “It’s simply nonsense.” leagues proposed that biotic pumps set up
happening aloft, and the pressure drop of The authors’ responses to criticisms were by the forests on land draw moisture-rich
condensation occurring closer to the sur- “really just mathematics that gave no one air away from the cyclone nurseries. This,
face, where it generates the biotic wind. any confidence that there was any point in she says, might explain why cyclones rarely
The other reviewer was Judith Curry, then continuing the dialogue.” Jose Marengo, a form in the South Atlantic Ocean: The Am-
an atmospheric physicist at the Georgia meteorologist in Brazil and head of the Na- azon and Congo rainforests between them
Institute of Technology, who has long had tional Centre for Monitoring and Warning draw so much moisture away that there is
concerns about the atmospheric dynamics of Natural Disasters, says: “I think the pump too little left to fuel hurricanes.
at the core of climate models. She felt it was exists, but it’s very theoretical right now. The Kerry Emanuel, a leading hurricane re-
important to publish the paper and says the climate model community hasn’t embraced searcher at the Massachusetts Institute
standoff was “very bad for climate science, it, but the Russians are the best theoreti- of Technology, says the proposed effects
which badly needs an infusion from hard- cians in the world, so we need proper field “while not negligible are very small.” He
core physicists.” After 3 years of debate, the experiments to test it.” Yet no one, including prefers other explanations for the lack of
journal’s editor overruled Held’s recommen- Makarieva, has yet proposed clearly what South Atlantic hurricanes, such as the re-
Downloaded from http://science.sciencemag.org/ on June 18, 2020
dation and published the paper, saying it such a test might look like. gion’s cool waters, which send less mois-
ture into the air, and its strong shearing
winds, which disrupt cyclone formation.
Rain parades Makarieva is equally dismissive of the tra-
So-called flying rivers are prevailing winds that pick up water vapor exhaled by forests and deliver rains to ditionalists, saying some of the existing
distant water basins. A controversial theory suggests forests themselves drive the winds (bottom). theories for hurricane intensity “conflict
with the laws of thermodynamics.” She has
another paper on the topic under peer re-
view at the Journal of the Atmospheric Sci-
ences. “We are concerned that, despite the
editor’s encouragement, our work will get
Some 80% of rejected once again,” she says.
China’s rain Even if Makarieva’s ideas are fringy in
Flying rivers comes from the the West, they are taking root in Russia.
west, thanks to
Last year, the government began a public
a trans-Siberian
flying river. dialogue to revise its forestry laws. Aside
from strictly protected areas, Russian for-
ests are open to commercial exploitation,
but the government and the Federal For-
The Amazon’s estry Agency are considering a new des-
flying river ignation of “climate protection forests.”
provides 70%
of the rain for
“Some representatives of our forest depart-
southeastern ment got impressed by the biotic pump
South America. and want to introduce a new category,”
CREDITS: (GRAPHIC) N. DESAI/SCIENCE; (DATA) HANSEN ET AL., SCIENCE, 342 (6160) 2013
she says. The idea has the backing of the
Russian Academy of Sciences. Being part
Sowing the wind of a consensus rather than the perennial
The biotic pump theory suggests forests not only make rain, but also wind. When water vapor over coastal outsider marks a change, Makarieva says.
forests condenses, it lowers air pressures, creating winds that draw in moist ocean air. Cycles of transpiration This summer, the coronavirus lockdown
and condensation can set up winds that deliver rains thousands of kilometers inland. put the kibosh on her annual trip to the
northern forests. Back in St. Petersburg,
she has settled down to respond to yet an-
other round of objections to her work from
Moist air Condensation Transpiration Condensation Transpiration
anonymous peer reviewers. She insists
the pump theory will ultimately prevail.
“There is a natural inertia in science,” she
says. With a dark Russian humor, she in-
vokes the words of the legendary German
physicist Max Planck, who is said to have
once remarked that science “advances one
funeral at a time.” j
Fred Pearce is a journalist in London.
SCIENCE sciencemag.org 19 JUNE 2020 • VOL 368 ISSUE 6497 1305
Published by AAAS
Weather makers
Fred Pearce
Science 368 (6497), 1302-1305.
DOI: 10.1126/science.368.6497.1302
Downloaded from http://science.sciencemag.org/ on June 18, 2020
ARTICLE TOOLS http://science.sciencemag.org/content/368/6497/1302
PERMISSIONS http://www.sciencemag.org/help/reprints-and-permissions
Use of this article is subject to the Terms of Service
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published by the American Association for the Advancement of
Science, 1200 New York Avenue NW, Washington, DC 20005. The title Science is a registered trademark of AAAS.
Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of
Science. No claim to original U.S. Government Works
You can also read
