Science - USA (2019-08-30)

SCIENCE sciencemag.org 30 AUGUST 2019 • VOL 365 ISSUE 6456 853

PHOTO: © VICTOR MORIYAMA/GREENPEACE

NEWS | IN DEPTH

from the proportions and the known half-

life of the uranium, he and his colleagues

could calculate an age. They ended up with a

date of 2.229 billion years old, plus or minus

5 million years.

That puts the impact at a turbulent time

in Earth’s history. Life had existed for more

than 1 billion years, but photosynthetic life—

cyanobacteria living in shallow waters—was

a recent evolutionary invention, one that

triggered a sharp rise in atmospheric oxy-

gen about 2.4 billion years ago. Previously,

high levels of methane in the atmosphere

had generated a greenhouse effect that

warmed the planet. But many scientists

think the methane was destroyed by chemi-

cal reactions with Earth’s first ozone, pro-

duced when ultraviolet light from the sun

struck the oxygen molecules. They suspect

loss of methane sent Earth crashing into a

set of severe and long-lived ice ages, even at

low latitudes. Three or maybe four of these

icy episodes took place between 2.45 billion

and 2.22 billion years ago, which means

Australia might have been covered in ice at

the time of the Yarrabubba impact.

Scientists have assumed that volcanic

eruptions ended the ice ages, by belching

carbon dioxide and warming the planet.

But Erickson and his colleagues speculate

that Yarrabubba could have helped. They

modeled the effect of a 7-kilometer-wide as-

teroid striking an ice sheet between 2 and

5 kilometers thick. They found the impact

could have spread dust thousands of kilo-

meters, darkening ice and enhancing its

ability to absorb heat. It also would have

sent half a trillion tons of steam into the

stratosphere—orders of magnitude more

water vapor than in today’s stratosphere—

where it would have trapped heat.

Andrey Bekker, a geologist at UC Riverside,

doubts that the water vapor would have per-

sisted for the centuries needed to thaw Earth.

“I’m not convinced that by itself it could do

this job,” he says. Christian Koeberl, an im-

pact expert and the director general of the

Natural History Museum in Vienna, shares

those doubts, but says paleoclimate research-

ers need to model the effects explicitly.

If the Yarrabubba impact did thaw the

planet, allowing life to reclaim icy conti-

nents and oceans, it wouldn’t be the first

example of life benefiting from a cosmic

blow, Koeberl says. Although the public

tends to associate impacts with extinctions,

he notes that impacts 4 billion years ago

could have jump-started life. Asteroids de-

livered phosphorus, a key nutrient, and the

impacts also created the protected, energy-

rich hydrothermal systems where some

biologists believe life began. “Impacts can

be bringers of life, impacts can be destroy-

ers of life,” he says. j

Amazon fires clearly linked to

deforestation, scientists say

Brazilian government deflects blame for rise in fire activity

ENVIRONMENTAL POLICY

B

razil’s government claims its policies

aren’t responsible for the fires that

are ravaging the Amazon rainforest

and triggered worldwide indignation

last week. President Jair Bolsonaro

suggested nongovernmental orga-

nizations were setting the forest ablaze to

discredit his government; his minister of the

environment, Ricardo Salles, tweeted that

“dry weather, wind, and heat” were to blame.

Scientists dismiss those claims. “There is

no doubt that this rise in fire activity is as-

sociated with a sharp rise in deforestation,”

says Paulo Artaxo, an atmospheric physicist

at the University of São Paulo in São Paulo.

Thousands of fires occur in the Amazon

annually, but the numbers have risen since

Bolsonaro became president on 1 January

and began to encourage development. In

satellite images, Brazil’s National Institute

for Space Research (INPE) counted more

than 41,000 “fire spots” between 1 January

and 24 August, compared with 22,000 in

the same period last year. The Global Fire

Emissions Database, a collaboration be-

tween NASA ’s Goddard Space Flight Center

in Greenbelt, Maryland and two universi-

ties, sees a similar trend. The numbers are

the highest since 2010, when the Amazon

experienced a severe drought triggered by

El Niño and a warming of the North Atlan-

tic Ocean. This time, climatic anomalies

can’t explain the uptick, scientists say.

Deforestation can, at least partly. To clear

land for farming, settlers fell trees, remove

valuable timber, and then set fire to the re-

mainder. Recent INPE data showed defores-

tation to be on the rise, although Bolsonaro

called the numbers “a lie” and had INPE

Director Ricardo Galvão fired (Science,

2 August, p. 419). The 10 municipalities

with the highest rate of fire activity are also

the ones where the forest disappeared most

rapidly this year, according to the Amazon

Environmental Research Institute in Belém,

Brazil. And many of the recently detected

spots are active for more than a day, burn-

ing with intense heat and producing tall,

thick, smoke pillars—all indicators that

trees are on fire, not overgrown pastures,

crop residues, or roadside vegetation.

Bolsonaro has so far rejected interna-

tional pressure to protect the rainforest.

He hurled insults at Norway and Germany

after they suspended contributions to the

Amazon Fund, which supports conserva-

tion and sustainable development, and re-

fused $20 million offered by the G-7 nations

to help fight the fires. But facing mount-

ing criticism at home, Bolsonaro ordered

the military to help combat the fires and

made a 5-minute TV address to profess his

“deep love and respect for the Amazon” and

promise that his administration would “act

strongly” against the blazes. j

Herton Escobar is a science journalist in

São Paulo, Brazil.

By Herton Escobar

A forest fire in Altamira, in Brazil’s Pará state. President Jair Bolsonaro has rejected help from G-7 members.

Published by AAAS