The scientists said their research did not
pinpoint when this threshold, which they described as a tipping point, might be
reached.

“But it’s worth reminding ourselves that if it
gets to that tipping point, that we commit to losing the Amazon rainforest,
then we get a significant feedback to global climate change,” said one of the
scientists, Tim Lenton, director of the Global Systems Institute at the
University of Exeter in England.

Losing the rainforest could result in up to 90
billion tons of heat-trapping carbon dioxide getting put back into the
atmosphere, he said, equivalent to several years of global emissions. That
would make limiting global warming more difficult.

Among previous studies there has been a large
degree of uncertainty as to when such a threshold might be reached. But some
research has concluded that deforestation, drying and other factors could lead
to substantial forest dieback in the Amazon by the end of this century.

Carlos Nobre, a senior scientist at the
National Institute of Amazonian Research in Brazil and one of the first to
sound alarm over the potential loss of the Amazon more than three decades ago,
described the new study as “very compelling.”

“It raised my level of anxiety,” said Nobre,
who was not involved in the research.

Covering more than 2 million square miles in
Brazil and neighboring countries, the Amazon is the world’s largest rainforest,
and serves a crucial role in mitigating climate change in most years by taking
in more carbon dioxide from the atmosphere than it releases. In its diversity
of plant and animal species, it is as rich as or richer than anywhere else on
the planet. And it pumps so much moisture into the atmosphere that it can
affect weather beyond South America.

But climate change, together with widespread
deforestation and burning for agriculture and ranching, has taken a toll on the
Amazon, making it warmer and drier. The region, one of the wettest on Earth,
has experienced three droughts since 2000.

Most previous studies of resiliency in the
Amazon relied on models, or simulations, of how forest health might change over
time. In the new research, the scientists used actual observations: decades of
remote sensing data from satellites that measure the amount of biomass in
specific areas, which corresponds to their health. Looking only at pristine
parts of the rainforest, the researchers found that overall since 2000 these
areas lost resilience. For example, it took increasingly longer for forested
areas to regain their health after suffering a drought.

“That lack of resilience shows that, indeed,
there is only so much of a beating that this forest can take,” said Paulo
Brando, a tropical ecologist at the University of California Irvine who was not
involved in the study. “It’s reducing the ability to bounce back.”

But Brando said this was not necessarily a
sign that a tipping point was unavoidable, and pointed to the need to stop
clear-cutting and forest degradation in the region. “These systems are highly
resilient, and the fact that we have reduced resilience doesn’t mean that it
has lost all its resilience,” he said. “If you leave them alone for a little
bit, they come back super strongly.”

The researchers found that more than
three-quarters of the untouched rainforest lost resiliency over that time, and
that the loss was greatest in areas that were drier or closer to human
activities like logging. The study was published in the journal Nature Climate
Change.

Chris Boulton, a researcher at the University
of Exeter and the study’s lead author, said that the Amazon was like a giant
water recycling network, as moisture from evaporation and transpiration from
trees is blown by winds. So the loss of some of the forest, and some of the
moisture, leads to more drying elsewhere.

“You can imagine that as the Amazon dries you
start to see that resilience being lost even faster and faster,” Boulton said.
Forests might then decline and die off relatively quickly and become more like
a savanna, with grasses and far fewer trees.

Not only would the loss of forest trees add
the carbon stored in their tissues back into the atmosphere, savannas would
also take up far less carbon than the large, broad-leafed trees they replaced.
Savanna habitat would also support far fewer species.

Nobre said the research shows that the Amazon
“is on the edge of this cliff, this switch to a different ecosystem.” And if it
were to happen, he added, “that would be the new ecosystem for hundreds of
years, perhaps thousands of years.”

About 17% of the Amazon has been deforested
over the past half-century, and while the pace of deforestation slowed for some
years in Brazil, it has picked up again more recently. The researchers said
their work showed that efforts to stop deforestation would not just protect
specific areas but have an effect on the resiliency of the Amazon as a whole.

“They are absolutely correct,” Nobre said. “We
have to get to zero deforestation, zero forest degradation,” adding, “We still
have a chance to save the forest.”

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