Amazon near tipping point of switching from
rainforest to savannah – study
Climate crisis and logging is leading to shift from
canopy rainforest to open grassland
Fiona
Harvey Environment correspondent
Mon 5 Oct
2020 10.00 BST
Much of the
Amazon could be on the verge of losing its distinct nature and switching from a
closed canopy rainforest to an open savannah with far fewer trees as a result
of the climate crisis, researchers have warned.
Rainforests
are highly sensitive to changes in rainfall and moisture levels, and fires and
prolonged droughts can result in areas losing trees and shifting to a
savannah-like mix of woodland and grassland. In the Amazon, such changes were
known to be possible but thought to be many decades away.
New
research shows that this tipping point could be much closer than previously
thought. As much as 40% of the existing Amazon rainforest is now at a point
where it could exist as a savannah instead of as rainforest, according to a study
published in the journal Nature Communications.
Any shift
from rainforest to savannah would still take decades to take full effect, but
once under way the process is hard to reverse. Rainforests support a vastly
greater range of species than savannah and play a much greater role in
absorbing carbon dioxide from the atmosphere.
Parts of
the Amazon are receiving much less rain than they used to because of the
changing climate. Rainfall in about 40% of the forest is now at a level where
the rainforest could be expected to exist as savannah instead, according to the
study, led by the Stockholm Resilience Centre, based on computer models and
data analysis.
Last year,
Brazil’s president, Jair Bolsonaro, was warned that the continued destruction
of the Amazon by fire and loggers would bring the region closer to a tipping
point where rainforest could turn to savannah. This year’s fires in the Amazon
are the worst in a decade, with a 60% increase in fire hotspots compared with
last year.
Arie Staal,
lead author of the study, said the ecology of rainforests meant that although
they effectively produce their own self-sustaining rainfall in the right
climate, they are also prone to drying out in the wrong conditions.
“As forests
grow and spread across a region, this affects rainfall,” he explained. “Forests
create their own rain because leaves give off water vapour and this falls as
rain further downwind. Rainfall means fewer fires leading to even more
forests.”
But if
large areas of rainforest are lost, rainfall levels in the region decline accordingly.
This reduced level of “atmospheric moisture recycling” was simulated in the
computer models used in the study.
“Drier
conditions make it harder for the forest to recover and increase the
flammability of the ecosystem,” Staal told the Guardian. Once rainforest has
crossed the threshold and converted to an open savannah-type mix of wood and
grassland, it is unlikely to revert naturally to its former state.
“It is
harder to return from the ‘trap’ caused by the feedback mechanism in which the
open, grassy ecosystem is more flammable, and the fires, in turn, keep the
ecosystem open,” he said.
The team of
researchers ran computer simulations of where forests might be expected to
exist across the earth’s tropical regions, given certain climatic conditions,
and looked at the minimum and maximum areas of likely forest cover.
They also
looked at what was likely to happen if greenhouse gas emissions kept rising,
and found that the ability of forests to grow back once trees were lost would
be much reduced.
Ingo Fetzer
of the Stockholm Resilience Centre, co-author of the paper, said: “We
understand now that rainforests on all continents are very sensitive to global
change and can rapidly lose their ability to adapt. Once gone, their recovery
will take many decades to return to their original state. And given that
rainforests host the majority of all global species, all this will be forever
lost.”
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