Amazon forest canopy at dawn. The loss of
forests as ‘carbon sinks’ is likely to make climate breakdown more severe.
Tropical forests losing their ability to absorb
carbon, study finds
Amazon could turn into source of CO2 in atmosphere by
next decade, research suggests
Fiona
Harvey Environment correspondent
Wed 4 Mar
2020 17.08 GMTFirst published on Wed 4 Mar 2020 16.00 GMT
Tropical
forests are taking up less carbon dioxide from the air, reducing their ability
to act as “carbon sinks” and bringing closer the prospect of accelerating
climate breakdown.
The Amazon
could turn into a source of carbon in the atmosphere, instead of one of the
biggest absorbers of the gas, as soon as the next decade, owing to the damage
caused by loggers and farming interests and the impacts of the climate crisis,
new research has found.
If that
happens, climate breakdown is likely to become much more severe in its impacts,
and the world will have to cut down much faster on carbon-producing activities
to counteract the loss of the carbon sinks.
“We’ve
found that one of the most worrying impacts of climate change has already
begun,” said Simon Lewis, professor in the school of geography at Leeds
University, one of the senior authors of the research. “This is decades ahead
of even the most pessimistic climate models.”
For the
last three decades, the amount of carbon absorbed by the world’s intact
tropical forests has fallen, according to the study from nearly 100 scientific
institutions. They are now taking up a third less carbon than they did in the
1990s, owing to the impacts of higher temperatures, droughts and deforestation.
That downward trend is likely to continue, as forests come under increasing
threat from climate change and exploitation. The typical tropical forest may
become a carbon source by the 2060s, according to Lewis.
“Humans
have been lucky so far, as tropical forests are mopping up lots of our
pollution, but they can’t keep doing that indefinitely,” he told the Guardian.
“We need to curb fossil fuel emissions before the global carbon cycle starts
working against us. The time for action is now.”
At this
year’s UN climate talks, known as Cop26 and to be held in Glasgow in November,
many countries are expected to come forward with plans to reach net zero
emissions by mid-century. But some rich countries and many companies plan to
reduce their emissions via offsetting, often by preserving, replanting or
growing new forest.
This
research shows that relying on tropical forests is unlikely to be enough to
offset large-scale emissions. “There is a lot of talk about offsetting, but the
reality is that every country and every sector needs to reach zero emissions,
with any small amount of residual emissions needing to be removed from the
atmosphere,” said Lewis. “The use of forests as an offset is largely a
marketing tool for companies to try to continue with business as usual.”
The uptake
of carbon from the atmosphere by tropical forests peaked in the 1990s when
about 46bn tonnes were removed from the air, equivalent to about 17% of carbon
dioxide emissions from human activities. By the last decade, that amount had
sunk to about 25bn tonnes, or just 6% of global emissions.
The
difference is about the same as a decade of fossil fuel emissions from the UK,
Germany, France and Canada put together.
Climate
scientists have long feared the existence of “tipping points” in the climate system,
which when passed will condemn the world to runaway global heating. There are
many known feedback mechanisms: for instance, the melting of Arctic ice leaves
more of the sea uncovered, and, as it is darker than the reflective ice, it
absorbs more heat, thus leading to more melting.
These
feedback mechanisms have the potential to accelerate the climate crisis far
ahead of what current projections suggest. If forests start to become sources
of carbon rather than absorbers of it, that would be a powerful positive
feedback leading to much greater warming that would be hard to stop.
Forests
lose their ability to absorb carbon as trees die and dry out from drought and
higher temperatures, but the loss of forest area from logging, burning and
other forms of exploitation is also a leading factor in the loss of carbon
sinks.
Tom
Crowther, founder of the Crowther Lab, who was not involved with the research,
told the Guardian: “This analysis provides concerning evidence that, along with
continuing deforestation rates, the carbon sequestration rate of tropical
forests could also be threatened by increasing tree mortality under climate
change. This is very important information, as the capacity of tropical forests
to capture anthropogenic carbon emissions could be severely impaired.”
The study,
published on Wednesday in the journal Nature, tracked 300,000 trees over 30
years, providing the first large-scale evidence of the decline in carbon uptake
by the world’s tropical forests. The researchers combined data from two large
research networks of forest observations in Africa and the Amazon, as well as
years spent travelling to remote field sites, including a week spent in a
dug-out canoe to reach Salonga national park in the troubled Democratic
Republic of the Congo.
They used
aluminium nails to tag individual trees, measuring the diameter and estimating
the height of every tree within 565 patches of forest, and returning every few
years to repeat the process. This enabled them to calculate the carbon stored
in the trees that survived and those that died. They found that the Amazon sink
started weakening first, but that African forests are now rapidly following.
Amazonian forests are exposed to higher temperatures, faster temperature
increases, and more frequent and severe droughts, than African forests.
Their
projection that the Amazonian forest will turn into a carbon source in the
mid-2030s is based on their observations and a statistical model and trends in
emissions, temperature and rainfall to forecast changes in how forests will
store carbon up to 2040.
Doug Parr,
the chief scientist at Greenpeace UK, said governments should heed the science
and make strong commitments to cut greenhouse gases at the Cop26 summit, and
agree to measures to protect and restore forests. “For years, we have had
scientific warnings about tipping points in the Earth system and they’ve been
largely ignored by policy and decision-makers,” he said. “That forests are now
seemingly losing the ability to absorb pollution is alarming. What more of a
wake-up call do we need?”
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