Bayaka people in the Central African Republic rainforest. Image: By JMGRACIA100, via Wikimedia Commons
Climate change so far has meant more vigorous forest growth as greenhouse gases rise. The tropical forests may soon change that.
LONDON, 6 March, 2020 – Within about fifteen years, the great tropical forests of Amazonia and Africa could stop absorbing atmospheric carbon, and slowly start to release more carbon than growing trees can fix.
A team of scientists from 100 research institutions has looked at the evidence from pristine tracts of tropical forest to find that – overall – the foliage soaked up the most carbon, most efficiently, more than two decades ago.
Since then, the measured efficiency of the forests as a “sink” in which carbon is sequestered from the atmosphere has been dwindling. By the last decade, the ability of a tropical forest to absorb carbon had dropped by a third.
All plant growth is a balancing act based on sunshine and atmospheric carbon and rainfall. Plants absorb carbon dioxide as they grow, and surrender it as they die.
In a dense, undisturbed wilderness, fallen leaves and even fallen trees are slightly less likely to decompose completely: the atmospheric carbon in leaf and wood form has a better chance of being preserved in flooded forests as peat, or being buried before it can completely decompose.
The forest becomes a bank vault, repository or sink of the extra carbon that humans are now spilling into the atmosphere from car exhausts, factory chimneys and power station furnaces.
Theory and practice
And in theory, as more and more carbon dioxide gets into the atmosphere, plants respond to the more generous fertilization by growing more vigorously, and absorbing more carbon.
But as more carbon gets into the atmosphere, the temperature rises and weather patterns begin to become more extreme. Summers get hotter, rainfall more capricious. Then trees become vulnerable to drought, forest fire and invasive diseases, and die more often, and decompose more completely.
Wannes Hubau, once of the University of Leeds in the UK and now at the Royal Museum for Central Africa in Belgium, and more than 100 colleagues from around the world, report in the journal Nature that they assembled 30 years of measurement from more than 300,000 trees in 244 undisturbed plots of forest in 11 countries in Africa, and from 321 plots of forest in Amazonia, and did the sums.
In the 1990s, intact tropical forests removed around 46 billion tonnes of carbon dioxide from the atmosphere. By the 2010s, the uptake had fallen to around 25 billion tonnes. This means that 21 billion tons of greenhouse gas that might otherwise have been turned into timber and root had been added to the atmosphere.
This is pretty much what the UK, France, Germany and Canada together spilled into the atmosphere from fossil fuel combustion over a 10-year period.
“We’ve found one of the most worrying impacts of climate change has already begun. This is decades ahead of even the most pessimistic climate models”
“Extra carbon boosts tree growth, but every year this effect is being increasingly countered by the negative impacts of higher temperatures and droughts which slow growth and can kill trees,” said Dr Hubau.
“Our modeling shows a long-term decline in the African sink and that the Amazon sink will continue to rapidly weaken, which we predict will become a carbon source in the mid-2030s.”
Tropical forests are an integral factor in the planetary carbon budget – a crude accounting system that climate scientists rely upon to model the choice of futures that face humankind as the world heats up.
Around half of Earth’s carbon is stored in terrestrial vegetation and the tropical forests account for about a third of the planet’s primary productivity. So how forests respond to a warmer world is vital.
Because the Amazon region is being hit by higher temperatures, and more frequent and prolonged droughts than forests in tropical Africa, Amazonia is weakening at a faster rate.
But decline has also begun in Africa. In the 1990s, the undisturbed tropical forests alone inhaled 17% of human-made carbon dioxide emissions. In the decade just ended, this proportion fell to 6%.
Catastrophic prospect
In roughly the same period, the area of intact forest fell by 19%, and global carbon dioxide emissions rose by 46%. Even so, the tropical forests store 250 billion tonnes of carbon in their trees alone: 90 years of fossil fuel emissions at the present rate. So their sustained loss would be catastrophic.
“Intact tropical forests remain a vital carbon sink but this research reveals that unless policies are put in place to stabilize the Earth’s climate, it is only a matter of time until they are no longer able to sequester carbon,” said Simon Lewis, a geographer at the University of Leeds, and one of the authors.
“One big concern for the future of humanity is when carbon-cycle feedbacks really kick in, with nature switching from slowing climate change to accelerating it.
“After years of work deep in the Congo and Amazon rain forests, we’ve found one of the most worrying impacts of climate change has already begun.
“This is decades ahead of even the most pessimistic climate models. There is no time to lose in tackling climate change.”
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