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Until now, of all the rainforests, the Amazon has been considered the most powerful carbon sink. But African forests have just dethroned it: they now absorb a larger part of the increase in global CO emissions2.
This is what a study published in the journal reveals Nature, Thursday, March 5, which warns of the weakening of these natural wells, essential to mitigate the rise in CO concentrations2 in the air. “The peak of carbon sequestration by tropical forests took place in the 1990s and, since then, their capacity has been decreasing”says Dr. Wannes Hubau, a researcher at the Royal Museum for Central Africa (Brussels) and first author of the article. The Amazon, whose sequestration capacity has decreased by a third in twenty years, could even release more CO2 that it will not absorb by 2040.
This decline is explained primarily by deforestation, but the internal dynamics of forest ecosystems under the effect of warming also comes into play. It is the originality of this study to have focused its observations on "Intact forests", to measure the impact of climate change on tree growth and dieback. 565 plots, with an average area of one hectare, distributed in the Amazon and Africa have been monitored at regular intervals over thirty years.
Massive withering episodes
Asia where "Intact forests" are residual has been left out. "The destruction of forests has so far attracted more attention from scientists, but there is also a more invisible transformation that needs to be realized"continues Wannes Hubau, questioned by The World Africa.
All forests do not react in the same way and the large massifs of the Congo basin resist better. On both continents, the increase in CO concentrations2 in the atmosphere first has a tremendous acceleration of tree growth. But rising temperatures and recurring droughts are increasingly counterbalancing this phenomenon. Particularly in the Amazon, where droughts have caused massive episodes of decline.
In contrast, the relative altitude – 200 meters above sea level – has protected African forests with temperatures 1.1 ° C lower on average than those in the Amazon. “African forests have a slower life cycle. Carbon remains trapped for another 15 years before mortalities occur. This also explains the gap between the two forest blocks ", continues the researcher. While the Amazon’s ability to store carbon began to decrease in the mid-1990s, the curve reversed only a few years ago in Africa.
Dreaded tipping points
No turning back should be expected. The mathematical model used to predict the future of these carbon sinks shows that they will reach saturation within a few decades, much sooner than predicted by the studies used by the IPCC (intergovernmental group of experts on the climate change) to establish its scenarios: around 2040 for the Amazon and 2060 for Africa.
These dates, although approximate, indicate tipping points (tipping points) feared by scientists because they mark the border from which nature itself becomes a factor in accelerating climate change. "This means that to contain global warming below 2 ° C, the objective set by the signatories of the Paris agreement, greater efforts must be made", warns Wannes Hubau. The use of the compensation mechanism for " to erase " partially the emissions of certain countries or large companies could thus find their limits faster than expected.
However, insists the professor of the Royal Museum for Central Africa to whom this study required seven years of work, "The tropical forests remain gigantic reservoirs and it is important to do everything to preserve them". In their trees alone, that is to say without taking into account the organic way of the grounds, they store 250 billion tons of carbon, which is equivalent to 90 years of world emissions of fossil fuels at the current level.
