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Tropical forests have a crucial role against climate change. They act as global carbon sinks , removing CO2 from the atmosphere and storing it in tree biomass. Until now, scientists have developed climate models under the premise that this process, known as carbon sequestration or sequestration, will remain stable.
But the ability of the world's tropical forests to neutralize carbon is declining, according to the results of a study published Wednesday in Nature , which has analyzed the behavior of 565 tropical forests over the past 30 years. .
The new analysis has monitored the evolution - or disappearance - of 300,000 trees in Africa and the Amazon since 1983. The results reflect that the global carbon uptake in the virgin tropical forests of the planet reached its maximum capacity in the 1990s and that , arrived in 2010, had already decreased a third.
The causes of this rapid fall are in the loss of biomass: the forest area decreased by 19% during that period, while global emissions increased by 46% .
"The additional carbon dioxide in the atmosphere drives the growth of trees. But, each year, this effect is increasingly offset by the negative impacts of higher temperatures and droughts, which slow down growth and can kill trees, "explains Wannes Hubau , a scientist at the Royal Museum of Central Africa in Belgium and lead author of the study, in which almost 100 institutions around the world have collaborated.
The report points out that we are immersed in a cycle change for tropical forests , which are going from being a sink to becoming another source of CO2.
Reassess climate models
The authors also indicate that the reversal of the trend will arrive earlier in the Amazon basin than in the forests of Africa . "Our model shows a long-term decline of the African sink, while the Amazon will continue to weaken rapidly: we expect it will become a source of carbon in the mid-2030s," says Hubau.
The difference, explains the researcher, is because South American forests are more dynamic: they are exposed to higher temperatures, more sudden temperature increases and also to more regular and severe droughts .
In view of the conclusions of the study, the researchers believe that current global estimates and targets on emissions should be revaluated "urgently" to adapt to this new cycle.
"The ability of tropical forests to absorb carbon is declining decades earlier than the most pessimistic climate models had anticipated, " says Spanish scientist Aida Cuni Sánchez, one of the co-authors of Nature's article.
"The consequence is terrible: climate change will be worse than had been estimated, since tropical forests will stop removing some of the carbon and start emitting it," adds this scientist.
Amazonia and the Congo
To analyze the evolution of the process, the authors have periodically measured the diameter of each tree and estimated its height, returning every few years to the same wooded areas to observe its transformation. Based on the accumulated data on growth, evolution and mortality, they have been able to calculate global changes.
Some of the authors have periodically traveled to remote enclaves, including one that requires a one-week canoe trip to the Salonga National Park in the Democratic Republic of the Congo.
"After years of work in tropical forests of the Congo and the Amazon, we have found that one of the most worrisome impacts of climate change has already begun , decades earlier than in the most pessimistic models," says Simon Lewis, a researcher at the Faculty of Geology at the University of Leeds.
After the last measurements, made in 2014, the authors applied a statistical model, with trends in carbon dioxide emissions, temperature and rainfall, to estimate the evolution of carbon storage in forests in the coming decades.
Combining data from two large research networks (AfriTRON in Africa and RAINFOR in Amazonia), they observed that the Amazon sink began to weaken first , starting in the mid-1990s, while the trend began in Africa 15 years later.
Virgin tropical forests eliminated 17% of the carbon dioxide emissions produced by mankind worldwide in the 1990s, but only 6% in the 2010s . That represents a difference of 21,000 million tons , which is equivalent to a decade of fossil fuel emissions from the United Kingdom, Germany, France and Canada.
"Virgin tropical forests remain a vital carbon sink," Lewis notes, "but what this research reveals is that unless policies are implemented to stabilize the planet's climate, it is only a matter of time that they cannot continue to capture more carbon . "
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