The Amazon rainforest less sponge in less carbon emitted by man
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The Amazon rainforest is sick and this is one of the worst news brought by science, in recent years, the climate front. Thursday, March 19, in the latest edition of the journal Nature, a hundred researchers publish the results of a project of gigantic research, led for over three decades and for monitoring in a warmer world, the evolution of Amazonian jungle. The highlight of the presented findings is that it sponge less carbon dioxide (CO 2) anthropogenic. About a quarter of it is currently absorbed by terrestrial vegetation, tropical forests play a key role in this process.
The bad news was expected, but the published figures are striking in their magnitude. In the 1990s, the Amazon was withdrawing from the atmosphere each year some 2 billion tonnes of CO 2. This rate would have fallen by a third in the 2000s and hardly exceed today a billion tonnes of CO 2. In more than two decades, the efficiency of "carbon sinks" Amazon would have been halved.
"The mechanical consequence of this observation is that the CO 2 will accumulate faster in the atmosphere," says Jérôme Chave, researcher at Evolution and Biodiversity (CNRS-Université Paul Sabatier Toulouse III) and co-author of this work . With the possible result of the worsening of global forecast for the end of the century, obtained through climate models. According to Roel Brienen, a researcher at the University of Leeds (UK) and first author of the work, "the climate models that include vegetation response assume that as long as CO2 levels continue to rise, Amazon will continue to accumulate carbon, but our study shows that this might not be the case. "
Higher mortality rate of trees by 30% in thirty years
To determine their findings, the researchers conducted a painstaking job. More than 320 forest patches, each measuring about one hectare, spread over six million square kilometers of the Amazon basin were visited at regular intervals since the mid-1980s growth and mortality of vegetation have were recorded at each visit all tree trunk diameter greater than 10 cm have been included. Result: since records began, the mortality rate has on average increased by 30%.
"The two exceptional drought that hit the region in 2005 and 2010, played a role in the increased mortality, says Damien Bonal, researcher at Forest Ecology and Ecophysiology (INRA), co-author of the study. But also see that this process has begun long before 2005. "
Role of climate change
The main suspect is of course the ongoing climate change, but the precise mechanisms by which it acts negatively on vegetation is not absolutely clear. In a commentary published by Nature, Lars Hedin (Princeton University, USA) believes "likely" that "the availability of water, the limits of available nutrients or heat stress" play a role.
What we are seeing, basically says Jérôme Chave, is probably the replacement of species of trees more susceptible to disruptions by other species. Upon completion of replacement, will be seen in the Amazon carbon sinks become as important as before? "It is unlikely, said Mr. Chave. For species that tend to replace those that have declined faster growth, a period shorter life and tend to store less carbon. »
The health status of the other major tropical forest basins - in South Asia, Equatorial Africa - is suddenly, too, a concern, even beyond the rapid rampant deforestation in these areas "From. severe droughts have also affected these areas, but this does not necessarily mean that the situation is identical to that of the Amazon, "cautions Mr. Bonal, adding that the results presented should be confirmed by other methods of measurement the forest inventory - especially direct measurements of CO2 fluxes over the forest.
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