Yes, the Amazon could become a savannah


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The origin of the rumor

There is a concept in physics and chemistry called “turning point” or “point of no return”. It is the threshold from which a system changes state: the simplest example is that of a liquid that is transformed into gas by the effect of temperature.

Applied to the climate of our planet, this concept implies that from certain thresholds — a higher average temperature, an abnormal level of precipitation, or drought — the conditions of an ecosystem or environment deteriorate irreversibly. The best-known example is the Arctic: beyond a certain amount of greenhouse gases (GHGs) in the atmosphere, the ice sheet will inevitably disappear, even if we drastically reduce our greenhouse gas emissions. greenhouse effect.

Could the Amazon move from the “state” of the forest to the savannah, without having to cut down all the trees?

Concerns

The turning point of the world’s largest rainforest – 5.5 million km2 – could be crossed in just five years, according to some experts recently quoted by New Scientist. The Amazon would be more precisely on the verge of reaching the stage from which it will be irreversibly transformed into a drier ecosystem.

At issue: man-made climate change, forest fires and logging that undermine the overall health of this immense carbon sink that is also a reservoir of biodiversity. More than three-quarters of the Amazon is less resilient than in the early 2000s, according to a study published in March by three British researchers. This would be especially noticeable “in the regions with less rainfall and in the parts of the forest closest to human activity.” This means that the rainforest is struggling to recover from the disturbances that have affected it.

It is true that the data collected by the satellites have revealed for several years an Amazon that is gradually deteriorating. A recent report from Scientific panel for the Amazon – researchers from eight countries brought together by the initiative Solutions for sustainable development of the United Nations – concluded that deforestation had amputated 17% of the forest territory since the 1970s. In Brazil, which hosts more than half of the Amazon, this figure would be 20%. Deforestation has even reached record levels there since President Jair Bolsonaro came to power in 2019.

But the whole point of a “point of no return” is to know exactly what that threshold is. In a report published in 2020 by the magazine Nature, researchers put it at between 20% and 25% deforestation. Not everyone agrees: the turning point could be higher. But this uncertainty is, for one of the researchers interviewed, the equivalent of playing “environmental Russian roulette”. The Amazon has already warmed by an average of 1.2 ° C since the beginning of the industrial era, which is slightly more pronounced than the average for the rest of the planet.

Newer climate models now add data on the “dynamics” of vegetation, with the aim of predicting its evolution over time more accurately. This is how one of the indicators of forest decline, highlighted in March by British researchers, would be the widening of the gap between the highest and lowest temperatures in a region.

A subtle mechanism

What makes the Amazon such a dense and rich environment is the result of a delicate balance in the water cycle. It is a forest called “humid” because the rains are abundant. An important part of this moisture in the air comes from the evaporation of the soil and the transpiration of the plants. The more trees we cut, the more we run the risk of stopping this subtle mechanism: after a certain threshold, the vicious circle of desiccation is activated.

And the mechanics already seem stuck: part of the forest releases more carbon dioxide into the atmosphere than it absorbs. Between 2010 and 2019, the carbon losses of the Brazilian Amazon were almost 20% greater than the gains, which technically made it a CO2 emitter.

The consequences of this savannah will be many: loss of biodiversity, loss of traditional cultural milestones, disturbances of the local economy.

And most importantly, an Amazon made up of less vigorous trees will capture far less carbon than the 120 billion tons it stores today. This is what makes the decline of the Amazon rainforest a threat to both the climate and the melting of glacial caps.

Stopping logging and restoring devastated land are among the proposed solutions to reverse the trend. At the COP26 in November 2021, Brazil signed a multinational commitment to stop deforestation in 2030. However, given past action, the news was received with skepticism.

Photo: Bruno Kelly / Amazônia Real, August 2020 / Wikipedia Commons

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