Could the Amazon become a Grassy Savanna?

Maxime Bilodeau – The Rumor Detector

Agence Science-Presse (www.sciencepresse.qc.ca)

Ravaged by humans, the “lungs of the planet” are running out of breath at a breathtaking pace. Is it possible that the Amazon rainforest could become dry grassland, in other words a savanna, by the end of this century? The Rumor Detector explains where this worry comes from. 

The Origin of the Rumor

In physics and chemistry there is a concept called “the tipping point” or “the point of no return.” It’s the threshold where a system changes its state: the simplest example is that of a liquid that changes to a gas under the effects of temperature.

Applied to our planet’s climate, this concept implies that given certain thresholds – a higher average temperature, abnormal rainfall or drought – an ecosystem’s conditions will irreversibly deteriorate. The best-known example is the Arctic. If greenhouse gases get above a certain level in the atmosphere, the polar ice caps will disappear forever even if we suddenly radically diminish our greenhouse gas emissions.

Could the Amazon go from rainforest to savanna without us having to cut down all the trees?

Worries

The tipping point of the world’s largest tropical forest – 5.5 million km² – could be reached within as little as five years, estimate certain specialists recently cited by the New Scientist. The Amazon may be on the verge of being at the stage where it irreversibly turns into a much drier ecosystem.

The cause: climate change caused by humans, forest fires, and the clearcutting of forests. The Amazon rainforest assures the health of the world through its immense carbon reserves, which also serve as a reservoir of biodiversity.

Over three-quarters of the Amazon is less resilient than in the year 2000, according to a research paper by three British researchers. This is particularly visible “in regions with less rainfall and in parts of the rainforest that are closer to human activity,” according to the study. That means that the tropical rainforest is finding it tougher to recover from the nasty effects that are hitting at it.

We’ve known for years from satellite images that Amazonia is disappearing little by little.  A report by the Science Panel for the Amazon – researchers from eight countries brought together by the UN’s sustainable development program – concludes that deforestation has cost the Amazon rainforest 17% of its territory since the 1970s. And in Brazil, which has over half the rainforest, that figure is up to 20%. Indeed, deforestation has reached record highs since President Jair Bolsonaro came to power in 2019.   

But the whole question of a “point of no return” rests on knowing just where that threshold lies. In a report published in 2020 in Nature magazine, researchers situated the tipping point at between 20 and 25% deforestation. Not everyone agrees: the tipping point may be higher. But this uncertainty is, for one of the researchers interviewed, the equivalent of “playing environmental Russian roulette.” The Amazon has already heated up by an average of 1.2°C since the industrial revolution, which is a bit higher than the rest of the planet.

The most recent climate models add data on the “dynamic” of vegetation, with the goal of more precisely predicting its evolution in time. Thusly one of the indicators of a perishing forest, according to British researchers, would be the widening of the gap between the highest and lowest temperatures in a region.  

A Subtle Dynamic

What makes Amazonia such a dense, rich environment is a fragile equilibrium in the water cycle. It is a humid forest because of its abundant rainfall. An important part of that humidity comes from soil evaporation and the transpiration of plants. The more trees we cut down, the more we run the risk of ruining this subtle dynamic: after a certain threshold, the vicious circle of drying is unleashed.

And this dynamic seems to have already started. Part of the forest releases more carbon dioxide into the atmosphere than it absorbs. Between 2010 and 2019, the Brazilian Amazon’s losses in carbon were close to 20 % greater than its gains, making it a net emitter of CO².

The consequences of this “savannization” are multiple: a decrease in biodiversity, a loss of cultural traditions, an upending of the local economy.

And above all, an Amazon composed of less vigorous trees will capture much less carbon than the 120 billion tons currently stocked there today. That’s what makes the disappearance of the Amazon rainforest a threat to the climate on the same level as the melting of the polar ice caps.

A halt to forest clearcutting and the restoration of ravaged land are among the solutions proposed to stop the trend. During the COP26 United Nations climate conference in November 2021, Brazil signed a multinational agreement pledging to end deforestation by 2030. Considering past actions, the news was greeted with scepticism.

French version on the Reflet de Société website

This article is part of the Rumor Detector series; click here for other articles in the series.