The South American monsoon is approaching a tipping point

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Observations and simulations suggest  the South American monsoon might be approaching an abrupt transition due to the combined threat of deforestation, fires, and droughts in the Amazon rainforest

Using observations and computer modeling Nils Bochow, University of Tromsø, Norway, and Niklas Boers, Technical University of Munich and Potsdam Institute for Climate Impact Research, Germany find that the South American monsoon could be nearing a critical transition to a substantially weaker state. The work was published in Science Advances today.

”People in South America should be worried about our result. Large parts of their economy and food supply depend on the rainforest and the monsoon system. South American agriculture is very dependent on the wet season there,” explains Nils Bochow.

Critical for ecology, societies, and climate

The South American monsoon and the Amazon rainforest strongly depend on each other. If one disappears, the other is likely doomed.

An abrupt weakening of the monsoon system would be critical for South America’s ecosystems and societies. The Amazon is home to thousands upon thousands of species that do not live elsewhere on the planet. The monsoon supplies water for this unique ecosystem. But also for agriculture, rural and urban areas, and electricity production in most of the continent.

Additionally, an abrupt transition of the Amazon Rainforest to a savanna-like state would impact the climate system globally, since historically the Amazon has been a large terrestrial carbon sink.

How it works

Two phenomena contribute to kickstarting the South American monsoon at the end of the dry season. The seasonal changes in the Intertropical convergence zone shift the trade winds southwards, bringing significant amounts of precipitation to the Eastern Coast of South America. Simultaneously, inland, billions of plants in the rainforest evaporate huge amounts of water into the air above the forest as they respond to the intense heat at the end of the dry season.

The moist air over the forest changes the energy in the atmosphere, giving rise to the formation of rain clouds and an overall updraft that enhances the winds from the east.

This establishes positive feedback between the rainforest and the monsoon system, facilitating a constant flow of moisture from the coastal regions westwards across the rainforest. In other words, the forest produces the energy that is needed to transport the moisture of the monsoon rain from the Atlantic coast across the continent.

Signs of instability

Analyses of satellite images have previously revealed that the Amazon rainforest has been losing stability over the last decades due to deforestation, wildfires, and droughts. Theoretically, this should additionally destabilize the monsoon system, bringing it at risk of passing a threshold where it would be severely weakened, with a prolonged dry season.

To estimate the risk Nils Bochow and Niklas Boers simulated forest loss in a coupled monsoon-rainforest model to the point where the combined system collapsed and the monsoon entered a weakened state.

Subsequently, they identified precursor signals for the collapse in the model and compared the results with the last 40 years of observations.

The idea was to use the model simulations as guidance on where to look for signs of stability loss in observations.

Stop deforestation

”It surprised me that we found remarkable similarities between our model simulations before the critical transition, and corresponding patterns in observations covering the last decades. It was not just one signal, it was several. They all matched our model. We didn’t expect to see it this clearly in the data,” says Nils Bochow

”Our simulation results suggest that the coupled system of Amazon rainforest and South American Monsoon has indeed moved closer to an abrupt transition to a much weaker monsoon state in response to deforestation, which would likely lead to degradation also of the so far untouched parts of the rainforest”, adds Niklas Boers.

”However, based on our results we can’t tell how close we are. It is thus a betting against time and we should stop deforestation, while there is still time,” says Boers.

  • Link to the article
  • Bochow, Nils, and Niklas Boers. “The South American monsoon approaches a critical transition in response to deforestation.” Science Advances 9.40 (2023): eadd9973.