A destabilizing link: Amazon rainforest degradation and deforestation endanger the South American monsoon

In their study, researchers of the Potsdam Institute for Climate Impact Research (PIK) and the University of Tromsø (UiT) investigated how changes in forest degradation and monsoon circulation are interlinked. “It turns out that forest loss caused by direct deforestation, droughts, and fires might vastly contribute to a changing climate in South America and could drive the coupled Amazon rainforest - South American monsoon circulation system past a tipping point. The results presented here suggest an upcoming regime shift of the Amazon ecosystem if deforestation and global warming are not brought to a halt,” says Nils Bochow, lead author of the study.

The moisture exchange between rainforest and atmosphere via rainfall and evapotranspiration is a key mechanism in South-American hydro climate and in stabilizing the Amazon as a whole: large parts of the moisture precipitating in the western parts of the Amazon and in southern South America stem from transpiration of the Amazon trees themselves. Degradation especially in the eastern Amazon, where deforestation rates have been highest in the last years, increase the risk of a shortcut of the moisture exchange that is crucial for the South American monsoon and, hence, for providing the moisture needed to maintain the Amazon rainforest. Using a dynamical computer model that was specifically designed to capture the complex interactions between rainforest and atmosphere, the PIK and UiT scientists were able to predict the impacts of deforestation on the moisture transport across South America. Guided by the simulations, they were then able to detect corresponding, significant and robust signs in observational data that the stability of the South American monsoon system has indeed declined in recent decades, likely in response to the ongoing climate and land use change and the resulting Amazon degradation.

Significant signs that monsoon system stability is decreasing

“A collapse of the coupled rainforest-monsoon system would lead to substantial rainfall reductions in large parts of South America,” explains PIK scientist and co-author Niklas Boers. Due to the complexity of this system, however, large uncertainties remain in the quantitative estimates of potential impacts of a collapse of the monsoon. Nevertheless, rainfall reductions especially in the western Amazon and further downstream of the atmospheric flow toward the subtropics would be severe. This would put the deep western Amazon rainforest at risk of a large-scale dieback, which would in turn lead to substantial additional global warming due to additional release of greenhouse gases from the degrading trees. Moreover, a South American monsoon decline would also induce potentially dramatic consequences for food security; for example in the La Plata basin with its extensive agriculture, rainfall depends crucially on the moisture supply stemming from the Amazon.

While the study provides significant evidence that a critical threshold for the coupled rainforest-monsoon system exists and this critical transition is approaching, no inference on the precise position of the tipping point or its timing can be made at this point, the authors emphasize. “Our study puts the South American monsoon on the map of potential Earth system tipping elements. It also confirms existing concerns about the Amazon rainforest. The transition would lead to substantially drier conditions, under which the rainforest could likely not be maintained,” Niklas Boers concludes.