The Amazon savanna? Rainforest teeters on the brink as climate heats up

Shifting rainfall patterns, especially those exacerbated by climate change, could drive large parts of the Amazon rainforest to become drier savanna, a new study has found.

Rainfall acts like a buffer in rainforest ecosystems, allowing forests to adapt to changes and keep destructive forces like fire at bay. But if the amount of rain dips below a certain level, rainforests could lose their ability to come back, instead turning into drier, savanna-like ecosystems.

That’s an issue because the climate plays a big role in where it rains and how much, and if we continue to add climate-warming greenhouse gases to the atmosphere, that could radically change the makeup of the tropics.

Even now, large portions of the Amazon sit on the precipice of remaining rainforest or becoming savanna, according to Arie Staal, the lead author of the study and a former postdoctoral researcher at the Stockholm Resilience Centre and the Copernicus Institute of Utrecht University.

“In around 40 percent of the Amazon, the rainfall is now at a level where the forest could exist in either state — rainforest or savanna, according to our findings,” Staal said in a statement.

The switch would likely be jarring, with the result “probably not even close to savanna, which is a very highly diverse system,” said Paulo Brando, a fire ecologist at the University of California, Irvine, who was not involved in the current study. “It will probably [be] a more impoverished forest in some regions, maybe without some of the large trees.”

Scientists have long debated the existence of a tipping point, beyond which the world’s largest rainforest would tumble irreversibly into a degraded, scrubby savanna. But puzzling out exactly what might push tropical rainforests past that point and cascading toward a drier, savanna-like ecosystem is complicated, involving a lot of different variables.

Staal and his colleagues used satellite data, rainfall models and climate projects to simulate a set of polarized scenarios: They gamed out what would happen if there were no forests across the tropics to see where they would pop up given current rainfall patterns. That gave the team a sense of the “minimal forest area” in the Amazon, the Congo and across Southeast Asia — that is, the smallest area necessary to sustain a rainforest.

“As forests grow and spread across a region this affects rainfall — forests create their own rain because leaves give off water vapour and this falls as rain further downwind. Rainfall means fewer fires leading to even more forests,” Staal said. “Our simulations capture this dynamic.”

The team also looked at a scenario in which forest covered all of the tropics. Their simulations revealed that this state wouldn’t hold for long, as there isn’t enough moisture in many places to keep them going.

Conversely, dwindling tropical forests can have the opposite effect, creating “a vicious cycle” in which less rainfall leads to drier forest more susceptible to fire and further loss, Staal said.

From these extremes, the team gained insights into how resilient different parts of the forest are now, leading to the striking finding that 40% of the Amazon rainforest is at risk of turning into savanna. When they plotted out how things might change in the future, particularly if greenhouse gas emissions increase as predicted by the Intergovernmental Panel on Climate Change’s high-emissions scenario, the team found that the minimal forest area in the Amazon would shrink by 66%.

Unlike the Amazon, the Congo rainforest may actually become more resilient as a result of climate change, the authors predict, as trees there grow more quickly with the addition of more CO2 to the atmosphere. And the ocean helps temper the rainforests of Southeast Asia and Oceania, making them less likely to change with the climate.

The study adds weight to the argument that the Amazon is indeed approaching a tipping point, and that approach might accelerate if we don’t act quickly to stem the emissions of carbon-containing greenhouse gases for manufacturing, transportation and agriculture. Though this study looked only at rainfall, experts have recently revised their estimates downward of what proportion of the Amazon would have to be deforested to push it over the edge, from 40% in 2007 to current predictions of 20% to 25% by scientists Thomas Lovejoy and Carlos Nobre.

Some scientists, such as Daniel Nepstad, president of the Earth Innovation Institute, and Oxford University rainforest ecologist Yadvinder Malhi, question the value of tipping points, arguing that they lend a fatalistic sense of inevitability to the Amazon’s demise. But few refute the need to use all the tools at our disposal — from halting deforestation to reforestation and afforestation efforts to improved firefighting and tracking to new models that better integrate the myriad variables involved into tipping point calculations — to keep what remains of the forest standing.

“We understand now that rainforests on all continents are very sensitive to global change and can rapidly lose their ability to adapt,” Ingo Fetzer, a coauthor of the study and an ecologist at the Stockholm Resilience Centre, said in the statement. “Once gone, their recovery will take many decades to return to their original state. And given that rainforests host the majority of all global species, all this will be forever lost.”

Liz Kimbrough contributed reporting.

Banner image of fire near the Branco river in the Jaci-Parana Extractive Reserve, in Porto Velho, Rondônia state, Brazil, by Christian Braga/Greenpeace.

Citation:

Staal, A., Fetzer, I., Wang-Erlandsson, L., Bosmans, J., Dekker, S., Van Nes, E., … Tuinenburg, O. (2020). Hysteresis of tropical forests in the 21st century. Nature Communications, 11(1), 4978. doi:10.1038/s41467-020-18728-7

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