El Niño hurts carbon storage in South America’s tropical forests, study says

South America has suffered through an intense and often record-breaking heatwave this year. Chile saw temperatures surpass 35°C (95°F) in August despite it being winter. In Bolivia it was 45°C (113°F). And in Brazil it was over 38°C (100.4°F).

Climate change and El Niño are the main forces behind the heat. The spiking temperatures across the continent have raised questions about countries’ preparedness for extreme climate events as well as what damage is being done to the environment in the process.

A new study revealed that, during extremely hot and dry conditions, tropical forests in South America stop acting as carbon sinks, meaning they’re no longer absorbing more carbon than they’re releasing — a key function that forests need for helping prevent climate change.

“Here in the southeastern Amazon on the edge of the rainforest, the trees may have now switched from storing carbon to emitting it,” co-author Beatriz Marimon, researcher at Brazil’s Mato Grosso State University, said of the study’s findings, published in Nature this month. “While tree growth rates resisted the higher temperatures, tree mortality jumped when this climate extreme hit.”

An El Niño event, the result of high temperatures on the ocean’s surface, occurs approximately every two to seven years. The last time El Niño hit was between 2015 and 2016, leading to record temperatures and drought in many tropical forests across South America. The result, the study said, was that the forests stopped absorbing carbon during those years.

Previously, they were storing around a third of a ton of carbon per hectare per year, the study said. But it stopped as forests’ biomass decreased due to tree death caused by the heat.

“Tropical forests in the Amazon have played a key role in slowing the build-up of carbon dioxide in the atmosphere,” said co-author Amy Bennett, research fellow at the University of Leeds. “Scientists have known that the trees in the Amazon are sensitive to changes in temperature and water availability, but we do not know how individual forests could be changed by future climate change.”

The study examined “tree-by-tree” records of the Amazon, Atlantic and drier forests in tropical parts of South America to determine whether they were acting as carbon sinks or emitters over the last 30 years. Except for the moments of extreme heat, they were almost always sinks.

The switch from carbon sink to emitter happens every time there’s an El Niño event — that includes 2005 and 2010 — but the impact is almost always the same. According to the researchers, that’s a good thing. That means intact tropical forests aren’t becoming more sensitive to drought, so conserving them can still be used to fight climate change.

“The big challenge is to keep forests standing in the first place,” said University of Leeds ecologist Oliver Phillips, who supervised the research. “If we can do that, then our on-the-ground evidence shows they can continue to help lock up carbon and slow climate change.”

Banner image: Dusk in the Peruvian Amazon. Photo by Rhett A. Butler.

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Citations:

Bennett, A. C., Rodrigues de Sousa, T., Monteagudo-Mendoza, A., Esquivel-Muelbert, A., Morandi, P. S., Coelho de Souza, F., Castro, W., Duque, L. F., Flores Llampazo, G., Manoel dos Santos, R., Ramos, E., Vilanova Torre, E., Alvarez-Davila, E., Baker, T. R., Costa, F. R. C., Lewis, S. L., Marimon, B. S., Schietti, J., Burban, B., … Phillips, O. L. (2023, September 4). Sensitivity of South American tropical forests to an extreme climate anomaly. Nature News. https://www.nature.com/articles/s41558-023-01776-4

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