Rainforest emissions shape the climate and cloud formation

The distinctive scent of a forest on a warm summer day is partly due to terpenes, a group of organic compounds found in tree resins and essential oils. Among these, isoprene is the most abundant molecule, released by plants on a massive scale. 

Global plant emissions are estimated at 500 to 600 million tons of isoprene annually, constituting about half of all gaseous organic compounds released by vegetation. The Amazon rainforest was found to be responsible for over a quarter of these emissions.

Rainforest emissions beyond the canopy

Previously, scientists believed that isoprene in the Amazon basin broke down rapidly and did not reach higher layers of the atmosphere. This breakdown was attributed to hydroxyl radicals – highly reactive molecules formed near the ground during daylight hours – that destroy isoprene within hours. 

“However, we have now established that this is only partly true,” said Joachim Curtius, an atmospheric scientist at the Goethe University Frankfurt and lead author of a recent study. 

“There are still considerable amounts of isoprene in the rainforest at night, and a substantial proportion of these molecules can be transported to higher atmospheric layers.”

Thunderstorms and rainforest emissions

Tropical thunderstorms play a pivotal role in elevating isoprene molecules. These storms act like vacuum cleaners, pulling isoprene to altitudes between 8 and 15 kilometers. 

When the sun rises, hydroxyl radicals react with the isoprene molecules, but the frigid temperatures at these heights transform the compounds differently than near the ground. 

The molecules combine with nitrogen oxides – produced by lightning during storms – to form clusters of aerosol particles just a few nanometers in size. Over time, these particles grow and serve as condensation nuclei for water vapor, significantly influencing cloud formation in tropical regions.

“We were able to shed light on these processes with the help of research flights that started two hours before sunrise and then continued through the day,” explained

Study senior author Jos Lelieveld is the director at the Max Planck Institute for Chemistry in Mainz and also head of the CAFE-Brazil research project.

For this project, an international research team collected data on the chemical processes in the atmosphere over the Amazon rainforest. 

“We were able to detect considerable amounts of isoprene in the air flowing out of the thunderstorms at high altitude, from which new aerosol particles rapidly formed after several chemical reactions,” said Lelieveld.

Impact on cloud formation over oceans

Curtius and Lelieveld are also part of the CLOUD consortium, which involves over 20 research groups studying climate-relevant atmospheric chemical processes. 

Using a specialized aerosol and cloud experiment chamber at CERN in Geneva, the researchers replicated high-altitude atmospheric conditions to analyze sunlight-triggered reactions in detail. 

“In this way, we were able to determine exactly the rate at which the aerosol particles form from the isoprene products,” explained atmospheric researcher Xu-Cheng He, who is in charge of the isoprene experiments. 

“Interestingly, it emerged that even extremely small amounts of sulfuric acid and iodine oxoacids commonly present in the atmosphere are sufficient to accelerate the formation of the aerosol particles by a factor of 100. These molecules may, therefore, jointly influence marine cloud formation – a critically uncertain process in climate projections.”

Long-distance particle transport and climate models

Sulfuric acid forms in the atmosphere through reactions involving sulfur dioxide and hydroxyl radicals. Within the CLOUD experiment, the Frankfurt team measured extremely low concentrations of sulfuric acid, while the Mainz team focused on hydroxyl radicals.

The winds that prevail at high altitudes above the Amazon rainforest can transport the particles that form from isoprene up to thousands of kilometers away from the sources. This means they may influence cloud formation at great distances. 

As clouds, depending on their type and height, both shield solar radiation and prevent heat from being radiated into space, they play a crucial role in the climate. The researchers, therefore, expect that their findings will contribute to improving climate models.

Deforestation’s dual threat to climate

The CAFE-Brazil project also underscores the environmental risks posed by continued deforestation in the Amazon rainforest. 

“On the one hand, greenhouse gases are released because the forest no longer stores carbon dioxide,” Curtius explained. “On the other hand, clearing the forest impacts both the water cycle and isoprene emissions, further propelling climate change.”

The research highlights the Amazon’s critical role in global climate dynamics and emphasizes the need for conservation efforts to mitigate its far-reaching impacts.

The study is published in the journal Nature. 

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