It was a long climb. I took almost an hour to ascend the 1500 steps of the tallest tower in Latin America, 325 metres above the floor of the pristine Amazon rainforest 鈥� a metre higher than the Eiffel Tower in Paris. Rising out of the steamy jungle, temperatures peaked just under the canopy, but then started to drop. Above 150 metres, a stiff cool breeze blew. From the top, the trees looked tiny, like a mass of broccoli heads stretching unbroken to the horizon.
I was met at the top by Meinrat Andreae, director of the Max Planck Institute for Chemistry in Mainz, Germany. In 2007, he had first proposed erecting the structure to sniff the Amazon forest鈥檚 breath and examine its interactions with the atmosphere. In this remote spot, 150 kilometres north of Brazil鈥檚 jungle city of Manaus, he hoped it could provide 鈥渁 window on our planet鈥檚 atmospheric chemistry before industrialisation鈥�.
Andreae got his wish 鈥� during the November to May wet season at least, when clean air from the Atlantic blows across 800 kilometres of largely intact forest. But our climb was in late September, the end of the dry season when the winds from the south cross the deforested areas of the Amazon. With forest fires in recent weeks burning faster than in the past decade, the air below us was thick with haze. And that haze was bringing trouble, even to such a distant corner of the world鈥檚 largest rainforest.
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Since the Amazon Tall Tower Observatory was inaugurated in late 2015, its instruments have been sampling the air above the forest hour by hour. Measurements of carbon dioxide, sulphur compounds, pollen, particulates, volatile organic compounds, methane, nitrogen oxides and any others are all logged in makeshift labs at the tower鈥檚 base. There, researchers derive other vital data, such as rates of photosynthesis in the trees and their exchange of carbon and moisture with the atmosphere.
Some of the chemistry discovered is unexpected. 鈥淲e know there are reactions going on in the air that we haven鈥檛 accounted for yet,鈥� said Andreae. 鈥淭hat means there are probably tree emissions we are ignorant about.鈥� Uncovering them may unlock the mystery of how the trees maintain rainfall so far from the ocean. Most continents have dry interiors, but South America鈥檚 contains the world鈥檚 largest rainforest.
We know rainforests recycle rainfall on a heroic scale 鈥� each tree releases about 500 litres every day into the atmosphere. More intriguing is the physics and chemistry that turn that moisture into rain clouds.
The key seems to be volatile organic compounds, such as isoprenes and terpenes, which most trees also emit. In the air around the tower鈥檚 summit, these compounds oxidise into the tiny molecular seeds around which raindrops form 鈥� but not enough to provide the volume of rain we see. Andreae鈥檚 hunch is that another type of compound, fast-reacting sesquiterpenes, could be the missing link, but they are hard to spot because they disappear quickly.
While straining to understand pristine climatic processes, Andreae鈥檚 team is increasingly worried about the polluted air coming in from the south. Back at the research camp, his colleague Matthias Sorgel told me about ozone generated by the fires. He is measuring up to 70 parts per billion (ppb), a level normally found in urban smogs. 鈥淲e don鈥檛 know the sensitivity of rainforest trees to ozone,鈥� he said. 鈥淏ut as a rule of thumb, levels above 40 ppb are toxic to plants. As the burning gets closer, it is poisoning pristine forest.鈥�
On the TV in the camp that evening, Brazilian president Jair Bolsonaro railed against foreign interference in the Amazon. Over beers, with howler monkeys growing in the background, Brazilian scientists worried that their German colleagues could soon become persona non grata.
