If you happen to brew beer in Brazil, consider selling a tank or two of carbon dioxide (CO2) to researchers building tall towers in the Amazon rainforest. Your business could help the scientific community understand the fate of the forest and predict just how much greenhouse gas will accumulate in the atmosphere by the end of the century.
As discussed in Science , a multidisciplinary initiative is underway in the central Amazon to understand how increased anthropogenic CO2 in the atmosphere will affect the spread or decline of the world’s largest rainforest. The potential knowledge gained from this research could inform more accurate climate models and tell us whether the Amazon is fated for devastation before it is too late to do something about it.
The tropical response
How will tropical forests respond to global warming? This simple question is maddeningly difficult for scientists to answer but incredibly important to understand planetary climate change. Terrestrial vegetation absorbs about 30% of all anthropogenic CO2 and rainforests worldwide account for about 2/3 of this sequestration. Since the Amazon accounts for over half of the world’s entire rainforest area, understanding the relationship between tropical forests and climate change begins with the 5.5 million square kilometers of lush forests and rich biodiversity in South America.
Unfortunately, little data exists to understand this complex CO2-forest relationship. On the one hand, increased atmospheric CO2 leads to higher temperatures, which models predict could create severe droughts that destroy much of the rainforest . However, it is well known that increased CO2 also encourages plants to absorb more of the greenhouse gas and induces even more photosynthesis, known as the fertilization effect. This process leads to more plant biomass and can expand the extent of the forest. When the same models predicting drought and devastation are modified to account for increased fertilization, the Amazon suddenly demonstrates a robustness against global warming .
Simulating the future
So which effect – hot droughts or thriving biomass – wins out? Researchers hope to collect the first evidence to answer this question over the next year using a ring of 35-meter towers spewing CO2 into a patch of rainforest just north of Manaus. The towers, known as a ‘Gas Ring’, encompass a circular area 30 meters in diameter and release enough CO2 to increase the regional atmospheric concentration to about 600 parts per million (ppm), 200 ppm more than current levels. The idea is to simulate atmospheric conditions that could occur by the end of the century due to anthropogenic emissions and then study how the rainforest reacts.
This type of experiment, known as free-air CO2 enrichment (FACE), has already been performed in numerous temperate forests to the north in the 1990’s. These studies demonstrated that increased CO2 does spur photosynthesis and increases plant biomass, as theory predicts, allowing more CO2 to be sucked out of the atmosphere . This is an early sign that temperate forests could buffer the planet against climate change, providing a negative feedback loop that slows the accumulation of CO2 in the atmosphere.
Some signs already suggest that the Amazon rainforest could be an even stronger buffer. The higher temperature in the tropics would work constructively with increased CO2 to increase photosynthetic rates. However, other factors, like the amount of phosphorus in rainforest soil, could be a limiting factor that determines just how much more biomass can be supported. Higher growth rates in the Amazon have already been observed but scientists have yet to find the causal link .
The FACE experiment could change that. This is “one of the most exciting experiments on the planet,” says David Lapola, lead scientist from Sao Paulo State University in Brazil . Beginning with a summit of scientists in Washington D.C. in 2013, the first tower ring is now being constructed. Researchers will measure a swath of variables like changes in tree trunk size, debris buildup, soil respiration, and root growth. Forest responses will be studied over two years and compared to a control region before expanding to four treated plots and four control areas.
The necessity to study multiple plots represents just one of many challenges for this unprecedented experiment. The unmatched biodiversity in rainforests means that results from one relatively small sample cannot be generalized to the forest as a whole. Also, the engineering challenge of building large enough towers to encompass the tallest trees of the Amazon has caused multiple delays and false starts. And then there is the CO2 problem. Annual shipment of the gas to supply the towers costs several million dollars. Researchers are now looking at business agreements with local soft drink and beer companies to provide the gas. There is also the question of whether to build pipelines to transport the gas or build roads for trucks.
Despite these challenges and delays, construction of the first Gas Ring should begin within the year and results will come in six months after its completion. With hard evidence finally available, climate models will hopefully provide estimates of future CO2 concentrations with less variability than current models. By 2100, models predict atmospheric CO2 concentrations between 699 to 1130 ppm, an enormous variation largely due to a lack of knowledge about fertilization. This range in predicted concentration corresponds to a 2.4 C global temperature range, the difference between drowned or surviving coastal cities, drought-ridden or thriving crops.
No data better emphasizes the importance of understanding the Amazon to gain insight to the planet’s future. Several rings of towers, a modern-day Stonehenge erected to push the limits of science and engineering, are one of our only bets to begin providing answers.
 Grossman, D. “Amazon rainforest to get a growth check.” Science, 352, 635-636, 2016.
 Cox, P.M. et al. “Acceleration of global warming due to carbon-cycle feedback in a coupled climate model.” Nature, 408, 184-187, 2000.
 Cox, P.M. et al. “Sensitivity of tropical carbon to climate change constrained by carbon dioxide variability.” Nature, 494, 341-344, 2013.
 Tollefson, J. “Experiment aims to steep rainforest in carbon.” Nature, 496, 405-406, April 25 2013.
Photo of tower rings from FACE experiment in temperate forests courtesy of David Lapola
Grossman, D. (2016). Amazon rainforest to get a growth check Science, 352 (6286), 635-636 DOI: 10.1126/science.352.6286.635