For a long time, people believed that the Amazon Rainforest was the planet’s lungs and an enormous drain of carbon, the gas associated with the rising temperature on the planet. Recent studies, however, have shown that the Amazon Rainforest does indeed absorb more carbon than it emits, but not in the proportion that had been imagined. Research studies conducted under the Large-Scale Biosphere Atmosphere Experiment in Amazonia/(LBA),an international project involving more than 300 researchers from Latin America, the United States and Europe, led by Brazilians, have shown that the rainforest absorbs only two more tons of carbon per hectare every year than it releases into the air (see Pesquisa FAPESP nº 72). And this amount might be even smaller – or even zero. This figure does not include the carbon gas emitted by the rivers in the Amazon Region, where 20% of the world’s fresh water reserves are concentrated.
In the last few years, the research team led by agronomy engineer Reynaldo Victoria, from the University of São Paulo/USP in Piracicaba, has analyzed in detail the quantity of carbon released by the forest’s rivers and swamps – especially in the form of carbon gas/CO2, which is associated with global warming. After recalculations had been conducted, the researchers observed that the acquatic environment of the Amazon Region releases approximately 470 million tons of carbon a year into the atmosphere, which corresponds to 1% of the total (49 billion tons) of greenhouse gases emitted by human activities around the world in 2004. Divided by the water-covered area in the region, these 470 million tons are equivalent to 1.2 tons per hectare, as detailed by the Piracicaba team in two recently published articles – one in Nature in 2002 and the other in Earth Interactions Journal in 2008. This data is expected to contribute to more specific knowledge in the future on the difference between what is absorbed and what is released in the entire Amazon Region.
The first study, the result of a partnership between USP’s Nuclear Energy Center for Agriculture/(Cena), and the team headed by Jeffrey Richey, from the US’s Washington University, suggests that the probable source of most of this carbon is the organic material (plants and animals) carried by rains on higher land that does not flood and areas of the forest that remain under water for most of the year flowing into the rivers and streams. Only a small part (approximately 10%) of the carbon gas dissolved in water reaches the Atlantic Ocean, says biologist Maria Victoria Ballester, a researcher at Cena and co-author of the articles. “On the basis of these findings, we suggest that, when the emissions from land and water environments are added up, the overall balance of carbon from tropical forests is fairly even,” she explains.
In the article in Earth Interactions Journal of June 2008, biochemist Maria de Fátima Rasera, also from Cena, estimated the amount of carbon gas released by small rivers, up to 100 meters wide from one side to the other, that comprise 92% of the Amazon Region’s river network. As the size of the Amazon basin makes it impossible to measure the emissions from each river, Maria de Fátima made the initial calculations for 28 rivers in the Ji-Paraná basin, in the state of Rondônia. Then she applied the data to the rest of the Amazon Region. Her numbers show that the small rivers release 170 million tons of carbon in the form of carbon gas (CO2) into the atmosphere every year. “This work underlines the importance of small rivers in this exchange of gases,” Maria de Fátima states.
The team from Cena has published 120 scientific articles in nearly two decades of investigating the role of the Amazon Region rivers in the carbon cycle – from the withdrawal of this chemical element from the atmosphere in the form of carbon gas and its incorporation by plants and its release into the atmosphere. “We wanted to understand the carbon transportation and transformation processes in the main bed of the Amazon River and its main tributaries,” explains Reynaldo Victoria. “In ten years of work, we accurately established the quantity of carbon that leaves the Amazon River basin and goes to the ocean every year. This amounts to 36 million tons of organic carbon and 35 million tons of non-organic carbon.”
In addition to the importance of the acquatic environment in the exchange of gases with the atmosphere, the research work conducted by Cena revealed another interesting form of behavior in the forest. The belief was that nearly all the carbon absorbed from the air was incorporated by the plants and used for their growth. Reynaldo Victoria’s team discovered that this is not quite true. “Apparently, the forest absorbs little carbon and even so it releases part of the carbon to the water ecosystems,” says the researcher. From the rivers, the carbon returns to the atmosphere and a small amount goes to the Atlantic Ocean. “The research work conducted in 2002 showed that the quantity of carbon which returns to the atmosphere is 13 times higher than the quantity that goes to the ocean,” says Reynaldo Victoria.
Initially, the researchers evaluated the concentration of CO2 dissolved in the Amazon River, in its main tributaries and in some of the wetlands, totaling an area of 1. million square kilometers, equivalent to one third of the Brazilian Amazon Region. “Information from remote sensors quantified the water cover in periods representing the dry and wet seasons,” explains Maria Victoria. “On the basis of this data, it was possible to calculate the flow of carbon gas from the water to the air in different environments.”
These findings fill a gap in our knowledge about the carbon cycle in tropical regions and its significant influence on the whole world. Maria Victoria explains: “With one fifth of the planet’s fresh water, the Amazon Basin emits into the atmosphere approximately twice the amount of carbon emissions released by the cutting and burning of the forest.”
“These results allow for a more accurate evaluation of the Amazon Region’s water systems’ influence on carbon emissions and, consequently, on our planet’s climate change,” says limnologist and ecology specialist José Galizia Tundisi, president of the International Ecology Institute in São Carlos. According to Tundisi, the research work conducted by the group from Piracicaba also helps understand how algae and earth plants extract carbon from the atmosphere.
The role of the Amazon Region’s river systems in the regional and global carbon balance: release of CO2 and interaction between the land and acquatic environments (nº 03/13172-2); Type Thematic project; Coordinator Reynaldo Luiz Victoria – Cena/USP; Investment R$ 1,080,118.96 (FAPESP).
RASERA, M.F. et al. Estimating the surface area of small rivers in the southwestern Amazon and their role in CO2 outgassing. Earth Interactions, v. 12, no. 6, p. 1-16, 2008.
RICHEY, J. E. et al. Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2. Nature, v. 416, p. 617-620, 2002.