Lack of rain affects Amazônia’s capacity to absorb carbon

Study published in the cover of Nature calculates the carbon balance in the Amazon Basin from 2010 to 2011.

By Karina Toledo

Agência FAPESP – The drought that hit the Amazon Basin in 2010 was so severe that it compromised the forest’s capacity to absorb excess carbon dioxide (CO2), widely considered to be the main gas behind the greenhouse effect. In 2011, a year with above-average rainfall, the vegetation managed to absorb not only the entirety of the CO₂ emitted through natural processes but also the emissions resulting from human activities, including fires.

The data are part of a study funded by the United Kingdom’s Natural Environment Research Council (NERC) and FAPESP (in the FAPESP Research Program on Global Climate Change), which was published in the cover of the latest issue of Nature magazine.

“These are two extreme scenarios that show how the lack of rainfall modifies the dynamics of the forest and the carbon balance in the region. Therefore, pluviometric precipitation is a factor that scientists working with climate forecasting will have to take into consideration in their models. If not, the results will be very far from reality,” stated Luciana Vanni Gatti, researcher at the Nuclear Energy Research Institute (IPEN).

Gatti is the main author of the study, along with Emanuel Gloor of Leeds University (U.K.), and John B. Miller of the National Oceanic and Atmospheric Administration (NOAA), one of the leading U.S. agencies focused on environmental matters. Several brazilian researchers participated in the study, from the IPEN, the National Institute for Space Research and the University of São Paulo. 

To reach this conclusion, the group collected 160 measurements throughout 2010 and 2011 in four locations in the Amazon Basin: Santarém, Alta Floresta, Rio Branco and Tabatinga. In each flight profile, the researchers collected 17 samples of atmospheric air at altitudes up to 4.4 kilometers above sea level.

“We made a flight plan telling the pilot at what altitudes the samples should be collected. He began from the highest point and travelled downward in a helicoid trajectory approximately 5 kilometers in diameter,” explained Gatti.

According to the researchers, the data become more representative as the altitude of the measurements increases because the samples offer information about the entire region through which the air mass has traveled since it arrived in South America.

“Measurements taken at ground level with cameras or from towers solely represent the reality of that location. Previous studies show that one cannot simply collect data from different locations and take an average because the Amazon has an immense diversity of habitats in its 6,000-square-kilometer extent,” commented Gatti.

“On the other hand, the flight profiles show the results of all processes that occurred from the coast to the collection site and do not allow us to comprehend all the sources and their escape hatches and dynamics. They are complementary approaches. The first is a top-down study; and the other, bottom-up. With one, we understand the macro, the Amazon region as a whole; and with the other, we understand the micro, each compartment of the forest and its dynamics,” she added.

With the help of portable equipment on board common planes (commuter jets), the researchers collected air samples and analyzed them in IPEN’s Atmospheric Chemistry Laboratory for concentrations of five different gases: CO₂, methane (CH₄), nitrous oxide (N₂O), carbon monoxide (CO) and sulfur hexafluoride (SF₆).

“CH₄ and N₂O are also important greenhouse gases that we are studying at the moment. The concentration of CO allows us to estimate how much of that emission results from biomass incineration. The SF₆ allows us to calculate what the concentration of carbon was when that air mass entered the continent,” explained Gatti.

Cross-referencing data

To understand the carbon balance during the period, the researchers cross-referenced data obtained from the measurements with information on pluviometric precipitation from 2010 and 2011 and the monitoring data on fires from the Aqua Tarde satellite.

“In 2010, the rainy season was far below the 30-year average. The hydric stress on the vegetation was so significant that mortality and the decomposition rates increased and modified the balance between photosynthesis and respiration. All this compromised the forest’s capacity to absorb carbon,” stated Gatti.

To make matters worse, added the researchers, the number of forest fire sites in 2010 was much greater than in previous years. According to the group’s calculations, biomass fires launched 510 billion kilograms of carbon into the atmosphere that year.

The forest absorbed only a quantity of carbon equivalent to the quantity that it produced naturally (excluding biomass fires) – around 30 billion kilograms of carbon – leaving a balance of 480 billion kilograms of carbon emitted into the atmosphere during 2010.

In 2011, in contrast, the fires produced approximately 300 billion kilograms of carbon, and the final balance for the Amazon Basin was approximately 60 billion kilograms of carbon.

“It was a year in which the forest offset practically everything that fire emitted. The majority of the climate forecast models are based on temperature. And we saw that both 2010 and 2011 were years with above-average temperatures. The main difference was the rainfall,” stressed Gatti.

For the researchers, the study’s results warn of the possible disastrous impacts that changes in the rainfall cycle – caused by fires, deforestation and the construction of reservoirs – can have on the environment.  

Amazon Mystery

For at least two decades, scientists from around the world have sought to understand the carbon balance in the Amazon Basin and discover whether the forest is, in fact, the escape hatch for carbon that has been imagined. “The Amazon concentrates 50% of the planet’s tropical forest, and this makes a huge difference in the global carbon balance. It is a very important mystery for climate models,” she adds.

Although the aerial measurements offer data with more regional representation, estimates the researcher, a study with temporal representativity is also needed: i.e., a longitudinal study.

“There is great variability from one year to the next. If we based this study solely on measurements from 2010, which was a completely atypical year, we would not have a precise idea of the Amazon’s carbon balance. For this reason, the project continues, and our goal is to collect ten years of measurements in order to have data that really represent the carbon balance in the Amazon Basin,” affirmed Gatti. 

The article Atmospheric science: Drought and fire change sink to source (doi:10.1038/506041a), by Gatti L.V., M. Gloor, J. B. Miller, C. E. Doughty, Y. Malhi, L. G. Domingues,  L. S. Basso,  A. Martinewski, C. S. C. Correia, V. F. Borges, S. Freitas, R. Braz, L. O. Anderson, H. Rocha, J. Grace, O. L. Phillips & J. Lloyd, can be read at www.nature.com/nature/journal/v506/n7486/full/nature12957.html.