Imprimir

Ecology

Little gas in the Amazon Forest

New calculations suggest that the tropical ecosystem may absorb less carbon dioxide than it used to be thought

FABIO COLOMBINIStudy indicates that, on a yearly basis, each hectare of preserved forest absorbs a maximum of 2 tons more than the carbon it emitsFABIO COLOMBINI

The weight of Amazon Rain Forest in the fight against the increase in the greenhouse effect, the overheating of the Earth’s climate, may be less than it used to be thought. New calculations of the flow of the main component in the atmosphere responsible for the increase in the average temperature of the planet, carbon dioxide (CO2), show that the quantity of this gas absorbed naturally by this tropical ecosystem is equal to, or slightly higher than the amount it gives off – and not absurdly more, as previous studies indicated.

The revision of the figures was carried out under the megaproject , the Large Scale Experiment in the Biosphere – Atmosphere of the Amazon (LBA), an international US$ 80 million which, since 1999, has put together over 300 researchers from Latin America, the United States and Europe, under Brazilian leadership. It points to a positive annual balance, in favor of absorption, of about 2 tons of carbon per hectare of preserved forest. Previous balances, some of them carried out by LBA itself, went as far as to indicate a net absorption, after discounting emission, of 5 to 8 tons per hectare. “It is possible that this amount may even be close to zero”, says Paulo Artaxo, from the Institute of Physics, of the University of São Paulo (IF/USP), responsible for a thematic project on the subject financed by FAPESP, and one of the coordinators of the LBA.

In each 3.67 tons of carbon dioxide, also known as carbonic acid gas, there is 1 ton of carbon, the chemical element. In general terms, one can say that the more CO2 a forest absorbs, the greater its biomass must be, measured in the form of carbon, given that the photosynthesis of vegetation is intensified. In other words, large absorption of CO2 is theoretically equivalent to the large growth of an ecosystem.

According to the new accounting, adding up all the known sources of entry (absorption) and exit (emission) of CO2 in the forest, the Amazon basin seems to trap a relatively modest quantity of this gas, per hectare of preserved forest. Even so, as the Amazon region is immense – covering just in Brazilian territory 5 million square kilometers (500 million hectares), of which about 80% are native forests – its impact on the world-wide balance of carbon dioxide may be quite respectable.

Neither a villain, nor salvation
A quick and simplistic calculation, taking into account an annual rate of carbon fixation between one and two tons a year per hectare of forest, shows that the Brazilian Amazon forest, which encompasses about 70% of this South American ecosystem, would be capable of taking out of the atmosphere something between 400 and 800 million tons of carbon every 12 months. This is the equivalent of something between 5% and 10% of the man-made world-wide emissions of carbon in the same period , basically the burning of fossil fuels and of deforestation. “Brazil is not the world’s villain because of the deforestation and the fires in the Amazon basin (which releases significant quantities of CO2 into the atmosphere), but nor does its main forest represent the salvation of the planet”, Artaxo comments.

According to meteorologist Carlos Nobre, from the National Institute of Space Research (Inpe), of São José dos Campos, the LBA’s scientific coordinator, it is estimated that every year the Amazon forest is responsible for pouring into the air roughly 200 million tons of carbon dioxide, as a result of the deforestation of areas hitherto preserved and forest burnings, above all in the months of drought, from July to November. To date, about 14% of the original cover of Amazon forest has already been deforested, at a rate of roughly 0.5 per cent a year, or a little over 20,000 square kilometers.

The balance in favor of the absorption of CO2 in Amazon basin has fallen because the LBA researchers discovered methodological inaccuracies in the way that they had been calculating the balance of carbon dioxide in the tropical forest. Basically, three points were carefully revised. The first topic reassessed: the specialists found that the 12 towers for measuring the flow of the gas in the tropical forest, installed in several points of the Amazon forest, were not recording properly the entry and exit of CO2 during the night, precisely the period when the emissions of carbon dioxide are highest, since plants’ respiration predominates.

“We estimate that about one ton of carbon per hectare, previously ignored, is emitted a night every year”, Artaxo reckons. The good news is that the researchers believe they have now succeeded in identifying the origin of the inaccuracy, and, best of all, of correcting the methodology employed. The second topic revised: the participants in the LBA found that, when it respires, the Amazonian vegetation emits, besides CO2, considerable levels of so-called Volatile Organic Compounds (VOCs).

Part of these VOCs, a group of gases that contain carbon, is transformed into carbon dioxide in the atmosphere, making this kind of emission an indirect source of the main agent responsible for the greenhouse effect. “Normally, the emission of VOCs is considered insignificant in most ecosystems of a temperate climate, but in the Amazon, it is important”, Artaxo points out. It is estimated that each preserved hectare of the region turns out into the air every year a quantity of VOCs capable of generating about half a ton of carbon in the atmosphere.

vFinally, the scientists managed to quantify a third source where CO2 escapes, hitherto little studied: the rivers and the meadows of the Amazon forest, which are saturated with carbon, and lose concentrations of this gas into the atmosphere by diffusion. “Nobodylooked at the role of the waters in the balance of carbon dioxide”, comments Reynaldo Victoria, from the Centerof Nuclear Energy in Agriculture, of the University of São Paulo (Cena/USP), in Piracicaba, and the coordinator of the biogeochemical of waters of the LBA and of a FAPESP thematic project.

“The rivers of the region are big reactors that process organic material”, he adds. In partnership with colleagues from Washington University, also taking part in the LBA, researcher Maria Victoria Ballester, from the Cena team, showed that about 1 ton of carbon per hectare leaves the rivers every year in the direction of the atmosphere.

Three Frances
Strictly speaking, the projected figures do not refer to the Amazon basin in its totality, but rather to a large part of the region, a quadrilateral of 1.7 million square kilometers, in the central part of the Amazon basin – equivalent to one third of the largest tropical forest in the world, or to three Frances. Even so, it is a figure that is quite representative of the exchanges of carbon dioxide that are processed in the Amazon.

In the scientific article in which she sets out her calculations on the escape of CO2 from the rivers of the region in the direction of the atmosphere, submitted for publication by a major international scientific magazine, Maria Victoria, jointly with the American researchers, says that the overall balance of the carbon cycle in mature and intact tropical forests, taking into account the sum of land and water environments, should be close to the point of equilibrium. In other words, close to zero.

To a large extent, the corrections to the figures for the carbon balance are due to the unique and pioneering nature of the LBA. In no other large tropical forest in the world is a systematic effort being made to measure the flow of CO2, on the lines that is being done in the Amazon basin. “This kind of work only exists in temperate ecosystems in North America and Europe, which are very different from the tropical forest”, says physicist Artaxo. “We don’t have any research model ready to copy and put into practice here. It all has to be developed and adapted to the reality of the Amazon basin.” In many cases, this adaptation can make all the difference in the final figures. The difficulties of interpreting correctly the data supplied by the towers installed in the region to measure the flow of CO2 between the forest and the atmosphere are a good illustration of this question.

Bought at a unitary cost of roughly US$ 200,000, the 55-meter tall towers were placed in 12 points in the Amazon basin, in an attempt to cover different realities of the immense region. There is a good reason for the towers’ tallness. Their top was planned to stand between 20 and 30 meters above the crown of the trees, a privileged position for the high precision sensors, instruments capable of measuring the vertical wind and the concentrations of CO2 ten times a second.

For the instrument, the CO2 that makes the upward movement, from the forest to the atmosphere, is counted as being emitted by the former in the direction of the latter. The gas that makes the reverse trip goes into the calculation as being absorbed by the vegetation or the soil (that is, as taken out of the air). In the course of time, the balance (the sum of the carbon dioxide that came in less what went out) provides the net flow of CO2 for the place.

Therefore, the towers measuring system works very well, provided that there isconstant circulation and vertical turbulence in the air at the point of measurement. This happens all the time in the temperate forests of North America and Europe, woodlands that are far more open than the Amazon forest. In the tropical forest, which is denser, the closed trees crown, constantly hampers the passage of light, may work as a lid capable of holding down the air in its domain.

During the day, this entrapment effect of the atmosphere brought about by the exuberant equatorial vegetation does not go so far as to hinder the workings of the measuring systems of the towers, since the periods of calm in the climate (with little turbulence in the atmosphere, free from wind and rain) are not predominant. Both the CO2 absorbed by the plants in the course of photosynthesis (the process by which the plants take carbon dioxide out of the atmosphere and convert it into energy and biomass) and the CO2 emitted as a result of exhaling are recorded satisfactorily.

At night, it all changes, and the periods of calm, without turbulence, become predominant – worsened by the fact that the air at this time of day becomes richer in CO2. Without the presence of light from the sun, the plants do not carry out photosynthesis. They just breathe, sending back into the air a part of the CO2 absorbed during the day.

Where did the CO2 go to?
This limitation on the measurement of the nighttime flow of CO2 is more intense in towers whose measurement range covers flooded areas and also those installed in places where the a topography is not entirely flat. This is because the slope of the land makes the CO2 slip down to the sides in the course of the night; because of the lid effect of the canopy of the trees, the gas finds it difficult to escape upwards. When this happens, the carbon dioxide emitted right below the tower leaves the area measured by it, so that it not properly accounted for by the equipment.

It is only in extremely flat areas that the carbon dioxide released during the night by the breathing of the plants does not slip away and stays blocked by the treetops until the following day. Accordingly, the nightly CO2 that has not been recorded by the tower ends up, with some luck, being picked up by the equipment’s sensor at the beginning of the following day, and ends up being accounted for adequately in the total flow of the day. “We discovered this after making a detailed analysis of the data from various LBA towers, in particular the two located in Manaus, operated by the National Research Institute of Amazonia (Inpa)”, says Artaxo.

Separated by only 20 kilometers, the two towers recorded different carbon balances. One would say that the forest absorbed from 20% to 30% more carbon dioxide than the other. In a detailed examination of the places where both items of equipment were located, the researchers found that the flooded parts in the action range of one of the towers were very different from the swamps in the area covered by the other. They also found that the land inclination where the towers were located showed significant variations. According Artaxo, these peculiarities were sufficient to intensify the horizontal escape of the CO2 emitted at night at one of the towers, and so giving an explanation for the apparently conflicting data supplied by the towers in the capital city of Amazonas.

To measure the quantity of CO2 given off at night correctly, the LBA researchers installed sensors of this gas at various points of each one of the 12 towers, as well as under the trees crown. Thus, the nighttime recording of the presence of carbon dioxide beneath the trees became easier to obtain. With the helpof more accurate techniques intopography, the researchers are going to determine this year the slope of the land in the areas where the towers are installed, another measure that aims at diminishing any inaccuracies in the data supplied by this kind of equipment and to give more credibility to the calculations of the overall carbon balance. “In the middle of this year, two towers will be installed on Bananal island, in Tocantins, the first in inundated areas of Amazon forest”, explains Artaxo.

With this enhancement, yet one more hole in measuring the flow of CO2 in Amazon forest begins to be filled. After all, 14% of the region is covered by rivers or waterlogged areas, a kind of habitat where the LBA still has no permanent form of permanent monitoring of the flows of CO2. Even before the revision of the numbers of the carbon balance in Amazonia, some researchers from the LBA itself were having reservations about the first figures raised by the project, which showed the region as a great whirlpool absorbing CO2.

The history of the forest and its physical constitution seemed, in a certain fashion, to contradict the numbers that signaled an annual balance on the side of absorption of up to 8 tons of carbon per hectare. After all, the Amazon forest is not a young formation of plants. It is relatively old and has reached its maturity. In theory, then, its carbon flow should be close to zero. That is, the quantities of carbon absorbed and emitted should be equivalent.

Growth of the forest
As one hectare of intact tropical forest contains between 140 and 200 tons of carbon in the form of biomass, the Brazilian Amazon forest, in theory, would have to double in size every 28 years, if its capacity for absorbing carbon dioxide were to be equivalent, for example, to 5 tons (of carbon) a year per hectare. “This is not happening”, says Artaxo. “The Amazon forest is not growing at this pace.”

Some scientists, however, argue that the Amazon forest today is no longer behaving like a classic mature formation of plants. Traditional ecology says that old forests absorb and emit the same quantity of carbon dioxide, as their growth in terms of biomass is close to zero. The cause of this attitude change is the high concentration of CO2 found nowadays in the atmosphere of the planet, the highest in recent history – the concentration of this gas has jumped up from 280 ppm (parts per million) in 1850 to today’s 370 ppm.

With more CO2 available in the air for absorption, the level of fixation of carbon dioxide by mature forests has risen, due to this increased supply of this component of the atmosphere. “In the overall balance, the majority of the towers in the Amazon basin show more carbon dioxide being absorbed than emitted by the preserved forest”, says meteorologist Carlos Nobre. “The debate is over whether this absorption is large or moderate.”

Directly involved in the process of revising the figures of the carbon balance for the Amazon, physicist Artaxo, from USP, stands among the LBA researchers who believe in a modest balance in favor of CO2 absorption of by the forest. On the other hand, biogeochemist Antonio Nobre, from Inpa, who is responsible for the operation of the two LBA towers in Manaus, is still not convinced that the balance between the quantity of CO2 absorbed and emitted in the region is so moderate. “It may even be that this balance is in the order of two tons a year of carbon (per hectare of preserved forest), but there are towers that show an annual net absorption of over 5 tons of carbon (per hectare of forest)”, says Nobre.

“It is logical that there are still uncertainties over the methodology applied in these measurements and over the carbon cycle, but this fact cannot simply be ignored. We still do not know the Amazon ecosystem well, and it is very complex. The plants adapt to the levels of carbon dioxide in the atmosphere and seek a new point of equilibrium.” Like almost everything in science, the new figures on the carbon cycle in the Amazon forest are neither definitive nor unquestionable.

Even more so, when one knows that these amounts are fueling a lively debate in international politics and diplomacy, on the Kyoto Protocol, an agreement signed and in the process of being ratified by the major part of the nations on the planet (with the noteworthy exception of the United States), which provides for targets for reducing the levels of emission of CO2 by the industrialized countries, as a way of reducing the greenhouse effect. “For us not to be dependent only on information coming from abroad, extrapolated from measurements taken in temperate forests, we Brazilians have to understand and to produce our own figures on the carbon cycle in Amazonia”, says Artaxo.

A reminder: the greenhouse effect, caused by a curtain of atmospheric gases, especially CO2, that prevent all the heat irradiated by the Sun in the direction of the Earth from returning to space, is a natural, desirable, phenomenon, without which there would not be a favorable climate on the planet for life to flourish. It is what makes the earthly globe hot enough to be inhabitable. The increase in the greenhouse effect, as a result of the exaggerated rise in the levels of carbon dioxide in the atmosphere and other gases, makes the temperature warm up more than is desirable, causing glaciers to melt, and, possibly jeopardizing the equilibrium of the planet.

All the range of the LBA
Unveiling the role of Amazon forest in the regional and planetary carbon balances is one of the big objectives of the Large Scale Biosphere/Atmosphere Experiment in the Amazon basin, the LBA, but it is not the only one. Under Brazilian leadership, the international project is also poring over the cycles of solar energy, water, gases, traces (compounds like methane and nitric oxide, which also contribute towards the increase in the greenhouse effect in the lower atmosphere), aerosols (solid particles in the atmosphere that influence the formation of clouds) and other nutrients.

This ambitious multidisciplinary venture, which started in 1999, is trying to understanding the workings and the interaction of all the parts of this unmatchable ecosystem: the fauna, the flora, the soils, the rivers, and man, its inhabitant. Generally speaking, the project seeks to understand and to quantify in an integrated manner how changes, caused by the hand of man or otherwise, in the use of the soil in the region – such as the clearing of forests to open up pastures – and in its climate alter the physical, chemical and biological components of the Amazon forest itself, and possibly of other part of Brazil and of the planet.

With a venture the size of the LBA, the researchers hope to find new elements to allow them to answer a series of questions on Amazon forest, and to help the country to draw up a strategy for the sustainable development of the region.

The mystery of the carbon lost on the planet
It is not just in the Amazon that understanding and quantifying the carbon cycle leads researchers to be constantly revising calculations and modifying methods of work. It is like that all the world over. At the moment, it is a scientific field in which there seems to be few certainties and many hypotheses, despite all the attention prompted by the proven increase in the concentration of CO2 in the atmosphere after the Industrial Revolution in the 19th century and the threat of an increase in the greenhouse effect.

Some points of the carbon cycle are still so open that scientists do not know how to give clear answers to many key questions for the future of the planet’s climate. One of the most important is about the question of the so-called “lost carbon”, and its whirlpools – places that absorb CO2 and so take this gas out of the atmosphere.

Global calculations made by the Intergovernmental Panel on Climatic Changes (IPCC), a sort of committee maintained by the World Meteorology Organization and by the United Nation’s environmental program, show that, on an annual basis, about 7.9 billion tons of carbon are emitted to the atmosphere, of which 6.3 billion due to the burning of fossil fuels (coal, oil and natural gas) and 1.6 billion to deforestation.

Where does all this carbon go to? Roughly 2.3 billion tons are absorbed by the oceans, and 3.3 billion tons goes to the atmosphere, above all in the form of CO2. As carbon does not disappear, it remains to find the destiny of the 2.3 billion tons that were emitted, but which it is not known exactly where they went. One of the possible destinations are the forests, the temperate ones, the boreal and the tropical, like the Amazon forest, which have the capacity, with photosynthesis, to convert this carbon dioxide into biomass.

The problem is that man does not yet know for sure what the real capacity for absorption – and for emission – of CO2 is nowadays in each one of these ecosystems. In addition, the status of an ecosystem may change with time. A young forest, still growing, is possibly a great consumer of CO2 from the atmosphere, using enormous quantities of carbon to increase their biomass. But, if one day it is cut down and burnt at a frenetic pace, this same plant formation will cease to fix carbon and start to emit it in the form of CO2, due to the combustion of its biomass. 

THE PROJECT
Studies of the Physical and Chemical Interactions in the Biosphere / Atmosphere Interface in Amazonia (nº 97/11358-9);  Modality
Thematic project; Coordinator Paulo Artaxo – Institute of Physics at USP; Investment R$ 718,131.52 and US$ 550,954.00

Republish