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The lords of rain

Gas discovered in the atmosphere of the Amazon intensifies precipitation, while slash and burning reduces it

PAULO ARTAXO / USPIn two scientific papers published in successive issues of Science magazine, Brazilian, European, Australian and Israeli researchers explain phenomena about the climate of Amazon that for many years intrigued scientists. The first study, which came out on February 20, shows why it rains so much in the world’s largest tropical forest, which covers a little over half the Brazilian territory.  By means of a chain of chemical reactions identified only now, a gas released abundantly by plants, called isoprene, is converted into another – recently discovered in the atmosphere – which has revealed itself as one of the key components in the process of forming rain clouds. The second work, from the following Friday, the 27th, reveals why the excess of particles inhibits the rainfall in the burning period of the forest, between August and November. The particles resulting from the burnings saturate the air and lead to the formation of clouds that are higher than those formed in the other months of the year, with far smaller raindrops, which remain in suspension in the atmosphere until they evaporate, instead of pouring down.

In order to rain, it is not enough to simply have a high concentration of humidity, which surpasses 90% in the atmosphere of the Amazon, in contrast to the drier regions, like the Center-West, where the water vapor dispersed in the air sometimes remains below 10%, at the most critical moments.  Another indispensable ingredient is particles in suspension in the air known as aerosols, which act as Cloud Condensation Nuclei (CCN): they attract and condense molecules of water, which grow until they are heavy enough to fall in the form of rain. But there was a problem.  Whenever the researchers from the Large Scale Biosphere-Atmosphere Experiment in the Amazon (LBA) – an international US$ 80 million mega-project that brings together over 300 specialists from Latin America, from Europe, and from the United States – would quantify the condensation nuclei, they would get low values, insufficient to explain why the Amazon is one of the wettest places in the world. The pluviosity of the forest varies from 2,500 millimeters a year per square meter in Manaus to 5,000 millimeters in São Gabriel Cachoeira, also in the State of Amazonas.  To get an idea of this volume of water, from 1,500 to 1,800 millimeters of rain a year fall in the city of São Paulo.

The answers  began to appear with the analyses of the measurements of the atmosphere taken in 1998 at one of the towers of the LBA, 70 kilometers to the north of Manaus.  It was when the researchers discovered the transformations undergone by the isoprene, a gas already known, produced by the plants. A simple molecule, with five atoms of carbon and eight of hydrogen, isoprene suffers a set of chemical reactions under the action of sunlight – it loses one hydrogen atom and gains four of oxygen – and converts itself into one of the two structural forms of one and the same substance, 2-methylthreitol, hitherto unknown as an atmospheric compound. This new substance is now seen as one of the principal pushers of the cloud condensation nuclei, for two reasons. Firstly, for being an alcohol, and thus capable of attracting molecules of water. Secondly, because of the quantity in which it is produced.  Although only 0.6% of the isoprene is converted into this substance, it is no small amount in absolute terms. It is estimated that the Amazon produces about 2 million tons of 2-methyltreitol a year, which makes this new compound one of the most common aerosols of organic origin produced by tropical forests in the world.

“As it has a low molecular mass, no one could imagine that isoprene could work as a precursor of a compound that, we now know, is one of the compounds that are important in the cloud condensation nuclei of the Amazon”, comments Paulo Artaxo, a researcher from the Physics Institute of the University of São Paulo (USP), who participated in the two articles in Science. There was another reason for which bets were not being laid on this chemical compound, produced by all plants in quantities that vary according to the each species, and also industrially, as a raw material for some kinds of plastic. Until the researchers managed to demonstrate the transformations in isoprene and its importance in the Amazonian ecosystem, it used to be believed that the task of forming clouds belonged to just one organic compound common in the temperate forests of Europe: terpene, a more full-bodied molecule.

The tropical forests, hotter, more humid, and sunnier than the temperate ones, work in a different way, since the plant populations are different.  According to the physicist from USP, in Balbina, the region of the Amazon in which the measurements were taken, from 40% to 60% of the rain clouds must be formed from one form of other of 2-methylthreitol, while terpene shows a modest participation, close to 20%.  In the forests of a colder climate, terpene accounts for something around 30% of the volatile organic compounds. Finally, about 10% of cloud droplets are born from organic particles emitted directly by the vegetation, such as pollen, bacteria, and funguses, equally capable of attracting water molecules.

Measuring drops
Coordinated by Magda Claeys, this first study from the issue of Science that came out on the eve of carnival, on the 20th, warns of the climatic changes caused by the loss of the tropical forest, either as a result of natural processes or of human action. The smaller the area of forest, the smaller the quantity of water vapor and of isoprenes released by the plants. Accordingly, there will fewer cloud condensation nuclei and possibly less rain.

But pluviosity already begins to diminish in an earlier stage, as an effect of the forest clearing that precedes the formation of pastures and crops.  “We discovered a very strong interaction between the smoke from the fires and the clouds that is interfering with the hydrological cycle”, comments Maria Assunção Silva-Dias, a researcher from the Astronomy, Geophysics and Atmospheric Sciences Institute (IAG) at USP, the director of the Weather Forecast and Climatic Studies Center (CPTEC), from the National Institute for Space Research (Inpe), and one of the authors of the second article in Science.

When the forest burns in flames to give room for pastures or plantations, the sky changes in a radical way. In billows of smoke, up to 30,000 particles per square centimeter enter the air – a concentration at least a thousand times greater than under normal conditions, and a hundred times greater than to be seen in the city of São Paulo in the most polluted days of winter.  The researchers from the LBA, this time under the coordination of Meinrat Andreae, from the Max Planck Institute for Chemistry, in Germany, demonstrated that the greater or lesser quantity of particles in the air makes all the difference in the process of forming clouds and rains.

A small number of aerosols, such as happens under normal conditions, without human interference, induces the formation of large raindrops, with a diameter that varies from 30 to 50 micrometers (1 micrometer is a thousandth part of a millimeter), which are massed in low clouds, the tops of which are from 3 to 5 kilometers from the ground, and they fall in a few hours. On the other hand, the excess of particles released when the forest burns produces smaller drops of water, from 10 to 20 micrometers in diameter, which form higher clouds, rising to as much as 16 kilometers, and, being lighter, they evaporate instead of gaining weight and falling in the form of rain.

The researchers established these differences, already outlined in more general terms by means of remote sensing, visiting the clouds themselves, in some 20 flights in two Bandeirante aircraft, one parallel to the other, between September and October 2000.  In one of them were Andreae and Artaxo, collecting information about the particles that form the clouds. The other plane, with at least one researcher from the Ceará State University (Uece) – sometimes Alexandre Costa would go, at other times, João Carlos Parente de Oliveira – and, always, Daniel Rosenfeld, a specialist from the Hebrew University, Jerusalem, Israel, would dive into the clouds with the purpose of analyzing the drops of water that were formed inside them. From the ground, Maria Assunção would accompany the two teams, advising them about the behavior of the climate.

The flights started in Ji-Paraná, in Rondônia, and would follow on to Porto Velho, in the same state, pass over Rio Branco and Cruzeiro do Sul, in the state of Acre, and end in Tabatinga, in the state of Amazonas.  As they followed on from a region of frequent forest fires to others where they were rarer, until landing at a spot where the forest remained reasonably preserved, the differences in the structure of the clouds would become clear. In Ji-Paraná, small drops and high clouds would predominate, while in the direction towards Tabatinga, it was large drops and low clouds that would become more common.

Less rain in the South
The measurements taken reiterate the estimate that the particles resulting from the burning reduce the quantity of rain by as much as 30%, but, according to Assunção, a little more work is still needed to arrive at a more precise figure: the aircraft covered only the smaller rain clouds, avoiding the larger ones, which usually bother pilots.  “It is possible that this same mechanism may also delay the rains, but this has not yet been demonstrated”, she says. In any case, the contrasts are now clear: “In Tabatinga, it rains every day, while in Ji-Paraná it rains less than it would if there were no emissions from the fires”, Artaxo observes.  It was already known that slash-and-burn forest clearing, as it covers the sky with smoke, reduce the temperature on the ground by at least 0.5° Celsius and luminosity by up to 50% (see Pesquisa FAPESP 86).

The data obtained strengthens the hypothesis that the forest burning in the Amazon may have a much more wide-ranging effect and inhibit rainfall in other regions of South America as well, particularly in the South and Southeast of Brazil, since the clouds of smoke are carried by the currents of air in this direction -–another part overcomes the Andes mountains and reaches the Pacific Ocean.  The LBA team has now found in the Andes and in São Paulo particles from the burning of the forest in the Amazon, but it still remains to be proved that they also reach other regions and manage to scare the rain away.

The Projects
1. Physical and Chemical Interactions between the Biosphere and the Atmosphere of the Amazon in the LBA experiment (nº 97/11358-9); Modality Thematic Project; Coordinator Paulo Eduardo Artaxo Netto – IF/USP; Investment R$ 1,814,179.30  (FAPESP)
2. Changes in the Use of the Land in the Amazon: Implications for the Climate and for Carbon Cycling (nº 03/02126-0); Modality Millennium Institute of the LBA Experiment; Coordinator Paulo Eduardo Artaxo Netto – IF/USP; Investment R$ 4,200,000.00 (MCT)
3. Interactions between Radiation, Clouds and Climate in Amazonia in the Transition between the Dry and Rainy Seasons/LBA (nº 01/06908-7); Modality Thematic Project; Coordinator Maria Assunção Faus da Silva-Dias – IAG/USP; Investment R$ 1,538,922.32 (FAPESP)