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Meteorology

The rain that comes from the Sahara

Dust that comes from the African Desert feeds clouds that are responsible for part of the rainfall in the Amazon Region

EUMETSATOver the Atlantic: winds carry dust from the Sahara to South AmericaEUMETSAT

In an intriguing paradox of the Earth’s climate system, one of the planet’s driest regions seems to have an important role in the formation of the rains that water one of the planet’s most humid regions. Experiments conducted during the rainiest season of the year on part of a preserved forest in the Central Amazon Region, in the vicinity of Manaus, indicate that dust from the Sahara Desert, transported over hundreds of thousands of kilometers by winds over the tropical Atlantic Ocean to South America, help form the clouds responsible for 80% of the rain in this region. The dust particles over the forest act like ice nuclei, microscopic platforms around which water in solid state is aggregated and creates the very high clouds full of rainwater.

The results of this study, published in the May issue of Nature Geoscience journal, are astonishing and still need to be refined, points out physicist Paulo Artaxo, from the University of São Paulo/USP), and one of the authors of the study. “We need to find out, for example, if this influence from the Sahara dust is also found in other regions of the Amazon Region. We also need long-term measurements, registered over several years, to understand how this effect varies according to the seasons of the year”, says the researcher. At any rate, the data obtained near the peak of the rainy season at the Reserva Biológica do Cuieiras, Biological Reserve, located 60 kilometers to the north of Manaus, suggests that dust from the Sahara makes an important contribution to the concentration of ice nuclei in the Central Amazon Region. Artaxo and researchers from the United States and Germany collected samples of the air in this part of the forest from February 9 to March 9, 2008 and found these dust particles in up to 80% of the ice nuclei.

The dust seems to alternate its function as main seeder of ice clouds with the so-called primary biological particles (bacteria, pollen particles, spores and fragments of leaves and insects), emitted by the forest itself. The dust or the particles, alternately, were responsible for forming the ice nuclei. Together, the two sources generated 99% of the cloud seeding – neither source contributed with less than 15% of the nuclei.

The analogy with seeds is very useful. In the Amazon Region, clouds that form at high altitudes are 15 to 18 kilometers deep and contain ice crystals. These clouds generate heavier and more abundant rain, essential for the region’s hydrologic cycle. Smaller clouds are 3 to 5 kilometers deep and are closer to the earth. They are formed of liquid droplets and have less influence on the rainfall over the Amazon Region.

In this study, the researchers collected particles in suspension – also referred to as aerosols – from the forest air at ground level and injected them into a chamber that simulates the formation of deep, convective clouds. “We used a chamber that reproduces atmospheric conditions up to 8 kilometers above ground and up to a temperature of minus 70 degrees Celsius”, said the physicist. The deep, convective clouds, the ones that sprout from ice nuclei and account for most of the rainfall in the Amazon Region, are formed in a similar environment, under low pressure and at a low temperature. “We’re planning to conduct experiments with airplanes in the period from 2010 to 2011, to take measurements in regions of the atmosphere where the ice clouds form. Measuring these particles at high altitudes is no trivial matter”, says Artaxo.

The contribution of the dust from the Sahara to the rainfall in the Amazon had never been witnessed, even though the journey of the dust particles over the Atlantic Ocean was relatively well known. Data provided by NASA, the US’s National Space Agency, suggest that 4% of the dust from each desert storm crosses the ocean to the Americas – while a higher proportion, nearly 20%, gets lost on the way, depositing iron particles that fertilize the sea water and increase the capacity of the algae to absorb the carbon from the atmosphere. Wind storms in the Sahara alone do not bring the dust here. There seems to be a permanent reservoir of particles floating over the north of Africa, which only moves forward towards the Americas if the wind conditions are appropriate.

A significant degree of mystery is still involved in the physical processes of the actions of the aerosols – whether they are dust aerosols or biological ones – such as the ice nuclei. “These processes are still not completely understood”, Artaxo acknowledges. The existence of certain metals – iron, in the case of the Sahara dust – and zinc, in the particles produced by the forests – seem to be important for the formation of ice nuclei. In addition, the existence and the proportion of chemical elements such as aluminum, silica, manganese and iron have allowed the Saharan origin of the dust analyzed by Artaxo and his team to be confirmed.

“The proportion of these elements in the particles found over Manaus is the same as the one found in the Sahara dust. And there is a correlation between the existence of these aerosols, which shows that this is not dust raised by a truck traveling on a road in the vicinity of the collection site, but of long-distance atmospheric transportation”, Artaxo explains.

In the physicist’s opinion, although the contribution from the Sahara to the rainfall is revealed as a general phenomenon in relation to the Amazon Region, it is difficult to envision the meaning of all this in the context of climate change. On a warmer planet, will dust get here in higher or lower quantities: “For the time being, we need to obtain more experimental data to try and answer this with quantitative forecasts”, he states.

In another recent article, published this time in Science journal, Artaxo left the specific context of the Amazon Region on the sidelines with researchers from other countries, to pore over the effects of fire on the planet’s climate and biosphere over time. Big and small fires, for example, helped create various savannas around the world in the course of millions of years. And these effects seem to be intensifying, says Artaxo. “We have witnessed an increase in fires breaking around all over the world in the last few years”, he states.

The team calculated that the effects of the greenhouse gases produced by the fires correspond to 19% of the human beings’ contribution to global warming since the pre-industrial era. “Fires in Brazil generate approximately 30% of the gases emitted by fires on the planet”, the physicist points out.

Regardless of the numbers, the practical point is that in terms of the cost/benefit ratio, eliminating fires is probably one of the best immediate investments against global warming caused by humans. Eliminating fires would be more efficient than extending the network of nuclear plants or substituting the current fleet of automobiles that run on fossil fuels with vehicles that run on biofuels or hydrogen. “If we controlled the fires, we would have a quick return in terms of reducing greenhouse gas emissions, with a very low investment. We would also gain other benefits, such as the preservation of the Amazon Region’s biodiversity”, the physicist points out. “The speedier construction of nuclear power plants or the global renovation of the automobile fleet would take decades to significantly reduce greenhouse gas emissions”.

According to Artaxo, there is an indirect relationship between the growing lack of control over fires on the planet and the hypothesis that the Amazon Region will turn into savanna. This possibility, which appears quite frequently in climate models that try to predict the future of the forest, is the consequence of the transformation of vast regions of virgin forest into forms of more open, ecologically impaired vegetation that resemble, to some extent, the Cerrado (subtropical savanna) region in central Brazil. “The advance of deforestation and the possible reduction in rainfall rates, might lead to the increase in vegetation that is more susceptible to fire, which would increase the number of fires”, says Artaxo. “This would generate a positive re-feeding that would speed up the transformation of the Amazon Region into a savanna”.

Fool’s gold
The intense exploitation of natural resources in the Amazon Region generates temporary prosperity

The predominant economic model nowadays in the Amazon Region – which includes deforestation, lumber exploitation, and transforming vast tracts of untouched land into pasture land or farmland – generates poverty rather than wealth, at least in the long term. This conclusion is the result of an analysis conducted by researchers from Brazil, the United Kingdom, New Zealand and Portugal, published in the June 12 issue of Science.

The municipalities of the Amazon Region that have not been affected by deforestation have a low Human Development Index/HDI, an indicator that takes into consideration the income, education, and life expectancy of the population. The opening up of the agricultural frontier has allowed these municipalities to enjoy a wave of prosperity; however, when the natural resources are depleted because of intense exploitation, the HDI goes back to the initial low levels.

The analysis did not evaluate the situation of the municipalities throughout the years because no timely series in this respect was available, explains Portuguese biologist Ana Rodrigues, from France’s Centre d’Ecologie Fonctionnelle Montpellier, who was one of the authors of the study. This study included the participation of Brazil’s Carlos Sousa Júnior and Adalberto Veríssimo, from the Instituto do Homem e Meio Ambiente da Amazônia/Imazon, Human and Environmental Institute. In view of this fact, the research team compared places that had not yet been overtaken by the agricultural frontier with other places where the agricultural frontier is very active, and with regions where deforestation and land occupation have nearly been concluded. “We used two variables in this classification: the percentage of deforested area until 2000, which provides an idea of the extent of the deforestation; and the proportion of the forest that was cleared in the period from 1997 to 2000, which indicated whether the municipality was at the active frontier or not”, says Ana. The year 2000 was used as a reference to coincide with the year of the Brazilian Census, which allowed the HDI of the municipal regions to be calculated.

Records show that the municipalities that deforested up to 60% of their area – and 0.5% of the total area in the period from 1997 to 2000 – achieved an HDI equivalent to the average Brazilian index. In contrast, the HDI where the proportion of deforestation was much higher and devastation was almost total, was similar to that in the regions of the Amazon Region where the forest is preserved – in both cases the HDI is lower than the average human development index in Brazil.

“We know about the existence of models that prevent economic decadence in spite of deforestation, although I suspect that they would depend on frequent investments coming from elsewhere”, says Ana. The challenge is to create a development model that would result in as little deforestation as possible. “Everybody would win: it would be good for people, for ecosystems and would reduce carbon emissions responsible for global climate changes”, she adds. “The situation is very bad on these three fronts”.

Scientific Articles
Prenni, A. J. et al. Relative roles of biogenic emissions and Saharan dust as ice nuclei in the Amazon basin. Nature Geoscience. v. 2, p. 402-405. May 2009.
Bowman, D. M., et al. Fire in the Earth system. Science. v. 324, p. 481-484. April 2009.

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