Climate change

Forest in a bind

Cleaner air in the Northern Hemisphere may lead to worse droughts in the Amazon Region from 2025 onwards

SEAWIFS PROJECT / NASA / ORBIMAGEAerosols over the eastern part of USA: pollutant cools the climateSEAWIFS PROJECT / NASA / ORBIMAGE

From May to September 2005, the western part of the Amazon region faced the most severe drought of the last 103 years. Rivers dried up, fish died, there were widespread forest fires and at least 250 thousand people were isolated and jobless in the states of Amazonas and Pará. In a region that is famous worldwide for housing the largest remaining part of the Earth’s “rainforest” – rainforest is the expression that is usually used in English to refer to tropical forests such as the Amazon – there was no rain at all for three months in a row in certain places. Up until now the distressing scenario that arises from extreme drought was a rare event, an exception, in the history of the recent climate of Brazil’s Northern Region. However, according to a study by Brazilian and English meteorologists, published in the May 8 edition of the British scientific journal Nature, what used to be an exception could become a lot more frequent 20 years from now and will simply become the rule in the second half of this century.

From 2025 onward, droughts such as the 2005 one, which used to occur every two decades, are likely to start devastating the local landscape every other year. In 2060, if this scientific study’s predictions are correct, there will be a sharp drop in the area’s rainfall in 9 out of every 10 years. “There could be a decrease of some 25% to 50% in the amount of rainfall over the Amazon region,” estimates meteorologist José Marengo, from Brazil’s National Space Research Institute’s (Inpe), Terrestrial System Science Center, who was one of the authors of the study, carried out jointly with English meteorologists from the University of Exeter and from the Hadley Meteorological Center. Since it rains some 2,500 millimeters a year, sometimes even more, across vast swathes of the country’s Northern Region, to speak of desertification is a downright exaggeration. However, with less water available, parts of the Amazon region may develop savannah-like vegetation in lieu of the lush forest. Pardon the play on words, but one can speculate that the severe drought of three years ago may have been a watershed point between the climate of the past and that of the future in the Northern Region.

And it is not just the scientists’ projections that seem shocking to those who automatically associate the Amazon Forest with abundant rainfall. Researchers have identified the “main guilty party” behind the entire mess that may well occur in the rainfall regime of Brazil’s Northern Region: the improvement in the quality of the air in the Northern Hemisphere. More specifically, the growing reduction in emissions of a type of atmospheric pollutant called sulfate aerosol particles in the United States and Europe. This type of aerosol can be produced spontaneously in nature, by volcanoes, for instance, as well as by man, as a result of industrial processes that involve the burning of sulfur and by means of car exhaust fumes. “The relationship between aerosols in the Northern Hemisphere and the lower rainfall in the Amazon Region is an indirect one,” explains climatologist Carlos Nobre, from Inpe, and one of the study’s authors.

Of all the different types of aerosols, which can be defined as a suspension of fine solid particles or liquid droplets in a gas, the sulfate aerosols are those that most reflect sunlight. They have a slight cooling effect on the climate, acting, in practice, as a counterbalance partially mitigating the rise in temperature resulting from the increase in the greenhouse effect. Therefore, the growing reduction in the amount of these aerosols over the Tropical North Atlantic Region, in a zone not far above the equator, makes this part of the ocean hotter than normal. The anomaly seems to shift a substantial part of the rainfall that would usually fall in the Western part of the Amazon Region to this area. In other words, in an environment where there is global ocean warming due to an increase in the greenhouse effect, droughts in Brazil’s Northern Region, such as the 2005 one, are a side effect of the progress that is being made in fighting pollution by aerosols, which comes largely from the Northern Hemisphere.

Lalo de Almeida/Folha Imagem 2005 drought in the Lower Solimões: increased drought frequencyLalo de Almeida/Folha Imagem

Supercomputer forecasts
The prognoses of more frequent droughts in the Western part of the Amazon region were issued by the Hadley Meteorological Center’s supercomputer. The English have one of the most complex climate models and one highly respected by the scientific community. It can produce long-term estimates on the effects of global warming and any other atmospheric anomaly in various parts of the planet. “Our supercomputer would also be able to run the model, but we didn’t have any machine time available for this,” explains Marengo. This restriction will be overcome in 2009 with the arrival of Inpe’s new supercomputer, which will be 30 times more powerful than its current one. However, one figure that was crucial for working out the long-term forecasts was supplied by the Brazilians. It was Marengo and Noble’s team that succeeded in linking the 2005 mega-drought in the Amazon region to the warming of surface waters in the Tropical North Atlantic Region. Usually, one associates a shortage of rain in Brazil’s Northern Region with the phenomenon known as El Niño, i.e., a temperature increase in the waters of the Equatorial Pacific. In the case of the extreme event of three years ago, Inpe’s researchers showed that the cause of the anomaly lay in another ocean, the Atlantic, rather than in the Pacific.

Bearing this assumption in mind, the meteorologists ran the climate model, which uses a series of variables, such as the ever higher levels of greenhouse gases and the declining rates of aerosol emissions, to come up with future scenarios. The result was unsettling: due to the progressive drop in sulfate particle levels in the Northern Hemisphere atmosphere, the Tropical North Atlantic Region will heat up more often. And, when that happens, scientists say, it rains less in the Amazon region. As nobody in their right mind could be in favor of an increase in the production of aerosols, a terrible pollutant that reduces the life expectancy of inhabitants in major cities by several years, in order, theoretically, to avoid changing the balance of the waters in the Northern Region, we are left with just one solution: to fight the increase in greenhouse gases. “There is no moral justification to maintaining aerosol levels just because they temporarily hold back the maximum effect of global warming,” states Noble. “What we have to do is to speed up as fast as possible the schedule for cutting greenhouse gas emissions.”