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The Amazonian Effect: how the rainforest sustains life in South America

Science has solved five mysteries concerning Amazonia. The answers are both remarkable and alarming for the future of South America.

No forest on earth is as ancient, complex or mystical as the rainforest. And the Amazonian forest is the largest, most legendary and famous of them all. South America’s verdant expanse is, quite simply, the jewel in the crown.

All of the statistics concerning Amazonia are impressive:

  • It is at least 50 million years old.      
  • It covers an area larger than the EU.
  • It provides the habitat for up to a third of the world’s animal and plant life.

It has been estimated that Amazonia formerly contained over 400 billion trees with a diameter of over 10 cm. But over 42 billion trees have been felled in Brazil alone. If all of these tree trunks were placed one after another lengthways, they would reach a distance of 635 million kilometres. That equals roughly 1,700 return trips to the moon.

Over the past 40 years, 34 trees have been felled per second on average in Brazil.

This has provided the authorities, private companies and a handful of individuals with a huge profit. The rainforest is full of natural resources which can be sold on the global market. Subsequently, agriculture has taken over the vacant space left in the forest. South America’s fertile climate, with its regular rainfall, has ensured both good crop yields and excellent grazing conditions for livestock.

But in an ironic twist of fate, these crops are now in danger. Recent research reveals that a high price will have to be paid for several decades of deforestation. A habitable climate in South America is, in all likelihood, dependent on the Amazonian rainforest.

We take a closer look at the five mysteries of Amazonia and the answers they provide. 

Read more: This article is based on The Future Climate of Amazonia Scientific Assessment Report. Download the report here.

Amazonia is renowned for heavy rainfall from low-lying clouds. Photo: Shutterstock

Mystery number 1: Why does it rain so far inland in South America?

South America is a vast continent with enormous tracts of land which lie far from the coast. Nonetheless, rainfall regularly reaches all the way to the foothills of the Andes in the west of the continent. This occurs because the root systems of all the trees in Amazonia draw up huge quantities of water from the ground, which is then transported up through the tree trunks and transpired through their leaves.

Like an Icelandic geyser, a large tree shoots over 1,000 litres of water up into the air each day.

The rainforest is, therefore, responsible for maintaining a steady level of atmospheric moisture far inland in South America. And it’s no small quantity of moisture that the rainforest transpires each day. A calculation of average evaporation per square kilometre indicates that Amazonia sends 20 billion tons of water up into the atmosphere each day.

That is three billion tons more than the daily amount of water which flows out of the world’s most abundant river, the Amazon, into the Atlantic Ocean.

The Amazonian forest has been dubbed the ‘green ocean’ by scientists who have proved that the atmosphere and clouds above the rainforest are the same as those found in a maritime climate. Photo: Shutterstock

Mystery number two: Why does Amazonia break all the rules for rainfall?

In earlier times it was said that there were two seasons in Amazonia – the wet season and the wetter season. However, with the increase in deforestation, a dry period now comes once a year. Nevertheless, Amazonia is still renowned for regular and heavy rainfall.

This has puzzled scientists; the heavy rainfall in Amazonia appears to contradict the laws of nature. The air above the rainforest is ‘too pure’ for this.

Scientists couldn’t explain why the ocean has such low rainfall while the ‘green ocean’ has just the opposite.

In order to create rain, warm vapour must rise and then condense on so-called cloud condensation nuclei; that is, tiny particles of, for example, dust, pollen or soot. There is as little of this in the air above the Amazonian rainforest as there is above the ocean. The scientists who proved this in 1999 therefore named Amazonia the ‘green ocean’. Even the clouds which hang over the forest canopy are reminiscent of a maritime climate.

Yet it still rains, because the forest emits volatile substances which react to the combination of solar radiation and moist air, forming tiny dust particles. This results in heavy rainfall from low-lying cloud.

In this way, Amazonia creates its own rain in a continual process. 

Extreme weather events will always pose a threat to the Earth’s ecosystems, but Amazonia has survived everything nature has thrown at it. This photo was taken while Hurricane Harvey ravaged Texas in autumn 2017. Photo: AMFPhotography / Shutterstock.com

Mystery number three: How has Amazonia managed to reach such an immense age?

The rainforest in South America is at least 50 million years old. But how has this particular forest managed to reach such an exceptional age when other ecosystems have collapsed as a consequence of floods, cold spells and other extreme events? Scientists have pondered the question of how the Amazonian forest has survived for millions of years without being eradicated.

In the case of Amazonia, it is not a matter of passive survival.

We have already established that Amazonia can produce its own rain. And when you control rainfall, you also control convection. Put simply, the Amazonian forest regulates the trade winds from the Atlantic Ocean and the moisture that they carry. The biotic pump theory was confirmed in 2007 and is steadily gaining wider recognition. This compares the rainforest to a pump that sucks moist air from the ocean. This is the result of atmospheric pressure being affected by evaporation and condensation processes in and above the forest.

This mighty forest both creates and maintains a favourable climate.

Thus, moist sea air will be transported to a greater extent over forested areas. As long as the forest survives, evaporating and condensing more water than the ocean, this process will continue to ensure regular rainfall.

If the forest is cleared, the opposite will occur. Evaporation and condensation will remain greater over the ocean, inverting the direction of the water supply – from the land to the sea.

Consequently, the forest might then become desert - which is what the next mystery is about.

The soya industry is responsible for destroying large areas of rainforest in Brazil. But the crops will probably dry out if the Amazonian forest disappears. Shown here is a soya crop in Mato Grosso in Brazil. Photo: Shutterstock

Mystery number 4: Why is there no desert south of Amazonia?

The rain that falls around the equator creates favourable living conditions for the vast forests which form the Earth’s green ocean. When the moist air of the equator has dispensed its rain, it moves on to subtropical areas where it sinks and is warmed up again. This drier air attracts moisture from the ground. As a consequence, we find many deserts along the Tropic of Cancer (in the northern hemisphere) and the Tropic of Capricorn (in the southern hemisphere).

Thanks to aerial rivers, Brazil’s two largest cities are not located in the desert

There are, however, some exceptions. One of them can be found to the east of the Andes in South America, where the Tropic of Capricorn runs adjacent to Rio de Janeiro and São Paulo.

Everyone knows of the River Amazon and its countless tributaries which supply huge amounts of water to large regions across the continent. But very few people have heard about the ‘aerial rivers’ that the rainforest also helps to create, due largely to its function as a geyser, as described in mystery number 1.

These ‘rivers’ occur when the moist air over the Amazonian forest meets the six-kilometre-high Andes, veering off in a south-easterly direction. After a long journey this moisture falls as regular rainfall, covering a vast area stretching from Buenos Aires in the south to Cuiabá over 2,000 kilometres to the north.

This is the centre of South America’s agriculture and production. Approximately 70% of the continent’s GDP is created in this enormous agricultural zone.

This green and fertile territory benefits directly from Amazonia.

Three hurricanes have formed in the Atlantic Ocean, but none of them move southwards towards South America. The reason is Amazonia. Photo: Shutterstock.

Mystery number 5: Why isn’t South America subject to hurricanes?

While powerful hurricanes are a frequent occurrence in the islands of the Caribbean and parts of the USA, they do not spread to South America. This is despite the fact that some of the conditions that create hurricanes, such as high temperatures and high levels of atmospheric humidity, are present in Amazonia and its adjacent coastal regions.

Recent research has shown, however, that the vast forests function as a barrier to the development of hurricanes and other extreme weather events, such as floods and droughts. The explanation can be found in two processes we have already mentioned:

  • The steady condensation of moist air above the forest canopy prevents wind from forming the destructive vortices that characterise hurricanes and tornadoes.
  • The biotic pump function of the rainforest is continually drawing moist air in from the Atlantic Ocean. This dries out the oceans beyond the forest, preventing hurricanes from forming there. The destructive paths of hurricanes which form in other places follow wetter routes.

As long as Amazonia continues to exist, it will provide South America with the world’s most effective storm prevention.

The theory that large forests protect us from extreme weather is proven by the paths of well-known tropical hurricanes.

South America’s beating heart

To conclude: The largest rainforest in the world collects enormous amounts of water, which it redistributes evenly across vast areas of South America with the help of ‘aerial rivers’. It produces its own rain and breaks all of the usual rules governing rainfall. In addition, the forest also provides an effective form of protection against extreme weather.

While Amazonia is being deforested by a minority, they are in the process of destroying the living conditions for millions of people over large parts of South America. Photo: ISA

It’s no surprise that Amazonia has been compared to a human heart pumping blood around the body. There is every reason to believe that South America’s inhabitants have the rainforest to thank for their favourable climate and excellent growing conditions.

Therefore, the destruction of Amazonia is an ill-conceived activity which will have disastrous and lasting consequences for large sections of the continent.

The expression ‘cutting off the branch you are sitting on’ has rarely been more appropriate than when describing the situation in South America.

This article is based on The Future Climate of Amazonia Scientific Assessment Report.