Amazon deforestation is stopping the forest from recycling rainwater

November 15, 2021

By: The Working Forest Staff

ABC NEWS AUSTRALIA — For 55 million years, heavy rain has brought life and splendour to the Amazon rainforest.

But a rapid increase in deforestation is threatening the very water systems that make the forest …  well, a rainforest.

Key points:
  • Experts have stressed the impact the Amazon has on global temperatures
  • Clear-cutting of the rainforest continued to rise last year, with over 11,000 square km of rainforest lost
  • Brazil has joined an international pledge to halt and reverse deforestation by 2030, surprising many observers

The destruction of the Amazon threatens water security, both in the rainforest itself, and further south in more temperate climes.

In the week leading up to the COP26 climate summit in Glasgow, Brazil surprised many observers by joining an international pledge to halt and reverse deforestation by 2030.

Despite South America’s dependence on the rainforest for water, there was a 34 percent increase in deforestation of the Brazilian Amazon in 2019, compared to 2018 when an estimated 10,129 square kilometres was clear cut.

Clear-cutting continued to rise last year, when an estimated 11,088 square km of rainforest in Brazil was lost.

CAPTION: The Amazon has been devastated by fires and deforestation in recent years.

Forests make clouds, which in turn create rain, explains Amazon expert Bill Laurance of James Cook University.

The Amazon rainforest is large enough to cover about three quarters of the Australian mainland and Professor Laurance says its sheer size allows it to have a profound impact on the world’s weather.

The dark forest absorbs energy from the sun and becomes a giant “heat and water engine”, Professor Laurance says.

“Hot, moist air moves high up into the stratosphere, and then that moist air goes poleward until it gets to the middle latitudes.”

Much of the rain falling in southern parts of Brazil and South America, including Brazil’s most populous city São Paulo, begins its journey in the Amazon.

How do forests make rain?

When it rains, trees capture the water with their roots.

During photosynthesis, this water is moved up the plant and released into the air via pores on the leaves.

This process is called “evapotranspiration”.

Each tree in the forest behaves like a giant fountain, moving large amounts of water from the soil to the air.

You can see this in action when a forest appears to steam, and clouds rise from the canopy.

Mist rises after rain in the Amazon in Ecuador. The rainforest’s water originates from the Atlantic Ocean.(Getttty Images: Wolfgang Kaehler/LightRocket)

There are billions of trees in the Amazon, and each one is pouring water into the air, creating clouds that eventually fall again as rain.

The rainforest’s water originally evaporates off the Atlantic Ocean; much of this water is kept in the system by the trees recycling it.

“On average a drop of water gets sucked up six times as it travels across South America from the Atlantic Ocean to the Andes,” Professor Laurance says.

Deforestation stops this cycling in its tracks.

Extensive deforestation in the tropics can reduce rainfall in a region by up 40 percent.

In Brazil, the rainforest is being replaced by soy fields and cattle pastures.

These landscapes do not cycle water like rainforests.

An area of the rainforest cleared to plant soybeans in western Brazil.(Reuters: Paulo Whitaker)

Fields are less complex, with fewer plants and lower biomass. This means there are not so many leaves pumping water back into the air, Professor Laurance explains.

“If you imagine a rainforest, then imagine measuring every single leaf [in that rainforest] and then compare that measurement to the leaves on a cattle pasture. It’s just a tiny fraction.”

If deforestation continues, the Amazon will reach a tipping point whereby the rainforest will not be able to produce enough rain to keep itself alive and it will become a savannah, Professor Laurance warns.

In 2018, researchers published a cautionary letter in Science Advances, writing that with the combined effect of deforestation, fire, and climate change, it may only take 20 to 25 percent deforestation to transform the east, southern and central Amazon rainforest into savannah.

We are edging close to this tipping point.

In 2019, the World Wildlife Fund reported that 17 percent of the Amazon had been deforested in the past 50 years.

Deforestation also reduces rainfall in Australia, according to Clive McAlpine from the University of Queensland.

Historical clearing has caused local climates to increase by up to 2 degrees Celsius, Professor McAlpine says.

This has been paired with declines in rainfall of up to 15 to 20 percent.

“We are losing that cycling function of our deep-rooted trees,” he says.

Water security in South America

Not all the water pumped into the air by Amazonian trees falls back on the rainforest.

A percentage of the water is carried away and falls in temperate regions south of the rainforest, explains Philip Fearnside, a biologist at Brazil’s National Institute of Amazonian Research.

“A tremendous amount of water is transported by these so-called flying rivers and winds that circle around through the Amazon and go [south] towards São Paulo,” Professor Fearnside says.

“During the rainy season in São Paulo, [about] 70 per cent of the water is coming from the Amazon, and that’s when you need to fill the reservoirs.

“The city of São Paulo has come close to running out of water several times, and that’s without destroying the Amazon rainforest, which makes it much more likely you’d have a major catastrophe.”

A map shows the size of the Amazon Basin, and its location in relation to Brazil’s most populated city, São Paulo. (Supplied: Wikimedia Commons)

Professors Fearnside and Laurance are among the many researchers and commentators who lay the blame for Brazil’s recent spike in deforestation at the feet of President Jair Bolsonaro, who came to power in Brazil in 2018.

“Bolsonaro has been pushing for new roads into the Amazon, which always create Pandora’s box ranging from deforestation, poaching, illegal mining, illegal logging, illegal land colonization, a lot of illegal burning for charcoal, other things,” Professor Laurance says.

Professor Fearnside adds: “You have a projection that São Paolo is going to be out of water; that’s a very drastic thing.

“It’s much more convenient to just deny it.

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