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ISIS Special Miniseries - Why Gaia Needs Rainforests

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The Institute of Science in Society

Science Society Sustainability

http://www.i-sis.org.uk

 

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ISIS Director m.w.ho

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ISIS Special Miniseries

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Why Gaia Needs Rainforests

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Losing the earth’s largest remaining tropical rainforests will greatly

accelerate global warming. Peter Bunyard reports.

 

Diagrams and sources for this article are posted on ISIS Members’ website.

Details here.

 

 

Vast amounts of greenhouse gases - carbon dioxide, methane and nitrous oxide -

are released into the atmosphere as a result of clearing and burning

rainforests. In recent years, deforestation has contributed as much as 30

percent of all anthropogenic emissions of carbon dioxide in the atmosphere.

Tropical deforestation therefore contributes significantly to global warming

both through the release of stored carbon and through the destruction of one of

the Earth’s prime ways of absorbing excess atmospheric carbon.

 

 

Moreover, by acting as a ‘heat pump’ that redistributes the energy of sunlight

from the equator to the temperate regions, tropical rainforests have another

vitally important role that has been largely ignored by climatologists. Tropical

rainforests, and particularly those of the Amazon Basin, warm the temperate

zones while cooling the tropics, and in the process, regulate the flow of

freshwater through the ecosystem, determining local and regional rainfall

patterns. Destroying the tropical rain forests will perturb climate in ways

every bit as powerfully as the addition of greenhouse gases.

 

 

Through evapotranspiration from the forest canopy, large amounts of rain-water

are returned to the atmosphere, generating clouds that reflect sunlight back

into the outer space, thus cooling the forested regions. Transpiration draws

water absorbed by the roots up through the entire plant, releasing it into the

atmosphere as water vapour from open ‘stomata’ or pores on the plant leaves.

This process accounts for 60 per cent of the humidity in the air over central

Amazonia; evaporation from the surfaces of leaves and stems of the vegetation

accounts for the remaining 40 per cent. In sharp contrast to forests in

temperature regions, virtually no evaporation occurs from Amazonian soils when

supporting mature forest.

 

 

Evapotranspiration over Amazonas involves enormous amounts of solar energy and,

according to Brazilian climatologist, Luiz Carlos Molion, takes up as much as 80

per cent of the energy of sunlight directed down over the forests. The hot,

humid air generated over the rainforest then rises rapidly and develops into

cumulo-nimbus thunder clouds that simultaneously water areas further downwind

and release the energy bound up in water vapour as ‘latent heat’ back into the

atmosphere, so driving the great air masses in their circulation patterns. The

hydrological dynamics of evapotranspiration fall apart when the rainforest is

destroyed.

 

 

One study in Nigeria shows up the difference between the forest and a clearing

just 50 metres apart. The day-time temperature just above the soil in the

clearing was 5oC higher than in the forest, and the humidity nearly halved. With

the Amazon forest totally destroyed, evapotranspiration is likely to fall to one

half of its original value and precipitation down by as much as 20 per cent.

 

 

Brazilian physicist, Eneas Salati has shown that up to 75 per cent of all the

water falling as rain over the Amazon is evaporated and transpired back into the

atmosphere, to fall again as the winds move from east to west. The energy flow

across the 5 million square kilometres of the Brazilian Amazon Basin is

equivalent to 5 to 6 million atom bombs exploding every day, Salati says.

Clearly a 10 or 20 per cent drop in the amount of water vapour being carried in

the system represents a reduction in energy flow equivalent to more than 20

times the total energy used in industry and agriculture across the entire

planet.

 

 

The moisture, originally picked up by the Trade Winds as they blow across the

tropical Atlantic Ocean (see Box 1), may therefore be deposited up to seven

times across the entire 4000 kilometre expanse of the Amazon Basin in an

unparalleled ‘leap-frogging’ cycle of evapotranspiration and precipitation. The

Amazon River, having collected the run-off from all its tributaries, carries

less than half the total rainfall that precipitates over the 7 million square

kilometres of the Basin. The rest is carried in the air mass travelling west

across the Amazon Basin until it hits the mountain chain of the Andes. There,

the air stream splits into three branches. The central part jumps over the Andes

into the Pacific and continues west along the Equator, following the convergence

of the warm northern sea current. The southern stream is deflected by the Andes

and passes over Patagonia via the Brazilian cerrado (savanna). The northern

stream crosses the Caribbean, touches the eastern seaboard of

the US and goes over the Atlantic towards northern Europe.

 

 

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Box 1

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How the earth’s atmosphere circulates

The circulation of the earth’s atmosphere modulates surface temperatures over

land and sea, and determines rainfall patterns (see Fig. 1).

 

 

 

 

Figure 1. The earth’s atmosphere circulates to distribute warmth and moisture.

 

 

The earth’s atmosphere is set in motion because the tropics are heated up more

than the poles. The excess heat in tropic is transported towards the poles by

circulation of the atmosphere and by ocean currents (see “Global warming & then

the big freeze”, this series).

 

 

At the Equator, the hot air with water vapour expands and become less dense, so

it rises, creating low pressure. But as the hot air rises, it cools, the water

vapour condenses and falls as rain. This creates high rainfall in the

Intertropical Convergence Zone in the tropics.

 

 

As the air mass cools, it increases in density and falls back towards the

surface in the subtropics (30oN and S), creating high pressure. The net

circulation is referred as the Hadley Cell, one on either side of the equator.

 

 

If the earth did not rotate, there would be a single circulation cell in each

hemisphere. Because of fluid motion on a rotating sphere, the single cell is

broken up into three circulation cells in each hemisphere, named in order from

the Equator: Hadley Cell, Ferrel Cell and Polar Cell.

 

 

This creates alternating bands of high and low pressures approximately every 30o

latitude. Wind arises as air moves horizontally between regions of different

pressures. Very little wind is present at the Equator because air rises

vertically as it heats up. Light, variable winds at the equator are known as the

Doldrums. Similarly, there is little wind at 30oN and S where the air descends.

Air always moves horizontally from an area of high pressure to low pressure.

 

 

Wind blows straight down the pressure gradient but is deflected by the Coriolus

Force, which is a consequence of motion on a rotating sphere. This deflects the

wind to the right of the direction of motion in the Northern Hemisphere and to

the left in the Southern Hemisphere.

 

 

The circulation of the earth’s atmosphere can be severely perturbed by

deforestation, with drastic consequences on climate and rainfall patterns.

 

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The Amazon rainforest, if undisturbed, is a self-contained, self-sustaining

system of extraordinarily rich biological diversity(see Box 2).

 

 

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Box 2

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The Amazon rainforest is self-sustaining

The Amazon rainforest, especially over the unflooded areas, is a remarkable

self-contained system that depends crucially on the integrity of the whole to

sustain itself. The soils are among the poorest on the planet — washed out after

millennia of heavy rains — yet the vegetation and the unparalleled richness of

living organisms would seem to suggest a luxuriance that derives from plenty

rather than from deprivation. That paradox is the miracle of the rainforest. In

the 1980s, one of the world’s most prestigious experts on Amazonia, Harald

Sioli, director of the Max Planck Institute for Limnology in Germany, told us

how the entire system serves to retain virtually all the nutrients within the

biomass. Leaks of vital nutrients, such as are common in temperate ecosystems

would spell disaster. A dense root mat system, combined with fungal mycorrhiza

bridges, literally sucks up any decomposing matter from the forest litter.

 

 

Above ground, the system of tall trees, with their extraordinary profusion of

epiphytes — the ferns, orchids and bromeliads that have attached themselves to

the stem and branches of the great trees — take up any nutrients that are

flushed down with the heavy rains. Most of the fauna lives in the canopy, and is

also perfectly integrated into the nutrient recycling system by providing the

sustenance for the lateral extension of the forest. As a result, said Sioli,

“the greatest number of plant and animal species we are aware of (estimated at

between 1.5 and 2 million species) divides the general nutrient cycle into an

immense number of sub-cycles.”

 

 

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If we continue to destroy the rainforests of Amazonia, as well as those

remaining in Africa and in South-East Asia, we will perturb climate and rainfall

patterns across the entire planet. Tropical ecosystems will undoubtedly

collapse, with all that that means for agriculture across Latin America,

South-East Asia and Africa. Northern Europe will also feel the chill that will

come with a drastic reduction in the energy flows from the warm tropics.

 

 

In 2002, an area of Amazonia the size of Belgium - some 25 thousand square

kilometres - went up in flames. Already more than half a million square

kilometres of the Brazilian Amazon have gone in a matter of a few decades:

one-fifth of the total three and a half million square kilometres of Brazil’s

rainforest. To make matters worse, when areas are cleared of trees the

surrounding forest suffers die-back and disintegration. Carbon emissions from

areas of Amazon that have been cleared are likely to be at least 7 per cent

higher than previously thought, because of that die-back — the equivalent of

felling one million more hectares than are actually felled.

 

 

Molion points out that the Amazon forest canopy intercepts on average about 15

per cent of the rainfall and that its removal would lead to as much as 4000

cubic metres (tonnes) per hectare per year hitting the ground. Because of soil

compaction much of that water would run off directly into the rivers, rather

than being retained and maintaining some soil moisture. The net result is

‘sandification’ whereby the heavy drops of rain hitting the ground cause the

selective erosion of finer clay particles, leaving behind increasingly coarse

sand. With time, the remaining ‘soil’ has virtually no water-retaining

properties and the forest is unable to regenerate itself. Soil under intact

forest absorbs ten times more water compared with nearby areas that have had

pasture for five years. Outside the forest and away from its soil-protecting

attributes, erosion increases a thousand-fold.

 

 

When the forest is cleared, the contrast between day and night temperatures

becomes more extreme, leading to gustier winds that dry out soils and send dust

swirling into the air. Even if some forest is left around the edges of clearings

it will be under siege from water-stress as the water table plummets. Large

areas of the Amazon Basin are far closer to water stress than scientists once

thought and the clear-cutting and burning of large areas of rainforest will

inevitably precipitate die-back and death of the nearby forest. We have no idea

just what proportion of forest must be left for the system to be

self-maintaining. It may be three-quarters; perhaps even less: if so, with 20

per cent already gone, we are terrifyingly close to those limits. How ludicrous,

as many international conservation bodies have done, to think that saving 10 per

cent of the Brazilian Amazon would be anywhere near adequate.

 

 

Oliver Phillips and his colleagues from around the world reported in Science in

1998 that, uniquely among tropical forest systems, the neo-tropical forests of

Central and South America, where they are intact, are showing growth that

amounts to as much as one tonne per hectare per year. If all the forests of the

Brazilian Amazon, covering some 360 million hectares, put on biomass in that

way, the Amazon would be an annual sink of up to 0.36 billion tonnes of carbon.

In contrast, burning a hectare of forest releases up to 200 tonnes of carbon,

and the destruction of 10 million hectares a year would release 2 billion tonnes

of carbon, five to six times more in carbon than is drawn down out of the

atmosphere by the entire Brazilian Amazon.

 

 

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