Science in Society Archive

I-SIS Special Miniseries - Life of Gaia

This miniseries is dedicated to our planet earth, so we may better appreciate how she lives and sustains all creatures large and small, that we may learn to dance to the complex rhythms of her life music without stopping her in her tracks.

Space scientist and inventor Jim Lovelock first proposed in the 1970s that the entire earth is a self-organizing, self-regulating entity, rather like an organism. He named the earth Gaia, after the Greek earth goddess.

The idea that Gaia is alive and has a life of her own immediately caught fire. It inspired many earth scientists to look for the dynamic processes that organize and regulate the currents of the earth, to make a congenial home for all her inhabitants. These scientists are richly rewarded.

Records from ice and deep sea cores show detailed globally correlated changes going back at least 800 000 years, leaving us in no doubt that the earth behaves from moment to moment as one coherent whole, just like an organism.

Not only can we can read Gaia’s life-history from her deep memory stores, we can also tune in to her life-force pulsing as she is living today.

Gaia spinning in her perpetual dance around the sun, her mighty breath tumbling from hot belly to the poles, swirling across the continents, bringing welcome rain to forests, grasslands and crops, or torrential downpours, floods and hurricanes. Vast slow vortices of water connect her oceans from the furthest northern reaches to the southernmost haunts, from the shimmering sea surfaces to the dark deep beds, distributing warmth and nutrients, sustaining life with life.

Gaia’s breath is our breath, her water our water. Let Gaia live that we may live.

Why Gaia Needs Rainforests

Losing the earth’s largest remaining tropical rainforests will greatly accelerate global warming. Peter Bunyard reports.

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 5o C 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.

Box 1

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).

The earth's atmosphere circulates to distribute warmth and moisture

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.

The Amazon rainforest, if undisturbed, is a self-contained, self-sustaining system of extraordinarily rich biological diversity (see Box 2).

Box 2

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."

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.

Article first published 08/10/03


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