Trees of the Amazon rainforest - in pictures Read more

Rainforests cover 6% of Earth’s land surface, yet they are home to 50% or more of all land-based species, perhaps half of which remain to be discovered and named by scientists. Relatively few people have experienced pristine, undisturbed rainforest at first hand, but many of those who have are awed by the soaring cathedral of life they find themselves in. The subdued light, the constant temperature and humidity, and often the quiet, add to the awe. The capacity for rainforests to moderate the climate embody the Gaia hypothesis – the concept that the Earth self-regulates to create the conditions to support life.

A constant supply of sunlight and water in rainforests has encouraged the proliferation of species. There is a parallel to be drawn between rainforests and great cities. Both require a constant supply of life’s essentials (imagine a city left a week without electricity), and both encourage specialists.

The extraordinary diversity of rainforests is without doubt their greatest value to humanity, for it has turned the rainforests into a great natural pharmacopeia. The warmth and humidity below the canopy allow pathogens to abound. If any one species becomes too abundant, disease transfers easily, and it will be ravaged by an epidemic. But the ubiquity of pathogens has also led to a proliferation of physical and chemical defences in every organism that inhabits the forest. As humanity gathers together in ever greater urban aggregations, the need for defences against pathogens grows. Around a quarter of all medicines originate from tropical forest plants, and it’s a fair bet that cures for many of humanity’s present and future ills lie undiscovered below the verdant canopy.

Rainforests fulfil other needs. They are the source of much food and other products. From Brazil nuts to bananas, and from rubber to resins, they enhance our lives. They are also vital sources of clean, reliable water, acting as great cleansing filters and sponges which provide a steady source for pristine streams that can flow right through the dry season. The Amazon rainforests store around half of Earth’s available fresh water, and right across the tropics many cities and farms depend on rainforests for their water supplies.

Facebook Twitter Pinterest Rainforests also play a larger role in the water cycle, in that they create their own rainfall. Photograph: Frans Lanting/Corbis

But rainforests also play a larger role in the water cycle, in that they create their own rainfall. The rainforest canopy transpires enormous quantities of water. This occurs when the individual leaves open tiny pores, known as stomata, to gain access to the CO2 in the air. While the stomata are open, the plant loses moisture through its leaves, which accumulates over the forest as water vapour, eventually to fall as rain. It’s been estimated that the Amazon creates as much as 80% of its own rainfall.

Rainforests are also crucial in the Earth’s carbon cycle. Through photosynthesis, the CO2 the plants absorb from the air is turned into leaves, bark, timber and fruit – indeed all the tissues of living plants. You can think of trees largely as congealed CO2. Up to half of all the carbon stored in land-based ecosystems resides in rainforests, in the form of living organisms as well as soil carbon. One thing rainforests are not, however, is the “lungs of the planet”. Although significant producers of free oxygen (via photosynthesis), they play a far smaller role in producing free oxygen than do single-celled algae in the ocean.

Facebook Twitter Pinterest Not everyone accepts that rainforests are important to humanity and the planet. Photograph: Bloomberg/Bloomberg via Getty Images

Beyond the functional reasons that rainforests are important must be counted the place they hold in our imaginations. Rainforests are so extraordinarily ancient that they allow us, imaginatively at least, to travel in time. Fifty-five million-year-old fossils from the lignite mines of Germany come from plants and animals whose barely altered descendants can be found today in the rainforests of Malaysia. The rainforests of central eastern Australia are even more ancient, telling of a time when the landmasses of the southern hemisphere formed the supercontinent of Gondwana. The harpy eagles of south America and New Guinea are among the world’s most awesome birds of prey. Genetic studies show that they are each other’s closest relatives. Yet neither the south American nor the New Guinean harpy eagle ever stray outside the rainforest, and they have failed to colonise even large, rainforest islands lying near the lands they occupy. They came to occupy their widely separated homes because their ancestors inhabited rainforests that flourished in Antarctica 60 million or more years ago.

Facebook Twitter Pinterest South American and New Guinean harpy eagles inhabited rainforests that flourished in Antarctica 60 million or more years ago. Photograph: Morley Read/Alamy

Not everyone accepts that rainforests are important to humanity and the planet in the ways I’ve outlined above. One Australian I asked thought about rainforests much like Groucho Marx did about future generations – why should I care about them: what have they ever done for me? He thought that rainforests could be put to better use, as timber and croplands.

There is no doubt that rainforest timbers bring high prices on markets, nor that there is money to be made in felling them and growing crops. In both instances the great common benefits that rainforests provide are exchanged for private gain for the few.

But even that gain is largely illusory. The soils of the rainforest are notoriously infertile, as was inadvertently illustrated in one of the most memorable scenes in the 1972 cult classic Aguirre: The Wrath of God. A beleaguered conquistador, starving, fever-racked, and hallucinating, gazes at a butterfly perched serenely on his finger.

In a surreal film steeped in violence and the alien world of the Amazon, the friendly butterfly seems as absurd as the galleon another conquistador sees floating in the treetops. But anyone who has lived in the rainforest knows that visits by the most glorious butterflies is an everyday occurrence. They come to drink our sweat, for the sake of the minerals it contains. Almost all the nutrients in a rainforest are incorporated in the plants and animals that comprise it, so animal sweat is one of the few mineral sources available. Cut, burn and carry them away, and the wealth we gain is as transient as a butterfly’s visit.