Sunday, November 7, 2010

Forests and Changing Climate (The Economist)

A special report on forests

Seeing the wood

Purveyors of water, consumers of carbon, treasure-houses of species, the world’s forests are ecological miracles. They must not be allowed to vanish, says James Astill

DAYBREAK is a heavenly time to look on the Amazonian canopy. From a Brazilian research tower high above it, a fuzzy grey sylvan view emerges from the thinning gloom, vastly undulating, more granular than a cloud. It is mind-bendingly beautiful. Chirruping and squawking, a few early risers—collared puffbirds, chestnut-rumped woodcreepers and the tautologous curve-billed scythebill—open up for the planet’s biggest avian choir.
In a slick of molten gold, dawn breaks and the trees awaken. In every leaf, chlorophyll molecules are seizing the day for photosynthesis. Using sunlight to ship electrons, they split water molecules and combine the resulting hydrogen with carbon dioxide extracted from the air. This produces carbohydrates that the trees turn into sugars, to be burnt off in respiration or, by another chemical process, turned into new plant-matter. The main waste product, oxygen, they emit through their stomata in a watery belch. Hence the rainforest’s high level of humidity, visible from the observation tower in diaphanous cloudlets drifting over the canopy.
That plants emit oxygen has long been known—since 1774, in fact, when Joseph Priestley, a British chemist, found a mouse not too “inconvenienced” by being trapped inside a bell-jar with a mint plant. Yet the importance of plants’ ability to store carbon in making the planet habitable is still not widely appreciated. On two previous occasions when the atmosphere contained very high levels of carbon dioxide, the early Carboniferous and Cretaceous periods, beginning about 350m and 150m years ago respectively, they were reduced by the expansion of carbon-sequestering plants. Industrial burning of the fossil fuels laid down in the Carboniferous period, in the form of decaying plant-matter, is the main reason why there is now more carbon in the atmosphere than there has been for 4m years.

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