Thomas L. Friedman, the New York Times columnist, wrote yesterday:
These intact forests — from those in the Amazon and Congo River basins to ones in Canada, Russia and Ecuador — are the world’s life-support system. They sponge billions of tons of carbon dioxide out of the atmosphere, generating oxygen [...].
A forest would, over longer time periods, only be a net carbon sink — and corresponding oxygen source — if its biomass increases and/or if dead organic material is removed from the global cycle of matter, e.g. by forming peat underground.
But, for example, parts of the Amazon rain forest stand basically on a relatively thin layer of humus above infertile quartz sand (here is a popular science article); since the rain forest is millions of years old, there is no significant sequestering of organic matter over time in the Amazon.
Of course I may have overlooked something, and rain forests in Africa or Asia, let alone forests in temperate climates, all in places with very different geologies, may indeed sequester carbon. After all, our fossil fuels once were plants.
Are forests overall significant carbon sinks or should I write a letter to the editor? I suppose that Friedman should at least remove the Amazon basin from the sentence.