Whatever life Mars had sustained could not overcome the forces that made Mars a frozen desert. The Gaia principle represents, at best, the tendency of a planetary biosphere to rebound from disaster. But its power is not limitless. Some form of the Gaia principle is probably at work on Earth now, but Gaia cannot take the place of intelligent foresight and action, nor overcome all of our excesses. The Amazon provides one quarter of the Earth s oxygen.
Rainforest is highly efficient in trapping light and thus promoting photosynthesis.
Photons that penetrate the top canopy of the forest have usually two more canopies to go before they can be absorbed by the ground or reflected. Anywhere they go they run into something green. The fact that carbon dioxide and oxygen are exchanged one for one in photosynthesis and combustion-respiration was part of what led to the discovery that oxygen levels are dropping. The drop of approximately 50-70 parts per million, presently estimated since 1958, when it was last measured, is miniscule compared to the 210,000 parts per million that exist in the atmosphere. But oxygen is so vital to life that any drop must be taken seriously.