Letter: Letters : Breathing easily

From Nicholas Pillans

London

David Prichard misses two crucial points when he argues that destruction of
the ozone layer would kill oxygen-producing organisms and thus increase global
warming (Letters, 11 October, p 58).

The first is that the stock of oxygen in the atmosphere is very large
compared to the annual turnover which occurs through photosynthesis. If
photosynthesis were to cease entirely on the Earth, the oxygen in the atmosphere
could keep all animals breathing for thousands of years before its concentration
fell significantly below the current 20 per cent. (This is not as reassuring as
it sounds: without photosynthesis there would be nothing to eat, so animals
would die of starvation—but certainly not of asphyxiation.)

The second point is that practically all the matter produced by
photosynthesis becomes a food source for one organism or another within a
relatively short time. The organisms that Prichard cites may well generate
oxygen when they are alive, but once they become food for something else, their
biomass is very rapidly converted back to carbon dioxide and returned to the
atmosphere.

The issue of oxygen production is frequently raised in relation to the
destruction of rainforests, with such emotive phrases as “the rainforests are
the lungs of the world”. The reality is that rainforests, or any other
vegetation for that matter, have almost no influence on the level of atmospheric
oxygen. Virtually all the biomass generated by the forests is consumed as food
in one form or another, and the oxygen created by photosynthesis is matched by
an equivalent uptake of oxygen used for respiration.

If the rainforests were to disappear, there would be a huge loss of habitat,
and many plants and animals would die, but the balance between oxygen supply and
demand would remain unchanged, both locally and globally.