Re: is our planet generating progressively less oxygen ?

Hi David:

That's a very good question! It's possible that the amount of oxygen produced by plants may be decreasing due to mortality caused by forestry practices and some pollution mediated plant deaths; as I'm sure you're aware, this is often given as one of the reasons that global CO₂ levels are rising. There are, however, a couple of reasons why we don't have to worry about running out of oxygen too soon.

Firstly, there is a heck of a lot of oxygen in our atmosphere! The atmosphere contains about 78% Nitrogen, 21% Oxygen, and 1% other trace gases (compare with Carbon dioxide at about 0.04%; for more on atmospheric composition, an excellent introduction by Dr. Michael Ritter of the University of Wisconsin- Stevens Point is available here). The fact that there is so much oxygen in the environment makes it very difficult to detect if there has in fact been any changes in how much there is, because the measurements would need to be very accurate.

Besides the truly huge amount of oxygen in the atmosphere, in the grand scheme of things, there is more-or-less an equilibrium between carbon and oxygen in the biosphere (which encompasses both land and sea). For instance, as a tree grows, it turns CO and H₂O into O₂ and CH₂O (which is "sugar" or "carbohydrate"), so over its lifetime it will "fix" a certain amount of carbon into its body. When that tree dies and rots, the carbon (i.e. the CH₂O, of which the tree is mostly comprised) is converted back to CO₂ and H₂O by the bacteria, fungi, bugs, etc that do the actual rotting, and consume oxygen.

For more on this equilibrium, I'd also refer you to an outstanding answer previously posted here at MADSCI, by Karen Culver-Rymsza:
Re: Why doesn't the earth's oxygen/carbon dioxide ever get out of balance?

Hope that answers your question!

Rob Campbell, MAD Scientist