Re: Has the quantity of pure Oxygen in the atmosphere been declining?

There is good evidence that over the course of geological history, 
atmospheric oxygen has built up from very low levels prior to 2000 million 
years ago to a maximum at present. There is a good, but rather technical 
discussion in Peter Warneck: Chemistry of the Natural Atmosphere, Academic 
Press 1988, pages 612-631.

There has been no measurable decline in atmospheric oxygen levels in recent 
times on a time scale of decades. There is no evidence that the level of 
oxygen in the atmosphere is declining at present.

Every time we burn any fuel, we also consume oxygen from the atmospheere. 
If we were to burn a large proportion of the total fuel reserves, there 
would have to be a significant oxygen loss from the atmosphere to support 
the combustion. But oxygen is being recycled in the atmosphere by the 
photosynthesis reaction in plants. There is even an argument that extra 
oxygen production is being stimulated by higher carbon dioxide levels in 
the atmosphere and warmer average temperatures.

If we rely too heavily on fossil fuels, there are many other aspects of 
atmospheric chemistry that will make life on earth impossible long before 
lack of oxygen becomes a problem. The increase in atmospheric levels of 
greenhouse gases, and evidence of global warming, are much more real and 
immediate problems that we may have to face. Scientists are currrently 
concerned with catastrophic effects of a possible rise of 2-5 degrees in 
global average temperatures. It is likely that if global average 
temperatures were to rise by 15 to 20 degrees, life would become 
impossible.