SIR – Mr Goldsbrough raises some interesting questions in his letter about oxygen in the atmosphere (T&A, November 20).

We certainly wouldn’t want to run out of it, but this is most unlikely as the percentage in the atmosphere has been very stable for the last half a billion years since multi-cellular life began.

However, it is true that oxygen levels are falling at a rate consistent with the burning of fossils fuels as the oxygen combines with this historical carbon to produce CO2.

Indeed, this decline is one of the indicators that would be expected if global warming is caused by man’s activities and not as a naturally occurring variation.

It isn’t a worry, though, because while oxygen makes up one in five of the molecules of gas in the atmosphere, there is much less carbon – only one in 3,000 molecules – so it won’t decline to a dangerous level.

Keith Thomson, Heights Lane, Bradford