This letter appeared in the 18 September 2012 print edition of the Canberra Times:

David Teather (Letters, September 14) made some good points. His concerns are justified and, as an example, he pointed out the 39 per cent increase of CO2 concentration using a simple ratio based arithmetic calculation to arrive at a correct answer.

Another way of looking at this example is to use a simpler arithmetic calculation – subtraction. This provides us another correct answer of 110 parts per million, i.e. .011 per cent increase of CO2 concentration in our atmosphere since pre-industrial times.

Of course 39 per cent looks a more impressive headline figure than a very humble .011 per cent. Certainly it is a far more worthy number to bring grist to the alarmist climate mill.

John Morland, Curtin

… to which I replied …

So John Morland (Letters, 18 Sept) prefers to express the recent increase in CO2 as 110 parts per million, claiming that this shows it is “modest”, putting climate “alarmist” figures into perspective.

But what perspective? Morland implies that the current level of nearly 400 parts per million is safe, presumably because 0.04% just looks like a small number.

A better perspective is to relate this to the planet’s past. For at least 800,000 years, probably much more, CO2 has never been over 300ppm.

A huge amount of work has gone into projecting the possible impacts of higher CO2 levels. Many scientists suggest that 350ppm should be regarded as a safe upper limit. By the World Health Organisation’s measure, we’re seeing 150,000 deaths a year caused by the current level of climate disruption. At higher levels – 500, 700 and beyond – we face nightmare scenarios.

However, we have more than enough known coal, oil and gas reserves to take us well over 1000ppm if we continue with “business as usual”. Our current path does not just make catastrophe a possibility, it makes it the most likely result. We need to get off the fossil fuel train, and quickly.

Matt Andrews, Aranda