Could C A Melhuish (Letters, December 21) or anyone else please explain how the proportion of CO2 in the atmosphere that has been, or is being, caused by human activity, has been measured.

Years ago I saw a figure of 10%, about three years ago a Government figure of 30% was given. Recently I have read that it is 40%.

Unless it can be proved otherwise, I think the last two figures are just exaggerated claims by politicians.

Climate changes naturally. In Roman times there were vineyards in the North of England and in contrast the Baltic froze over in 1658.

I have seen a graph of CO2 in the atmosphere and earth temperature over thousands of years. The CO2 rose about ten years after a rise in the temperature.

This is probably, perhaps obviously, caused by the temperature increasing the animal population that breathes out the CO2.

It seems to me that the sun’s activity causes the rise in temperature and the rise in CO2 follows, not the other way round.

C S Gould Georgia Avenue Worthing