Regarding the letter querying the weight of carbon dioxide being heavier than the petrol producing it, I don't know the exact figures but bear in mind the oxygen part of the CO2 molecule comes from the air, not the petrol.

In a similar way, rust on an iron object is heavier than the original iron because it is combined with a certain weight of atmospheric oxygen.

But I really wouldn't worry about it because most of the global warming arguments fall between "unknown" and "rubbish".

-M Boyask, Hove