DAVID Ainsworth (Letters, July 18), remarking on your earlier article stating that global temperatures are rising faster now than at any time in the last 1,000 years and that the carbon dioxide levels are higher than at any time in the last 400,000 years, asks if this is due to human activity, why, then, were they so high then, when there wasn't any?

But there are many factors which contribute to high levels of carbon dioxide other than human activity. One of them is that when the rain forests are thriving, they absorb and store it, often for long periods of time.

However, in times of drought, particularly if severe and affecting large areas, the many thousands of trees which die as a result, as they decay, give off that amount which they have absorbed in their growth period, and this then finds its way back into the atmosphere.

Of course, as Mr Ainsworth observes, the fact that these effects coincide with human activity does not necessarily mean that one is the cause of the other.

Human activity, however, including burning fossil fuels on a global scale and coupled with the ever-increasing destruction of the rain forests, does have a cumulative effect on the amount of carbon dioxide in the atmosphere, thereby increasing global warming.

ALBERT MORRIS, Clement View, Nelson.