PHILIP ROE is right about one thing (Letters December 12): the earth’s climate has changed, sometimes dramatically, over 4.5 billion years.

Not all these climatic phases could have supported human life. During the first billion years, no life of any kind existed.

Mr Roe describes as “meaningless” the report that 97 per cent of climate scientists confirm that not only is rapid climate change occurring but is driven by human activity because he doesn’t know how many climate scientists there are.

Well, I’m not sure either, but the evidence for human-driven climate change is overwhelming. Would Mr Roe regard it as “meaningless” if the British Dental Association reported that 97 per cent of dentists are confident that deposits of sugar in teeth and gums exacerbate tooth decay unless he knew how many dentists there are?

The concentration of carbon dioxide (CO2) is what is leading to climate change. Human activity since the beginning of the industrial revolution has resulted in an increase of about a third in the level of CO2 in the atmosphere.

This huge change can be measured, for example, by examining ice cores in Greenland and other ancient ice sheets. Some of the additional CO2 is absorbed in the oceans. This has led to acidification of seawater.

We fail to grasp how intimately dependent we are on the earth continuing to exist in the very particular conditions in which we have come into being. We really could be sawing off the branch we’re sitting on.

Chris Walker-Lyne, Millfield Road, York.