IN THE theory of man-made climate change, two-thirds of the predicted warming comes from changes in humidity and clouds, and only one-third comes directly from the extra carbon dioxide (CO2) and other greenhouse gases.

The theory assumes humidity and clouds amplify the warming directly due to CO2 by a factor of three: extra CO2 warms the ocean surface, causing more evaporation and extra humidity. Water vapour, or humidity, is the main greenhouse gas, so this causes even more surface warming.

Not many people know that. It is the most important feature of the debate, and goes a long way to explaining why warmists and sceptics both insist they are right.

The warmists are correct that CO2 is a greenhouse gas and it causes warming, that CO2 levels have been rising, and that it has been warming.

Serious sceptics agree with all that, but point out that it does not prove that something else isn't causing most of the warming. By way of illustration, if the main cause of warming was actually Venusians with ray guns, then all those things would still be true.