That said, I must admit I am strongly tempted to agree that, since I am not a climate scientist, I should from now on remain silent on the subject — on the clear understanding, of course, that everyone else plays by the same rules. No more statements by Ed Davey, or indeed any other politician, including Ed Milliband, Lord Deben and Al Gore. Nothing more from the Prince of Wales, or from Lord Stern. What bliss!

But of course this is not going to happen. Nor should it; for at bottom this is not a scientific issue. That is to say, the issue is not climate change but climate change alarmism, and the hugely damaging policies that are advocated, and in some cases put in place, in its name. And alarmism is a feature not of the physical world, which is what climate scientists study, but of human behaviour; the province, in other words, of economists, historians, sociologists, psychologists and — dare I say it — politicians.

And en passant, the problem for dissenting politicians, and indeed for dissenting climate scientists for that matter, who certainly exist, is that dissent can be career-threatening. The advantage of being geriatric is that my career is behind me: there is nothing left to threaten.

But to return: the climate changes all the time, in different and unpredictable (certainly unpredicted) ways, and indeed often in different ways in different parts of the world. It always has done and no doubt it always will. The issue is whether that is a cause for alarm — and not just moderate alarm. According to the alarmists it is the greatest threat facing humankind today: far worse than any of the manifold evils we see around the globe which stem from what Pope called "man's inhumanity to man".

Climate change alarmism is a belief system, and needs to be evaluated as such.
There is, indeed, an accepted scientific theory which I do not dispute and which, the alarmists claim, justifies their belief and their alarm.

This is the so-called greenhouse effect: the fact that the earth's atmosphere contains so-called greenhouse gases (of which water vapour is overwhelmingly the most important, but carbon dioxide is another) which, in effect, trap some of the heat we receive from the sun and prevent it from bouncing back into space.

Without the greenhouse effect, the planet would be so cold as to be uninhabitable. But, by burning fossil fuels — coal, oil and gas — we are increasing the amount of carbon dioxide in the atmosphere and thus, other things being equal, increasing the earth's temperature.

But four questions immediately arise, all of which need to be addressed, coolly and rationally.

First, other things being equal, how much can increased atmospheric CO2 be expected to warm the earth? (This is known to scientists as climate sensitivity, or sometimes the climate sensitivity of carbon.) This is highly uncertain, not least because clouds have an important role to play, and the science of clouds is little understood. Until recently, the majority opinion among climate scientists had been that clouds greatly amplify the basic greenhouse effect. But there is a significant minority, including some of the most eminent climate scientists, who strongly dispute this.

Second, are other things equal, anyway? We know that, over millennia, the temperature of the earth has varied a great deal, long before the arrival of fossil fuels. To take only the past thousand years, a thousand years ago we were benefiting from the so-called medieval warm period, when temperatures are thought to have been at least as warm, if not warmer, than they are today. And during the Baroque era we were grimly suffering the cold of the so-called Little Ice Age, when the Thames frequently froze in winter and substantial ice fairs were held on it, which have been immortalised in contemporary prints.