Tap any sleeping meteorologist on the shoulder and ask him, as he stirs to consciousness, about any weather incident that might be blamed on global warming, and in the clipped, staccato tones of an automaton, he will recite his mantra: "Such occurrences are well within the range of natural climatic variability." Needless to say, lest Weather Eye stands accused of gender bias, the response would be the same if the drowsy meteorologist were female.
This cautious attitude stems from a knowledge that the Earth's climate changes over the years without any help from CO2 or human beings. On the grand scale, planet Earth is subject to the whims of a climatic pendulum, so that not once, but several times during every million years, great glaciers move southwards across the continents in a colder world, only to retreat pole-wards again as the global temperature increases. On a shorter time scale, the benign Mediaeval Optimum a thousand years or so ago was followed by the Little Ice Age from whose rigours we have only recently, in climatic terms, recovered. What reasons are there for these changes in our global weather?
One cause of climatic change be variations in the energy output of the sun. Until comparatively recently the solar constant was assumed to be just that - constant. Now scientists are reasonably sure that it is not, and the amount of energy available to fuel the world's weather machine changes over anything from decades to millennia.
But even with the sun's output constant, the amount of its energy absorbed by Earth may vary. The path of our planet's orbit is known to change slightly over long periods; this affects our distance from the sun at any given time of year, and hence the amount of energy received. Secondly, the inclination of the Earth's axis changes slightly over the centuries, and this alters the proportion of the sun's energy which is absorbed - as opposed to being reflected back to space.
And another phenomenon known to affect the global climate is the amount of volcanic activity at any given time. More volcanoes mean more dust in the atmosphere, and this interferes with the transparency of the air to radiant energy from the sun - and thus affects our weather.
Scientists have devoted much effort to assessing these processes - and others - to establish which are the most important for climatic change. Ultimately, of course, the best way to test any scientific hypothesis is to initiate a possible cause and then observe the effect. The human race, albeit unintentionally, is currently conducting precisely this experiment.