Signatories of the UN Framework Convention on Climate Change have committed themselves to do something about global warming, and the purpose of their current meeting in Japan is to decide exactly what. In this context, let's examine what we know, and what we do not, about this controversial subject. Theories about climate change can be conveniently divided into three broad categories. There are the facts - or at least those findings we believe to be the facts; there are suspicions about what may be happening, but for which any evidence is purely circumstantial; and there are the Armageddon theories - bizarre, unthinkable scenarios which, while not impossible, have little real support in mainstream meteorology.
We know, for example, that the average temperature of the world has increased by about half a degree Celsius since 1860, and that the rate of warming has increased significantly in recent years. There is also now a consensus in the scientific community that this increase in global temperature is more than might be expected from natural and normal variations, and that, as the experts put it, "the balance of evidence suggests a discernible human influence on global climate".
And we also know that the best climate models available today predict that unless action is taken to restrict greenhouse gas emissions, the average temperature of the world is likely to increase by a further 2 Celsius or so in the next 100 years, with sea level rising by a foot or two.
This is what we know. What is feared by some scientists is that many of the unusually severe weather events of recent years - the increase in the frequency and ferocity of hurricanes, the unusual droughts and floods in many places, and the recent premature resurgence of a very strong El Nino - may have been triggered by the observed increase in global temperature. There is no firm evidence that they have, and for every such event a previous occurrence of similar severity can be identified, but there is a suspicion that they may be happening, just now, because of global warming.
And finally, there are doomsday theories. These are the ones that headline writers love - those that predict an interruption or diversion of the Gulf Stream, bringing Baltic winters to the coasts of western Europe, or those that would have a melting polar ice shelf inundate us all as the levels of the oceans rise inexorably. Such wild scenarios are long shots, generally regarded as unlikely, and promoted seriously only by a small minority - but, in general, scientifically sound to the extent that they may not be impossible.