I am sorry to plunge straight into what is complicated science, but the first thing I want to learn from the report on global warming by Sir Nicholas Stern, due to be published today, is what the objective should be in terms of the level of concentrations of carbon in the atmosphere. Like many people I cannot think straight without some numbers in my head. Carbon dioxide in the atmosphere is usually expressed as parts per million by volume (ppm). Today atmospheric carbon dioxide stands at 380 ppm. It is growing all the time. How high can we safely let it be?
According to pre-publication leaks, Sir Nicholas argues that a concentration of 550 ppm by the middle of the century would be "potentially dangerous territory". And on unchanged policies it seems likely that CO2 gas levels will rise well beyond that level. I hope that the report itself is more precise. None of us wants to be in dangerous territory. Mark Lynas, a campaigner who writes well about this subject, states that stabilising at 400 ppm yields a three-to-one chance that global temperature increases would level off on this side of 2C. What is a sensible numerical target?
It is hard, too, without an authoritative guide such as I hope Sir Nicholas will be, to make sense of various estimates of what is required to avoid the danger zone. On Mr Lynas's calculations, only 80 billion tonnes more carbon can be emitted by humanity over the next few decades. He thinks that this means that we must reduce our emissions by 90 per cent. On the other hand, a group of scientists were quoted last week as arguing that by 2050 global emissions of greenhouse gases need to be cut by about 40 per cent of what they were in 1990. And Friends of the Earth say that there should be legally binding targets for reductions of about 3 per cent a year. What we need is an agreed method of measuring the problem and of counting progress, or the lack of it.
I should also like to understand whether we Britons really waste more energy than anyone else in Europe. It is important to know this when it comes to framing government policy. At all events, this was the finding of a poll of 5,000 Europeans who were asked questions about their energy saving habits. Are appliances left on standby? Yes, in my case; I shall have to stop. Is more water boiled in a kettle than is needed for the pot of tea or whatever? No, I am pretty accurate.
Once measured, we can then confront the most difficult aspect of the problem. At what level must counteracting policies be agreed? Clearly not by a single nation acting alone. However virtuous Britain becomes, we would not make a significant difference to the concentration of carbon in the atmosphere. We should have done our best and obtained no reward.
This is one of the relatively rare problems that cannot be solved at the level of the nation state if its citizens so wish. Compare it with a global phenomenon such as avian flu. We could protect ourselves from an epidemic solely by our own efforts if we were prepared to spend substantial sums of money - though co-ordinated international action would be much better. With global warming, the possibility of going it alone simply doesn't exist.
At the moment some nations do their best and hope their good example will be imitated - even parts of nations do, such as California, which has imposed a mandatory cap on greenhouse gas emissions. The European Union, too, has been energetic in tackling the problem. In January 2005, Europe created the biggest market in the world for trading greenhouse gas emissions. Fine, but the US government won't engage, and big developing economies such as China, India and Brazil don't see that they should be hobbled. Indeed it is hard to see that a solution will be found without a global treaty that lays down targets and time scales and an enforcement mechanism.
Such a thing on such a scale has never been done before. Yes, there have been conferences such as the Treaty of Versailles in 1919 which did difficult things like redrawing national boundaries in the Middle East and shifting a few European frontiers. Yes, the United Nations' charter was negotiated in 1945 by 50 nations with 142 more countries subsequently signing up. But then it asked nothing so problematic as stopping global warming in its tracks. Commenting on this may be beyond Sir Nicholas's remit, but I hope his report provides a blueprint for global negotiations.
In discussing action against global warming, people shy away from stating that rationing will be necessary, whether implicit (by increasing taxation on wasteful uses of energy) or explicit (by setting physical limits like food rationing in the Second World War). Some people want to forbid the mention of rationing and talk instead of allowances or allocations of emission rights. They say that our discussion should be conducted in terms of carbon trading and carbon credits rather than using the dread word. Please, it is rationing that is required, and let us not deceive our fellow citizens.
Rationing takes us to the heart of the problem. I shall not believe that we can avoid the dire consequences of unchecked global warming unless I hear that the nations of the world have agreed how to share the burden. Go back to the 80 billion tonnes more carbon that Mr Lynas believes can be safely emitted by humanity over the next few decades. I am waiting for a treaty that divides up this 80 billion tonnes (or whatever is the right figure) between nations - China can have this much, the US will have to make do with this, the British ration is so many tonnes and so on.
At the same time there would have to be a mechanism for enforcing the rationing system - just as we had food inspectors during the war who were sent out to check that shopkeepers were not selling goods without the relevant coupons being given.
How far will Sir Nicholas's report go? Will he warn us that a global rationing system would be extremely difficult for North America, western Europe, Japan and other industrialised nations, albeit necessary. For it is very unlikely that China, India and Brazil and the rest of the developing world would agree that the basis for a rationing scheme would be existing levels of national wealth.
They would say to us - you have created much of the problem by unchecked consumption of coal and petroleum for 200 years, so you must clean up your share of the mess, which is most of it. Admittedly, until recently, industrialised nations didn't understand the consequences of their actions, but tant pis.
I don't myself see how we shall easily be able to resist the formula that rationing will be in proportion to population and that Americans who put 20 tonnes of carbon emission per head per annum into the atmosphere and Britons who are responsible for ten tonnes each per year will have exactly the same ration as Indian citizens, who emit only one tonne. I think it will come to that.