Over the past half-century we have become used to planetary scares of one kind or another. But the latest such scare -- global warming -- has engaged the political and opinion-forming classes to a greater extent than anything since, a little over 200 years ago, Malthus warned that, unless radical measures were taken to limit population growth, the world would run up against the limits of subsistence, leading inevitably to war, pestilence and famine.
This is partly perhaps because, at least in the richer countries of the world, we have rightly become more concerned with environmental issues. But that is no excuse for abandoning reason. It is time to take a cool look at global warming.
It is frequently claimed, by those who wish to stifle discussion, that the science of global warming is "settled". Even if it were, that would not be the end of the matter. But in fact, while some of the science is settled, there is much that is not. So let's start with the facts. It is customary to focus on three of them.
The first is that, over the past hundred years, the earth has become slightly warmer. To be precise, there has been a rise in global mean annual temperature of about 0.7°C. The second is that, over the past hundred years, the amount of carbon dioxide in the Earth's atmosphere has risen sharply, by well over 30 per cent, largely as a result of carbon-based industrialisation, in particular, electricity generated in coal and oil-fired power stations and motorised transport.
And the third fact (and this is the settled science) is that carbon dioxide is one of a number of so-called greenhouse gases -- of which far and away the most important is water vapour, including water suspended in clouds -- that in effect trap some of the heat we receive from the sun and thus keep the planet warmer than it would otherwise be.
So is it not clear that the warming we have seen over the past hundred years must be due to the massive rise in man-made carbon dioxide emissions, and that unless we substantially decarbonise the world economy the warming will continue, bringing doom and disaster in its wake? No: it is not at all clear.
In the first place, while atmospheric carbon dioxide concentrations have grown steadily over the past hundred years, and indeed continue to grow briskly, the warming has occurred in fits and starts. To be precise, it has been confined entirely to two periods: from 1920 to 1940, and from 1975 to 1998. Between 1940 and 1975 there was a slight cooling, and so far this century (and contrary to all predictions) there has been no trend one way or the other.
So clearly carbon dioxide is only part of the global temperature story: it is very far from being the whole story. This is borne out by the longer term historical record.
It is well established, for example, that a thousand years ago, well before the onset of industrialisation, there was what has become known as the medieval warm period, when temperatures were probably at least as high as, if not higher, than they are today. Going back even further, during the Roman Empire, agricultural records suggest that it was probably even warmer.
So we are left with a double uncertainty. First, while we know that, other things being equal, rising atmospheric concentrations of carbon dioxide will warm the planet, we have no true understanding of how much they will do so. And second, we know that in fact other things are very far from equal. So even if we did know the answer to the first question, we would still be unable to predict what the world's temperature will be a hundred years from now. These uncertainties clearly have a profound bearing on the economics of global warming, and thus on the policies it is sensible to pursue.
For while we can do our best to make an estimate of the cost of substantially decarbonising the world economy, we have no idea of what benefit that will bring in terms of a lower mean global temperature than would otherwise be the case. Not that it is clear, even if we could predict the temperature of the planet a hundred years from now (which we can't), how much economic damage a given rise in temperature would do.
It was to advise governments on these issues that the Intergovernmental Panel on Climate Change was set up in 1988, under the auspices of the UN. The IPCC concludes, on the basis of to say the least very slender evidence, that "most" -- note, not all -- of the warming that occurred during the last quarter of the 20th century was very likely to be due to the growth of atmospheric carbon dioxide concentrations. But even if -- and there is clearly a case for erring on the side of caution -- this is so, and even if, as the IPCC blithely assumes, the natural forces that affect the world's temperature in often unpredictable ways can be safely ignored, the policy conclusions which are widely believed to follow from this are very suspect indeed.
Is it really plausible that there is an ideal average world temperature, which by some happy chance has recently been visited on us, from which small departures in either direction would spell disaster? Moreover, while a sudden change would indeed be disruptive, what is at issue here is the prospect of a very gradual change over a hundred years and more. In any case, average world temperature is simply a statistical artefact. The actual experienced temperature varies enormously in different parts of the globe and man, whose greatest quality is his adaptability, has successfully colonised most of it. Two countries that are generally considered to be economic success stories, are Finland and Singapore.
The average annual temperature in Helsinki is less than 5°C. That in Singapore is in excess of 27°C, a difference of more than 22°C. If man can successfully cope with that, it is not immediately apparent why he should not be able to adapt to a change of 3°C, when he is given a hundred years in which to do so.
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