Is human activity increasing the carbon dioxide content of the atmosphere? Does an increased carbon dioxide content trap heat in the atmosphere?
A powerful prevailing opinion answers 'yes' to both these questions, and demands huge expenditure to reduce our carbon dioxide production, stressing urgency if global climate catastrophe is to be avoided.
This is the theory of Anthropogenic Global Warming - AGW.
In Britain, Europe, Canada and (increasingly) America politicians are convinced by the prevailing opinion, so AGW is the issue dominating political, economic and social policies.
Contrary opinions are heard, but so far have failed to raise significant doubt in the minds of our rulers. Indeed doubters are often dismissed as ignorant, cranks, or worse, and dubbed 'deniers'.
A sound theory is established by analysis of data, and critics rebutted similarly. Attacks on the probity or sanity of critics must always arouse suspicion that their criticism cannot be answered by argument based on evidence.
So what evidence is there to support the theory of AGW?
There is useful summary of the various data sets available here. [The slider above the diagrams permits the time span to be adjusted.] The following facts are difficult to challenge.
1. Global temperatures increased steadily from 1960 to 2000. After 2000 temperatures stabilised.
Between 1880 and 1940 temperatures were reduced, with a minimum around 1912.
The total temperature increase between 1910 and 2000 was approximately 0.7 degrees. The increase during those 90 years was not regular: between 1940 and 1970 the net change was about zero.
2. Atmospheric carbon dioxide increased from 320 to 390 parts per million during the 50 year period 1959 to 2009. The upward trend shows some acceleration during this time.
The increase is about 30%, but is only 70 parts per million in the atmosphere. The contribution to atmospheric heat content is negligible, especially set beside the contribution of water, also a 'greenhouse gas', and much more abundant in the atmosphere.
I am sure that a statistical analysis would show an excellent correlation between global temperature and atmospheric carbon dioxide concentration, but correlation is no proof of causation - and the naive belief that it is can cause serious errors. Thus in the twentieth century I would expect good correlation between global temperature and the number of muslims in Europe, and a good inverse correlation between atmospheric carbon dioxide and the number of whales in the oceans, but it would be absurd to suggest such correlations prove cause and effect.
The observed correlation of temperature and carbon dioxide is consistent with at least 4 hypotheses, as follows.
1. Increasing atmospheric carbon dioxide increases global temperatures.
This hypothesis is basic to any attribution of global warming to human activity.
2. Increasing global temperatures increase atmospheric carbon dioxide.
This is the inverse of 1.
Warming oceans release carbon dioxide. Studies of polar ice cores give some support to this hypothesis. Over thousands of years warm periods show higher atmospheric carbon dioxide values than cool periods. There is also a hint that temperature changes lead carbon dioxide changes.
3. Global temperature and atmospheric carbon dioxide are not linked directly, but each is affected by some third factor.
4. The correlation is a coincidence.
This seems unlikely.
The data do not permit a decision between these alternatives. Clever computer manipulations may add detail, but there is insufficient information to allow a firm conclusion.
Similarly computer modelling is prone to errors, including programmers' bias, ignorance of the relative importance of the many factors affecting climate, lack of data, and the critical impact of unpredictable items.
Correct predictions improve confidence in computer models. Prediction performance so far is, well, unimpressive.
Our climate has never been constant. Climate change is a fact of our environment. Human activity may now be sufficient to influence climate, but this remains a hypothesis. Doubt and scepticism are rational responses at our present level of knowledge and understanding.
I strongly support policies and engineering to reduce human pollution of the atmosphere, not least because such measures will demand more sanity in our economics. But I am not convinced that the AGW theory is correct. Certainly the case for huge expenditures and dramatic forced changes in human society cannot be justified on present evidence.
Our climate is driven by the sun. Solar energy output is not constant. Solar radiation and magnetic fields vary with the sunspot cycle. Solar maxima became exceptionally large during the second half of the twentieth century, in the period when data showed global warming.
We know from past data that periods of absent solar maxima are associated with global cooling. The current solar cycle, number 24, is currently running at least 2 years late, and solar activity continues to be well below the predicted values.
The number of days free of sunspots was much greater during the recent solar minimum than it was in the previous one.
We may be entering a period of reduced solar activity, after the intense activity in the past century. We may be at the beginning of a 'Maunder Gap' - another period free of sunspots. If that proves to be the case the problem will be global cooling, certainly sufficient to affect human life, maybe seriously.
But on this too, there is insufficient evidence to be sure. But I note that weather forecasting which includes solar data has proved more successful than the conventional techniques of our Meteorological Office.