It is a fundamental principle of formal logic that a correlation, however convincing, is  not, of itself,  evidence of a cause and effect relationship.
It is surprising how frequently this principle is ignored. We are for ever being told that there is a "link" between one factor and another, followed by an assumption that this proves a relationship even in the absence of other evidence.
Of course, a correlation is a stimulus to further investigation, which may confirm or deny the value of the correlation.
My statistics textbook  ("Modern Elementary Statistics" by John A Freund, 1967) Gives two examples.
It was found that there was a correlation between the number of storks in Holland and the number of babies delivered.  Does this prove that the storks deliver the babies? Further investigation would show that there are no observations supporting the hypothesis.
The other example given was the correlation between the salaries of schoolteachers and the consumption of alcohol. Does this prove that schoolteachers are the main drinkers? Further study would show that alcohol consumption correlated with general salary increases.
The procedure for checking the reliability of a correlation is called Validation.
Validation might include some or all of the following
The Intergovernmental Panel on Climate Change has featured the use of compute-generated climate models and has argued that since they correlate, with climate observations, to a greater or lesser degree,  they are a correct representation of climate behaviour, and are an effective way to predict future climate.
They have, however, completely failed to validate any model.
The first IPCC Report (1990) had a Chapter entitled  "Validation of Climate Models"
The "Executive Summary" begins with  "The validation of the present day climate simulated by atmospheric general circulation models shows that there is considerable skill in the portrayal of the large scale distribution of the pressure, temperature wind and precipitation"  In other words, there is a correlation.
If you take a look, it doesn't seem so convincing, and in the "Policymakers Summary" they admit, for temperature,.that
"The size of this warming is broadly consistent with predictions of climate models". But since the models show a steady increase of temperature over time and the measurements do not, this conclusion is dubious.
The Chapter on "Validation of Models" nowhere defines what "validation" implies, and it carries out none of the "validation" exercises suggested above. The conclusions of the Report are based entirely on correlation, without any supporting information that could legitimately be called "validation"
The Second IPCC Report (1995) circulated its first draft for comment to the expert reviewers, including myself. It included Chapter 5 entitled "Validation of Climate Models". I made the comment that there was no discussion in the Chapter on what should be done to validate the models, and no actual measures taken to carry out such an exercise. I therefore considered that the term "validation" was not appropriate,
To my surprise, they accepted this criticism. Not only did they change the title of the Chapter to  "Climate Models - Evaluation", they altered the words "validation"  and "validate" to "evaluation" and "evaluate" no less than fifty times, throughout the Chapter. Since then, "validation"  has been a dirty word, never to be mentioned. This applies to the whole of "Climate Change 2001". although. It slipped through once or twice.
Chapter 5 of "Climate Change 2001", "Climate Models- Evaluation"  starts with a heading "What is Model Evaluation and Why is it Important?. They state
"we are trying to demonstrate the degree of correspondence between models and the real world they represent. We will therefore avoid the use of the word "validation", although this term is commonly used in the sense of evaluation in the climate modelling community"
In the "Conclusion" they say
"Confidence in climate models depends partly upon their ability to simulate the current climate and recent climate changes, and partly upon the realistic representation of the physical processes that are important in the climate ate system"
This makes it plain that they are not interested in finding out whether the models can predict climate changes as they occur. They are  concerned exclusively with correlation, and do not intend to validate any model.
The Chapter  contains no discussion of what measures need to be taken to provide a climate model that is sufficiently reliable to be used to forecast future climate. I have consistently argued that until a procedure has been agreed and applied the models should not be used for this purpose.
All there is, is correlation. Since the parameters in climate models are uncertain, and sometimes extremely uncertain, it is a relatively simple matter to adjust them so that the model fits almost any sequence of climate observations. We used to call this procedure "fudging", and it usually called for low marks. Even with the most desperate fudging the models cannot explain why the Northern Hemisphere warms up faster than the Southern Hemisphere, or why the poles do not heat more rapidly than the equator.
A crucial validation test of climate models would be a proven capability of predicting future climate. All  users of computer generated models except those connected with the IPCC automatically check whether their models are in line with what is actually happening, and adjust the model accordingly.The IPCC refuse even to consider this.
Computer models of the climate have now been going for a number of years. Comparisons between the predictions of models and actual climate observations, such as those published by myself (see my book "The Greenhouse Delusion") invariably show that the models exaggerate. But IPCC scientists are just not interested in correcting their models to  agree with actual climate trends.
The IPCC is about to launch its fourth Report. As before, they have a Chapter on " Climate Models and their Evaluation"  which includes no paragraph dealing with a genuine validation, and no mention of possible success in prediction of climate trends. 
Vincent Gray
75 Silverstream Road
Crofton Downs
Wellington 6004
New Zealand
Phone/Fax (064) 4 9735939
"It's not the things you don't know that fool you.
It's the things you do know that ain't so"
Josh Billings