There have been numerous comments and inquiries about this statement of Myles Allen and myself: in our nature-online piece (no longer freely available): "The e-mails do not prove, or even suggest, that the main product of CRU, namely the record of global surface air temperature based on thermometer readings, has been compromised. Indeed, the thermometer-based temperature record has been verified by results from other groups." I want no to take the opportunity to explain my arguments. These are my arguments.
However, in the public, and also on this blog, such doubts are now raised. I do not think that they are warranted – even if some questions on technical issues (related to the homogeneity of sub-data sets and their corrections) may need some additional analysis (a process normal in science). But given, these doubts, a re-analysis by an independent group is in any case required – to further demonstrate the validity of this product.
Personally, I am convinced, insofar as is possible in an empirical science, that anthropogenic climate change is taking place and will emerge more strongly in the future. For explanation, a few comments are needed:
1) The assessment that elevated greenhouse gas concentration contributes to most of the recent warming since, say 1970, is made up of two steps, a "detection" step and a "attribution" step". Both steps operate under some assumptions – and the assessment to what extend these assumptions are valid or not, is to some extent subjective.
2) The detection step reveals that the warming trend extending across the recent few decades is more rapid than warming or cooling trends what would be expected from internal variability alone (from phenomena such as El Nino, the Pacific Decadal Oscillation and so on). The statement is not that the present level of warmth is unprecedented, even though it may very well be, but that the speed of warming is remarkable. The description of the warming in recent decades ("the signal") is based on thermometer data, including the CRU data. Even if this data may not be perfect, the description of the recent warming is robust. The "detection" is based on a rigorous statistical analysis, but depends on our understanding about the natural variability. The latter, the level of natural variability, is estimated from the thermometer-based temperature record, and from long climate model simulations.
That the data base is really good enough for estimating the range of internal variability can not rigorously be demonstrated. However, given the quality of our climate models in reproducing various aspects of the global climate and its change, and the consistency of model and thermometer-based large-scale temperature variability, I am confident that our present estimate of internal variability, derived from thermometer-data and long control runs, is about realistic. But, while I am unable to prove positively that my estimate is correct, any doubt will essentially be based on a general gut feeling. Only time will eventually help us to overcome this remaining, unavoidable uncertainty.
3) Attributing observed temperature variations to specific causes relies more on climate models, as they are needed to discriminate between the response of the climate system to different ’drivers‘, such as solar activity, greenhouse gases and volcanoes. It turns out that the best, and really the only, satisfactory explanation of the history of surface air temperature change particularly over the last few decades is obtained when the warming influence of anthropogenic greenhouse gases is taken into account. These gases are behind most of the recent decades' warming.
This attribution step is more uncertain that the detection step, because it relies on the skill of present day climate models to describe the large-scale response of the climate system to various external factors. Most climate scientists find the evidence that the models do a reasonable job sufficient, but chances remain for future revisions. In principle it may even be that there are external factors, we do not know of.
In Summary: Most climate scientists are convinced that a warming is going on, which can not be explained by internal dynamics (detection). The best explanation for this is –given knowledge gathered and re-examined during many years of research – the effect of elevated greenhouse gas concentrations (attribution). In case of detection, the uncertainty is mostly of statistical nature, while in case of attribution it is also of epistemic nature.
... Now, when I say, "most scientists", we should ask Dennis Bray for a quantification.