95% certainty or 95% opinion?

On September 27, 2013 the IPCC declared that the “human influence on the climate system is clear” in the AR5 Summary for Policymakers. In the current report they declared more than 95% certainty (extremely likely) that greenhouse gases account for more than half of the increased global average surface temperatures, while they declared only 90% (very likely) for something similar in the previous report.

For us, the great unwashed, this seems really impressive. We assume it is a accurate picture of reality and the science made quite some progress in the last six years and hey, 95% certainty isn’t that more than enough for action?

It made me wonder what this “certainty” exactly means and how it is determined. This is what the AR Summary for Policymakers says about it (page 2):

The degree of certainty in key findings in this assessment is based on the author teams’ evaluations of underlying scientific understanding and is expressed as a qualitative level of confidence (from very low to very high) and, when possible, probabilistically with a quantified likelihood (from exceptionally unlikely to virtually certain).

That seems more like the subjective interpretation of a certain group of scientists (that probably are selected for having these opinions in the first place).

But look at reality. The “pause” was not foreseen and unexplained. We were told many times (also in this report) that human produced CO2 drives temperatures. If this really is the case then a more than a decade long standstill in temperature increase, together with a record amount of CO2 in the atmosphere, must tell us something: there is at least one forcing or feedback with at least the (combined) strength of CO2 that is not accounted for.

That alone should have humbled the scientists and adjust their certainty DOWNwards in stead of upwards. Self declaring more certainty in the face of a glaring gap in their understanding of the climate system doesn’t make much sense.

95% certainty or 90% certainty, what does it even matter? It is both as impossible to evaluate anyway.


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