Climate variability

RealClimate has a guest column on climate variability. This is an important concession for them since they have spent years and considerable effort trying to prove that CO2 is the only driver for climate change. Al Gore, who cites them as a major source of his information is on record as saying that the only reason that Venus is hotter than the Earth is because of CO2. This shows how badly the cooling of the last few years has hurt their case.

On its surface, the column continues to defend global warming theory. It starts out with this statement:

A climate that is highly sensitive to radiative forcing (i.e., responds very strongly to increasing greenhouse gas forcing) by definition will be unable to quickly dissipate global mean temperature anomalies arising from either purely natural dynamical processes or stochastic radiative forcing, and hence will have significant internal variability. The opposite also holds. It's painfully easy to paint oneself logically into a corner by arguing that either (i) vigorous natural variability caused 20th century climate change, but the climate is insensitive to radiative forcing by greenhouse gases; or (ii) the climate is very sensitive to greenhouse gases, but we still are able to attribute details of inter-decadal wiggles in the global mean temperature to a specific forcing cause. Of course, both could be wrong if the climate is not behaving as a linear forced (stochastic + GHG) system.

That last disclaimer is interesting but the thrust of this paragraph argues that variable climate proves warming theory because it shows that the climate can be changed. As I said, it represents a major policy shift for RealClimate to allow anyone to suggest that the climate can change without human forcing. I will get to the implications of this later.

The rest of the paper deals with the period from 1998 to the present which they refer to as an episode. Their point is that something natural happened to the climate in 1998 skewing the averages. Because of this, only data prior to 1998 should be used in charting global warming.

We hypothesize that the established pre-1998 trend is the true forced warming signal, and that the climate system effectively overshot this signal in response to the 1997/98 El Niño. This overshoot is in the process of radiatively dissipating, and the climate will return to its earlier defined, greenhouse gas-forced warming signal. If this hypothesis is correct, the era of consistent record-breaking global mean temperatures will not resume until roughly 2020.

While this sounds quite reasonable there are two problems with it. The obvious one is that you can't just throw out data that doesn't fit your model and claim that the remaining data is correct. There is a prediction that we will see a return of the warming signal around 2020. This is valid scientific method but we have to wait another decade in order to validate it.

The other problem is the assumption that, with the exception of the 1998 event, world temperatures would have been naturally stable without human-induced warming. Now that it has been admitted that the climate can change without human intervention, it has to be admitted that other, natural, long-term trends can happen. RealClimate tried to keep this line of inquiry closed. That's why allowing any admission of it is such a policy change for them.