In a recent post, Judith Curry highlighted a new paper by Shaun Lovejoy called Return periods of global climate fluctuations and the pause. This is a follow-up of an earlier paper that concluded that
the probability of a centennial scale giant fluctuation was estimated as ≤0.1%, a new result that allows a confident rejection of the natural variability hypothesis.
In other words, the rise in temperature over the last 100 years or so is almost certainly anthropogenic. This new paper also says
The hypothesis is that while the actual series Tnat(t) does depend on the forcing, its statistics do not. From the point of view of numerical modeling, this is plausible since the anthropogenic effects primarily change the boundary conditions not the type of internal dynamics and responses,
which is interesting given the discussion that prompted an earlier post. The basic result of this paper is essentially that the variability we’ve seen in the instrumental temperature record is simply a consequence of natural variability around a long-term anthropogenic trend. Additionally, the estimated climate sensitivities are entirely in line with IPCC estimates.
So, I should quite like this paper as it is not only a follow-up to a paper that ruled out that the rise in temperature over the last 100 years or so could be natural, but also illustrates that the variability in the instrumental temperature record is simply natural variability around a long-term anthropogenic trend; something I’ve been stressing on this blog. But I don’t really. This is a rather odd paper and – unless I’m missing something – am rather surprised that it got published.
Why? Well the fundamental equation is essentially the one below
This seems to be a rather unusual form of such an equation, as it suggests that the temperature rise is a linear function of the increasing forcing, plus some term representing natural variability. So, if the forcing stops rising, the forced response stops instantly, rather than continuing to rise to equilibrium. The coefficient in front of the first term on the right-hand-side is, therefore, then really only some kind of effective transient response. The bigger issue, in my view, is that the forcing is CO2 only. The paper actually says
Two things should be noted: first, Tnat includes any temperature variation that is not anthropogenic in origin, i.e., it includes both “internal” variability and responses to any natural (including solar and volcanic) forcings.
So, as stated, the natural variability being investigated in this paper is both external forcings and internal variability. What’s more, this isn’t even quite right because if the only forcing included is CO2, then the natural variability term also includes other anthropogenic influences (aerosols, black carbon, land use). Therefore, not only is the natural variability in this paper not what Judith would regard as natural variability (i.e., it normally refers to unforced natural influences), it’s not even natural in any reasonable interpretation of the term as includes anthropogenic influences.
So, what this paper seems to have done is determine the variability associated with non-CO2 anthropogenic influences and both forced and unforced natural influences. Given that some of these influences are stochastic, some have cycles (solar), and some are monotonically increasing or decreasing (aerosols, land use, black carbon) how can any kind of pattern really make any sense. If there is a pattern, it surely has be purely coincidental. Furthermore, what’s of real interest is the magnitude of the influence of internal variability, which this paper appears entirely unable to determine (and is what I think some have thought it is doing).
What it really should be doing, I think, is using a standard one-dimensional model
where is the heat content of the system, is the change in external forcing (natural and anthropogenic), is the climate sensitivity term, and could be some term representing internal variability. If this was done (and it may already have been) I think one would find that the unforced variability is much smaller than indicated in this paper.
So, I think Judith likes the paper because it suggests that natural variability could be quite large and because it suggests “pauses” could return every 20 years or so. Given that “natural” in this paper doesn’t really mean natural and that any kind of pattern is presumably entirely coincidental, it’s a rather unconvincing result. The paper finishes with
To be fully convincing, GCM-free approaches are needed: we must quantify the natural variability and reject the hypothesis that the warming is no more than a giant century scale fluctuation.
I don’t really agree with this. There’s only so much one can do with simple models. They’re very useful, but the idea that we can completely characterise the anthropogenic and natural influences using simple models, seems a little unrealistic. In my view, the role of simple models is to provide a way of checking that the results of more complex models make sense. Of course, those who don’t like GCMs, appear to like this conclusion.