Watts Up With That (WUWT) reproduces a post from the Hockey Schtick that discusses a recent paper in the Journal of Climate. The paper is called The Surface Climate Response to 11-Yr Solar Forcing During Northern Winter: Observational Analyses and Comparisons With GCM Simulations. The implication in the WUWT post is that this is more evidence that global warming/climate change (if it is indeed happening) is driven by the Sun.

So let’s see if that interpretation is consistent with what this paper actually suggests. The paper says,

Specifically, the temporal behaviour of a given surface cimate variable *X*(*t*) is represented as

Equation from Hood et al. (2013).

Unless my maths has deteriorated in the last few years, this equations has many terms – one for Solar, one for ENSO, one for Volcanic aerosols, one for the Change-of-Trend and one for the Trend. It also has a term

*μ*(

*i*) that is the long-term mean for either the

*i*th month or season. So what are these trend terms. The paper goes on to say

These observed long-term trends are likely influenced also by non-linear greenhouse gas forcing and so are not adequately represented by a simple linear trend term plus a volcanic term.

So, not only is there a greenhouse forcing term in their model, it is changing sufficiently with time that it can’t be assumed to be linear and so an additional change-of-trend term is needed.

So, they determine the coefficients of each term by linear-regression, making these coefficients independent of time. They then have various time series representing the various time-dependent terms in the above equation. These are all shown in the Figure below. So, according to this figure the Solar, ENSO, volcanic aerosol time series are oscillatory but with a long-term mean that is close to constant. These can’t drive any long-term changes. The only terms that can drive a long term change are the *Trend* term and the *Change-of-Trend* term which are straight lines because they are *t* and *t’*. I presume they could have had a single term that produce a non-linear dependence on *t* to represent the greenhouse forcing and how it varies non-linearly with time. Instead, they chosen to introduce a second time that produces some kind of offset at *t* increases.

Figure 1 from Hood et al. (2013)

So, as far as I can tell, this paper is saying that the climate is influenced by the 11-year Solar-cycle but that the only way to explain the long-term trend in whatever surface variable they’re considering, you also need to include a greenhouse gas forcing term that has a non-linear dependence on time. So, it’s not just the Sun then.

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The seems to be a link to a PDF copy of the paper (without figures) here.