Watt about the Sun?

A recent Watts Up With That (WUWT) post presents results from a Russian study suggesting that we may be in for a a period of global cooling due to changes in the Sun. As far as I’m aware, this is essentially an empirical projection into the future suggesting that we’re heading for another Maunder Minimum which is thought to have caused a little ice age (LIA) in the late 1600s and early 1700s.

Something that immediately struck me is that if you think climate modelling is difficult, then modelling the solar dynamo should be regarded as virtually impossible. Understanding solar variability requires understanding convection, magnetic reconnection and how differential rotation influences both of these processes. The prediction could be correct, but it appears to be based on empirical evidence (looking at past trends to predict the future) rather than a model of the solar dynamo. There is a very good review paper, that discusses some of this, by Michael Lockwood who is Professor of Space Environment Physics at the University of Reading.

The paper is called Solar influences on global and regional climates. I’ve read through quite a bit of the paper and it seems quite good and reviews, very clearly, our current understanding of how the Sun influences our climate. I don’t want to go into too much detail, but I thought I would highlight a few key points. One point that is made is that our estimates of the change in the Total Solar Irradiance (TSI) since the Maunder Minimum till today has been steadily decreasing. It appears now that it is probably similar to the total variation across a single solar cycle (about 0.1%). If so, the net forcing due to this would be about 0.24 W m-2. This is an order of magnitude lower than the expected forcings due to greenhouse gases – since pre-industrial times – and hence, even if we are heading into another Maunder Minimum, is unlikely to have much of an effect.

There is also often a claim made by skeptics that man-made global warming has only really been happening since about 1950. This, I think, is a rather confused claim. There has been an increase in CO2 since about the mid-1800s. What this paper by Mike Lockwood explains quite nicely, is that if you want the increase in temperature since the mid-1800s to be due to greenhouse gases alone, it would require an amplification factor of about 2. The increase in temperature is driven by a forcing term (i.e., an excess energy per square metre per second). One can determine what the forcing would need to be to explain the observed warming (5.15 W m-2) and one can determine how much can by contributed by the increase in greenhouse gases (2.45 W m-2). The ratio tells you what amplification of this forcing is required. On the other hand, for solar forcing (0.24 W m-2) to contribute 60% of the required forcing would imply an unrealistically high amplification factor of 12.8 and would require that the greenhouse forcing is lower than actually expected. If I understand it correctly, more reasonable numbers are that solar forcing provides 10% and greenhouse gases provide 90%. What has happened since the mid- to late-1900s is that we’ve essentially disconnected from the solar variations and our temperature rise is being driven primarily by greenhouse gases.

I recommend, however, that you read the paper yourself. I believe the link above is to an open-access version of the paper. I thought I would, however, quote a bit of the final section

Lockwood (2010) and Barnard et al. (2011) have deduced that there is an 8% chance that the Sun will return to Maunder Minimum conditions within 50 years. The recent evolution of solar cycle 24 indicates that the Sun may well be following such a trajectory (Owens et al. 2011). Feulner and Rahmstorf (2010) and Jones et al. (2012) have used GCMs and EBMs to
predict that this will offset anthropogenically rising global temperatures by no more than about 0.2 C in the year 2100, relative to what would happen if the solar output remained constant. Similarly, Lean and Rind (2009) find that the solar decline would delay the arrival at a given temperature level by no more than about 5 years. Thus, these predictions show that continued solar decline will do little to alleviate anthropogenically driven global warming. However, the decline should do much to end the debate about the fraction of global warming that can be attributed to solar change. For the first time since about 1900, long-term solar and anthropogenic trends are now in opposite directions. Non-robust fits will fail sooner rather than later because of the change in solar behaviour. Thus, the next few years will give us much better estimates of the solar contribution to both global and regional climate change. For global temperature rise, there is every indication that these new estimates will, if anything, be smaller that previous estimates.

What this appears to be saying is that there is only a small chance (8%) that we’re heading into another Maunder Minimum. Furthermore, even if we are, the effect will small. It will add something like a 5-year delay to the temperature rises. What I found particularly interesting was the claim that anothropogenic and solar trends are now in opposite directions. This suggests that very soon it should become obvious that current trends are not being driven by solar variations and that it is likely that we will be able to make stronger claims about the solar contribution, which is likely to be smaller than previous estimates. Of course, whether or not skeptics accept this if/when it happens, is another issue.

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