Replies

This one is interesting:

http://www.anelegantchaos.org/cru/emails.php?eid=710
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At 09:01 18/07/2006, Fortunat Joos wrote:

Hi Tim and co,

Thanks for the figure. I like the figure showing the model results and the general outline/graphic style.

However, I am concerned about what is shown in the forcing figure.

1) Volcanic panel: I strongly believe that we should show what was used by the model and not some 40 year smoothed curves for volcanic forcing or any other forcing. So please use the original data file. Scientific honesty demands to show what was used and not something post-processed.

2) solar panel:

2a) We must show the Wang-Lean-Shirley data on the original resolution as used to drive the models. In this way, we also illustrate the magnitude of the 11-yr annual cycle in comparison with the background trend. The record being flat, apart from the 11-yr cycle, during the last decades is a reality.

2b) Do not apply any smooting to the Bard data. Just use them as they are and how they were published by Bard and used in the model.

2c) It is fine to supress the Bard 0.08 case after 1610 (not done in my figure version) 2d) the emphasis of the figure is on the solar forcing differences. So, please show solar somewhat overproportional in comparison to volcanic and other forcings.

3) other forcings: again no smoothing needed here. It would be hard to defend a double smoothing.

4)- normalisation of solar forcing to some period mean. If the different solar forcings disagree for today as in your option, we may send the signal that we do not even know solar forcing today. Thus, I slightly prefer to have the same mean forcing values for all solar records during the last few decades as shown in the attached version. However, I also can see some arguments for other normalisations.

To illustrate points 1 to 4, I have prepared and attached a version of the forcing panel.

other points

- Your choice of colors is fine

- time range 1000-2000 AD is fine - suggest to remove the text from the y-labels except the units W/m2.

Sorry for this additional comments coming a bit late. However, I did not realise that you planned to smoothed the model input data in any way.

With best wishes,

Fortunat
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Tim Osborn wrote:

Hi all,

thanks for the responses, Peck and Fortunat.I drafted the new figure 6.14 following as closely as possible the approach used for the original forcing/simulation figure (now 6.13).

This is why I smoothed all series and used a common anomalisation period for all curves across all panels. It can greatly help to interpret why the simulated temperature responds in the way it does, because the zero (or “normal” level) is comparable across plots and because the strengths of different forcings can be compared *on the same timescale* as the simulated temperatures are shown. And, for 6.13, with so many different forcings and models shown, it would have been impossible to use unsmoothed series without making the individual curves indistinguishable (or indeed fitting them into such a compact figure).

Now that the EMIC panels are separate from the original 6.13, we do have the opportunity to make different presentational choices. But I think, nevertheless, that some of the reasons for (i) proportional scaling, (ii) common anomalisation period; and (iii) smoothing to achieve presentation on comparable time scales, that held for 6.13 probably also hold in 6.14.

However, I also appreciate the points raised by Fortunat, specifically that (i) it is nice to be able to compare the magnitude of the 11-yr solar cycles with the magnitude of the low-frequency solar variations; and (ii) that using a modern reference period removes the interpretation that we don’t even know the forcing today.

So we have various advantages and disadvantages of different presentational choices, and no set of choices will satisfy all these competing demands. One thing that I am particularly perturbed about is Fortunat’s implication that to show smoothed forcings would be scientifically dishonest. I disagree (and I was also upset by your choice of wording). If it were dishonest to show smoothed data, then presumably the same holds for 6.13 (but its impossible to distinguish all the different volcanic forcings if shown unsmoothed), but also to every other graphic... should I be showing the EMIC simulated temperatures without smoothing too, so you can see the individual yearly responses to the volcanic spikes? But annual means are formed from the temperatures simulated on the model timesteps, so we still wouldn’t be showing results that had not been post-processed.

Most climate models, even GCMs, respond in a quasi-linear way, such that the smoothed response to unsmooth forcing is very similar to the response to smooth forcing. So if we are interested in the temperature response on time scales of 30 years and longer, it seems entirely appropriate (and better for interpretation/comparison of forcings) to show the forcings on this time scale too, because the forcing variations on those time scales are the ones that are driving the temperature response (even though the forcing may be intermittent like volcanoes or have 11-yr cycles like solar). The choice of smoothing / no smoothing is not, therefore, anything to do with honesty/dishonesty, but is purely a presentational choice that can made accordingly to what the purpose of the figure is. Here our purpose seems to be long-term climate changes, rather than response to individual volcanoes or to the 11-yr solar cycle.

So the position is:

(1) smoothing or no smoothing: there are arguments for both choices, though clearly I prefer smoothing and Fortunat prefers no smoothing. I could make a figure which kept the smooth lines but put the raw annual histogram volcanic spikes underneath in pale grey, as Peck requested anyway (and possibly put the 11-yr solar cycles in pale brown underneath the smoothed brown solar series). This would be a compromise but the main problem is that the scale of the largest volcanic spikes would far exceed the scale I am using to show the smoothed series (so the panel is not large enough to do this)!

(2) pre-industrial or present-day anomalisation reference period: again there are arguments for both choices. Whatever we choose, I firmly believe it should be the same for *all* curves in this figure (which can make a dramatic difference).

(3) exaggeration of solar scale or proportional vertical scales: this is the one that I have the firmest opinion about. I see no reason to exaggerate the scale of the solar forcings relative to volcanic or anthropogenic forcings. The difference between the forcings looks clear enough in the version of the figure that I made. Exaggerating it will wrongly make the Bard 2.5% case look (at first glance) bigger than the anthropogenic forcing, and make it look more important than volcanic forcing.

I’ll hold off from making any more versions till decisions are made on these issues.

Cheers
Tim
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At 13:45 18/07/2006,

Fortunat Joos wrote:

Dear Tim,

Sorry, that was a very careless and a totally inappropriate choice of words. I seriously apologize.

Of course smoothing is not dishonest (I do it also all the time). To the contrary, I very much apreciate all your hard work to do these figures. I know that it is very time consuming from own experience ... (that is perhaps why I did not reflect on my wording when writing the e-mail).

What I wanted to say is that if one has the opportunity to show directly what forcing was used by the model than I very much prefer to do so.

I hope there remains no misunderstanding. I realize now that I should have used more modest wording at various places.

Let us see what Eystein, Peck and Keith are thinking about it.

With best wishes,
Fortunat

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Keith Briffa wrote:

Fortunat et al

My opinions were consistent with Tim’s expression - we discussed his response. The importance of consistency between different modelling Figures (time response of filters and in the absolute magnitude of forcing scale) are the most important aspects. To start showing apparently different volcanic spikes (in the sulphate and EMIC Figure ) will lead to confusion also.

Ultimately we should remember that the point of this Figure is to show that you can not get simulated temperatures to match observations without anthropogenic forcing - not to show proportional responses to different solar or volcanic events.

cheers
Keith