How large is the effect? Once again, I agree with Stephen that this is difficult to figure out. To estimate it, we need to first decide how we estimate much effect the individual volcanoes had. We could blank out–as Stephen did. We could do a multivariate analysis using the Dust Veil index (one measure of obscuration). We could do one using the optical depth (a second measure of the obscuration.) We could try to find information for time varying distributions of aerosols over the globe. (Like I believe those would be precise. Heh!)

Then, we do math or run model to compare trends with and without the volcanoes. So, in the end, the magnitude of the effect depends on how we estimate their impact.

But I think everyone agrees they did have an effect. My gripe with EW in this paragraph is that, when connected to what they claim to show, it obfuscates. They claim to be engaging discussions at blogs and forums. Climate blog-war addicts are aware that interpretations of recent short term cooling by “most” at blogs and forums all take into account the fact that the recent periods is not trendless or cooling due to the eruption of any volcano. Later in the paper, E&W make much of the “no volcanoes” issue with ECHAMP and model projections into the 21st century.

So, the organization of that paper is structured to cause less knowledgeable readers to be unaware that the example trend in the empirical record are due to volcanic eruptions and so irrelevant to the full argument. Whether E&W did this intentionally or unintentionally, it is unfortunate. Because clearly they will not achieve their apparent goal of convincing anyone in the blogosphere that there is any decent evidence that 20 year negative trends could be embedded in much longer (say 100 year) periods of brisk warming.

I know I am not at all convinced of E&W’s argument. It is very weak on the empirical front and equally weak on the interpretation based on models.

In relation to attempting to adjust the data as if the volcanoes hadn’t happened Steve Goddard attempted this on WUWT (Parts I & II)

http://wattsupwiththat.com/200.....mperature/

http://wattsupwiththat.com/200.....ds-part-2/

I agree entirely!

-Chip

There is a similar situation with the 1945-1975 cooling. The experts claim it was due to man-made aerosols

Do they explain the drop around the end of the century (19th) the same way? If they do what happened in between?

EW don’t state the cause of cooling during the 80s and 90s. They just mention it happened while advancing the case that 20 year periods with zero or cooling trends not caused by volcanic eruptions could be embedded during periods with brisk warming.

Readers who are not aware of the issue associated with volcanoes are likely to infer that the cooling during the 80s and 90s happened during periods unaffected by volcanic eruptions. Otherwise, the reader would need to stop and say “huh?” After all, unless writer (i.e. EW) were to advance a much more nuanced or sophisticated argument about correcting for volcanic aerosols etc. they have just introduced an irrelevant fact into their argument to suggest that absent volcanic eruptions 20 year long periods of no-warming can be embedded on longer term periods of warming.

I recognize that those who believe 20 year volcano-free periods without warming can be embedded in longer periods with brisk warming may be frustrated by lack of data. Maybe they believe believe empirical evidence for such things would exist if only a) the earth had experienced 100 year long periods of brisk warming, b) volcanoes conveniently did not erupt and c) we were there to measure such things. The fact is, we have no direct empirical evidence of any such thing.

There is a similar situation with the 1945-1975 cooling. The experts claim it was due to man-made aerosols, but that claim obscures whether there were other causes of cooling. They’ve adjusted their models to ascribe that 30 years of cooling as being due to aerosols, and related research also doesn’t detect other cooling effects.

The result is that if we correct empirical data for what we think is the plausible effect of volcanic activity, we are no longer looking at purely empirical data. Such comparisons are useful– but one must make it clear that we are actually making that comparison.

So, with regard to EW, we have the difficulty that they pose their discussion as showing these sorts of negative trends can happen even without volcanic intervention because they actually did happen. But, at best one might plausibly argue that they might have happened even if the volcano not erupted. The first argument would be very, very convincing evidence that such trends can occur and have occurred.

The second one is speculative, requires us to agree to the precise level of correction for the volcanic eruption and suggests we cannot exclude the possibility that they might occur. Would I say I can prove it’s impossible for such trends to occur? I can’t and I don’t. That would be overstating just as much as EW overstated.

We can estimate the probability of such occurances a variety of ways. But EW don’t do that. They substitute an argument that appears simpler, but which is wrong.

I am not so sure I agree with your answer to your qeustion:

With regard to the introduction in EW, the blogosphere is likely to respond with “can you give examples of negative trends not caused by volcanic eruptions that are also embedded during periods of brisk warming”?

The answer to that question is: “Ehrmm… no”.

While you are correct that it is difficult to find a negative trend not associated with a volcanic eruption in recent decades, the influence of volcanic eruptions occurs at random places in the dataset and perhaps masks other forms of natural variability. So, how about this question: Had the volcanic eruptions not occurred could you find negative trends embedded during periods of brisk warming?

While it may be impossible to answer this question (after all the volcanoes did occur), I think there is some evidence (based on observed ENSO variability) that the answer to that question isn’t such a slam dunk “no.”

-Chip

globalwarmingquestions.googlepages.com/trendvolc.jpg

So could the very high variability of this model data not be use to show that EW exaggerates the probability of 10 year (least square linear fit) trends after 2000?

BTW – I was really shocked that such a silly paper was not shot down by reviewers.

How goes the tip jar?

The other difficult issue as you approach the ’40s ’50s is possible sudden changes in bias in sea surface temperatures measurements associated with changing from measuring with buckets to jet inlets, back again, back again etc.

http://img509.imageshack.us/im.....pactst.png

The volcanic forcing estimates are really just made up by Hansen and his top pal, Dr. Makiko Sato. The satellite optical depth and Short Wave radiation data shows that the impact of Pinatubo was over by the end of 1993 – only 18 months – versus the 9 years they have built into the models.