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How large were the past changes in the sun?

Filed under: — rasmus @ 23 August 2011

We only have direct observations of total solar irradiance (TSI) since the beginning of the satellite era and substantial evidence for variations in the level of solar activity (from cosmogenic isotopes or sunspot records) in the past. Tying those factors together in order to estimate solar irradiance variations in the past is crucial for attributing past climate changes, particularly in the pre-industrial.

In the May issue of Astronomy & Astrophysics, Shapiro et al. present a new long-term reconstruction of the solar irradiance that implies much greater variation over the last 7000 years than any previously reconstruction. What is the basis for this difference?

Their results do not deviate much from previous work when it comes to the most recent period with modern instrumental observations, but Shapiro et al. obtain substantially lower radiance during the Maunder minimum than previous estimates.



Figure 1. TSI reconstructions from Schmidt et al (2011) with the new Shapiro et al (SEA) values all scaled to the same values in the last 30 years.

Shapiro et al estimate the total solar irradiance (TSI) during the Maunder minimum to be about 6 W/m2 less than at present, and hence the solar radiative forcing difference of about 1 W/m2. For comparison, the corresponding radiative forcing in the most recent IPCC report, however, ranges between +0.38 and +0.68 W/m2

How good are the new estimates? A major weakness of all such estimates – and Shapiro et al. is no different – is that the results cannot be tested on the basis of the last 30 years of solar observations. The paper is candid about this issue, and explains that this is partly because the time scale, on which their analysis rests, is 22 years. In any reconstruction, part of the analysis involves determining the degree which “magnetically enhanced contribution” contributes to the irradiance of the present “quiet sun”. These estimates were then scaled with the proxies for levels representing “quiet” sun activity.

However, the long term variance in Shapiro et al is based simply on an assumption. Specifically, they assume that there is a correspondence between the minimum state of a “quiet” sun (The article “Solar Myth” from NASA discusses the implications of a “quiet” sun in more depth) and the observed quietest area of the sun. The calculation for this area of the sun comes from Fontelna et al (1991), and Shapiro et al assume that during the deepest Grand Minima this controls the irradiance. But whether the sun could ever physically achieve this is unknown.

Another recent paper, Schrijver et al (2011), makes a similar argument by analogy, but they simply assume that the Grand Minima are like the recent minima 2008/2010. Again, no physically argument is given, and no evaluation of the assumption is performed.

So how do these new TSI-estimates relate to past climate variations and other forcings?

Through paleo-climate simulations for the last millennium with climate models, a number of alternate forcing histories for volcanic and solar changes have been proposed to see their effect on past climate variations. However, such model studies cannot provide definite answers, as there is a range of possible model outcomes because the solar forcing is just one of several forcings (e.g. aerosols, greenhouse gases, land surface) that are not well-constrained by observations.

Nevertheless, in a recent paper, Feulner used the TSI reconstruction of Shapiro et al. to drive a climate model. He compared the simulated temperature with temperature reconstructions, where the a number of climate model simulations were carried out, fed with different TSI estimates. His conclusion was that the climate model

… yields climatic conditions during past solar minima that are too cool and excessive fluctuations on timescales of several decades for Shapiro et al.’s [2011] TSI reconstruction.

His results were more consistent with temperature reconstructions if more traditional TSI estimates were used, with moderate difference between today and the Maunder Minimum, and that are in better agreement with the results presented in the recent IPCC report.

Of course, these evaluations rely on the models being able to mimic the sensitivity of the real climate system and assume that paleoclimatic reconstructions of the temperature do adequately describe the past climate variations. For the global or hemispheric mean temperatures, either a reduction in sensitivity or an underestimate of past temperature variability could also explain the discrepancy. Using models to distinguish between the forcing histories is thus likely to require a tighter focus on regional changes, or in climate patterns, more than the just the mean temperature.

There is still a great deal of uncertainty surrounding the issue of TSI reconstruction, though progress is more likely to come from a better understanding of the physics of the solar cycles than from simple, hard-to-verify assumptions.


20 Responses to “How large were the past changes in the sun?”

  1. 1
    ccpo says:

    progress is more likely to come from a better understanding of the physics of the solar cycles than from simple, hard-to-verify assumptions.

    So, you’re saying this new paper doesn’t really tell us much of anything?

    Rasmus, some errors in the text (assume you don’t mind the feedback):

    1st pp: “past climate changes particularly in the pre-industrial” > “past climate changes, particularly in the pre-industrial.”

    2nd pp: “any previously reconstruction” > “any previous reconstruction.”

    5th pp: “because the time scale, on which their analysis rests, is 22 years” > “because the time scale, on which their analysis rests, is 22 years”

    7th pp: “recent paper, Schrijver et al (2011) makes” > “recent paper, Schrijver et al (2011), makes”

    “no physically argument is given” > “no physical argument is given”

    10th pp: “where the a number of” > “where a number of”

    11th pp: “His results were more consistent with temperature reconstructions if more traditional TSI estimates were used, with moderate difference between today and the Maunder Minimum, and that are in better agreement with the results presented in the recent IPCC report.”

    This is hard to cipher, probably due to a lack of knowledge on my part. Might be: “If more traditional TSI estimates were used, his results would be more consistent with temperature reconstructions, with moderate difference between today and the Maunder Minimum, and would be in better agreement with the results presented in the recent IPCC report.”

    [Response: Thanks! I'll fix the errors. -rasmus]

  2. 2
    Didactylos says:

    Am I crazy, or does Schmidt et al (2011) use recent TSI studies, while this new TSI study is in closer agreement with older studies (considered by many to be wrong, outdated and generally superseded by the newer studies)?

    WLS: Wang et al. (2005)
    MEA: Muscheler et al. (2007)
    DB: Delaygue and Bard (2010)
    SBF: Fröhlich (2009); Steinhilber et al. (2009)
    VSK: Vieira et al. (2010)

    And the older papers:
    Lean (2000)
    Hoyt and Schatten (1993?)
    Beer et al. (2000) ← This one is notable for proposing a Maunder Minimum even lower than Shapiro et al.

    [Response: You are not crazy. The problem with the older papers is that they based their calibrations on bits of the science that are no longer valid. For instance, the Lean (2000) paper based their MM value on a paper by Baliunas and Jastrow comparing the irradiance of cycling and non-cycling sun-like stars. Unfortunately, their correlations did not hold up when lots more stars were examined. Similarly, Hoyt and Schatten used proxies that that they thought were calibrated to irradiance (like solar cycle length), that actually aren't. Reid (2000) used a calibration based on his estimate of the temperature change and the sensitivity (so that is completely backwards) etc. The more recent reconstructions (WLS, VSK, SBF, DB, MEA) are based on physical models (or calibrated to them) for the open flux and proxies of the same. There are still assumptions of course, but their MM to present changes are not so simply determined as in the Shapiro or the Schrijver papers. - gavin]

  3. 3

    Well, I’m glad you did this article now :)

    I have some estimates I have been playing with for contrast based on the quiet sun for the MM. This will cause me to add some new material for additional context possibly.

  4. 4
    Edward Greisch says:

    So the sun can vary by almost 0.6%, which is 6 times what we thought before. I used the minimum in the 15th century vs the present. 8/1366=.0059

    The solar maximum in the 1950s could have a lot to do with our overconfident expectations for food availability. I think food might be reduced by a minimum like the years near 1460. There could be linkages to history. There was a higher peak in the 14th century. The Renaissance started in that century.

    Thanks for the linked papers such as Schmidt et al (2011) and thanks for this excellent article. Figure 1 is an awesome result.

  5. 5
    chris colose says:

    1 W/m2 forcing seems quite large from Maunder Minimum to modern. IPCC 2007 had an order of magnitude less; Gray et al (2010) increased that to about 0.24 W/m2 (this depends on where you choose your start date too), but the Shapiro results don’t seem right…

  6. 6
    Yvan Dutil says:

    Since there is almost no difference between reconstruction past 1950, this as little impact on our understanding of the recent climate change. However, if large TSI excusion are true, this would also means that sensitivity is much lower for TSI than C02, which is hard to explain.

  7. 7

    The answer is probably no for good physical reasons, but I’ve got to ask:

    Is there any chance that the models would have to produce temperature mismatches using the Shapiro reconstruction because they were calibrated and developed using the other, much-less-variable TSI reconstructions?

    [Response: No. None of the large scale models used for the IPCC projections have been calibrated on the last millennium - because of uncertainty in the temperatures and uncertainties in the forcings. Additionally, for the GCMs, it is almost impossible to 'just' tune the climate sensitivity. - gavin]

  8. 8
    Doug Proctor says:

    According to the skeptics, the solar irradiance isn’t very important, it is the strength of the sun’s magnetic field (that allows or stops cosmic rays from coming in which then causes more or less clouds, which increases or decreases the Earth’s albedo, which then causes warming or cooling of the Earth’s surface).

    The really good thing about the climate “debate” is that the causations proposed by both sides – CO2 or cloud cover – will reveal themselves in the next few years. By 2015, by my estimate. By then either the Earth will cool – the sunspot/cosmic ray theory – or there will be a huge ramp-up in temps to account for the last 10 years of stability.

    With both sides so committed to their positions, there is little to no wiggle room. All to be decided soon.

    [Response: What last 10 years of stability?--eric]

  9. 9
    Sekerob says:

    Via Greg’s TSI site, just stumbled on a 2011 Kopp and Lean paper putting up an explanation of why TIM/SORCE measure a lower TSI, and why the old were off. http://europa.agu.org/?view=article&uri=/journals/gl/gl1101/2010GL045777/2010GL045777.xml&t=gl,2011,kopp

  10. 10
    ozajh says:

    The AGW Denial-o-Sphere is going to treat this reconstruction as Received Truth.

  11. 11

    I’m a bit puzzled by this set of numbers:

    The difference in TSI between Shapiro et al and most other recent reconstructions (between Maunder minimum and the present) is about a factor of 10, but the difference in radiative forcing as quoted in this post (see the paragraph below) is only a factor of 2. Is it so strongly non-linear over this range or is something missing here?

    “Shapiro et al estimate the total solar irradiance (TSI) during the Maunder minimum to be about 6 W/m2 less than at present, and hence the solar radiative forcing difference of about 1 W/m2. For comparison, the corresponding radiative forcing in the most recent IPCC report, however, ranges between +0.38 and +0.68 W/m2″

    [Response: Those are the numbers taken from Table 2.10 in AR4 - but that includes old reconstructions that have been superseded because of the downgrading of the sun-like stars comparison etc (see Foukal et al, 2006; Gray et al, 2010). Only the range of 'new reconstructions' as listed in Schmidt et al has current support - and Shapiro et al is around 4 times as large as the largest of those. - gavin]

  12. 12
    Didactylos says:

    Edward Greisch: I get the sense you are taking away the wrong conclusion from this article. My reading is that Shapiro et al. is based on a big assumption, and that it doesn’t produce plausible results in climate models.

    The consensus position is still that TSI has varied little historically, although nobody has had the last word on the subject yet.

  13. 13
    vukcevic says:

    Accuracy of the solar activity and pale-climatic reconstructions is questionable, since both are affected by changes in the geomagnetic field.
    http://www.vukcevic.talktalk.net/LL.htm

  14. 14
    vukcevic says:

    ( Doug Proctor says: 23 Aug 2011 at 9:25 PM …………….)

    Not necessarily. North Atlantic may be a good guide to the future. There are good data for the most parameters since 1950/60. Strength of the N. Atlantic’s currents index NAP (I have constructed) is on down-slope; the N. Atlantic pressure differential has parted the way around year 2000, while the SST is still holding the line.
    http://www.vukcevic.talktalk.net/NorthAtlantic.htm

  15. 15
    David Miller says:

    Doug, in #8 opines:

    According to the skeptics, the solar irradiance isn’t very important, it is the strength of the sun’s magnetic field (that allows or stops cosmic rays from coming in which then causes more or less clouds, which increases or decreases the Earth’s albedo, which then causes warming or cooling of the Earth’s surface).

    Well, that’s one reason given. “It’s the sun” is also very common. As are many other reasons, none of which are consistent with each other other than the “anything but carbon” theme that ties them all together.

    The problem with it being either the sun or cosmic rays is that we’ve measured both for 30+ years and neither show a trend that could possibly account for direct observations.

  16. 16
    Hank Roberts says:

    The link for the Feulner paper in the main post is to the AGU abstract.
    The illustration available there is helpful: compare volcanos and TSI:

    http://www.agu.org/journals/gl/gl1116/2011GL048529/2011gl048529-op01-tn-350x.jpg

    Here’s a full text copy:
    http://www.atmos-chem-phys-discuss.net/11/2297/2011/acpd-11-2297-2011-print.pdf

    That was found with: http://www.google.com/search?q=Foukal++2006+Gray+2010
    (which turns up a lot of other papers, as those are much cited)

  17. 17
    spyder says:

    It is beginning to appear that the more we know about the Earth’s climate, the more we discover we need to know. I suppose somewhere down the line there will be a new study linking the one discussed above with a new one concerning El Nino and human warfare.

  18. 18
    Garry S-J says:

    Typo in 2nd last par, I think: “to adequately describe” should be
    “do adequately describe”.

    [Response: fixed. thanks - gavin]

  19. 19
    Geoff Beacon says:

    If I’ve read it correctly the draft paper by Hansen et. al., Earth’s Energy Imbalance and Implications suggests that many climate models underestimate the effect of positive climate forcings but also underestimate the effects of negative forcings due to aerosols.

    If correct how would this modify the reults of Shapiro et al.?

    P.S. Is mentioning a draft paper a breach of any protocol?

  20. 20
    charles "chick" keller says:

    If we accept the recent Lead and Rind paper that determines (via linear regression) a solar influence on global temps, over an 11 yr cycle to be about 0.1°C, then there may be an indirect solar forcing about equal to that from TSI. that being said, perhaps the sun could be responsible for a larger variation in global temperatures without requiring a larger TSI forcing. This would btw adequately explain the lack of temperature increase in the past 6-8 yrs?


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