En una charla en el blog de Anthony Watts, a Leif Svalgaard, eminente físico solar, le tiraban de la lengua para que explicara su postura sobre las posibilidades de las variaciones solares de producir los cambios climáticos. Al final, se lanzó al ruedo:

Leif Svalgaard (14:11:34) :

DAV (13:46:01) :
Is it simply coincidence that the deepest part of the LIA began at the start of a very quiet sun period and both cycles ended at about the same time? Amazing coincidence in my view. It shouldn’t be summarily dismissed as mere coincidence.

It is NOT being summarily dismissed. It is dismissed for several good reasons. I have given those in another thread, and will post them here again [Anthony permitting]. First, I have already mentioned the Oort Minimum during the MWP, but here are my solar reasons:

Leif Svalgaard (16:43:03) :
setting forth several lines of inquiry and evidence:

Line 1:
The Total solar Irradiance (TSI) has several sources. The first and most important is simply the temperature in the photosphere. The hotter the sun, the higher the TSI. Some spectral lines are VERY sensitive to even minute changes in temperature. Livingston et al. has very carefully measured the line depth of such temperature-sensitive lines over more than 30 years spanning three solar cycles [Sun-as-a-Star Spectrum Variations 1974-2006, W. Livingston, L. Wallace, O. R. White, M. S. Giampapa, The Astrophysical Journal, Volume 657, Issue 2, pp. 1137-1149, 2007, DOI; 10.1086/511127]. They report [and I apologize for the somewhat technical turn my argument is taking, but if you really want to know, there is no avoiding this], “that both Ca II K and C I 5380A intensities are constant, indicating that the basal quiet atmosphere is unaffected by cycle magnetism within our observational error. A lower limit to the Ca II K central intensity atmosphere is 0.040. This possibly represents conditions as they were during the Maunder Minimum [their words, remember]. Within our capability to measure it using the C I 5380A line the global (Full Disk) and basal (Center Disk) photospheric temperature is constant over the activity cycles 21, 22, and 23″. I have known Bill Livingston [and White] for over 35 years and he is a very careful and competent observer.

Line 2:
Since the 1960 we have known that the sun’s surface oscillates up and down [with typical periods of ~5 minutes]. These oscillations are waves very much like seismic waves in the Earth [from earthquakes] and just as earthquake seismic waves can be used to probe the interior of the Earth, they can be used to probe the solar interior. There are millions of such solar waves at any given time and there are different kinds (called ‘modes’) of waves. The solar p-modes are acoustic [sound waves] normal modes. You can imagine a frequency increase with an increasing magnetic field, due to the increase in magnetic pressure raising the local speed of sound near the surface where it is cooler and where the p-modes spend most of their time. Of course one can also imagine higher frequencies may result from an induced shrinking of the sound cavity and/or an isobaric warming of the cavity. Another kind is the solar f-modes that are the eigenmodes of the sun having no radial null points [i.e. asymptotically surface waves; again I apologize for the technical mumbo-jumbo]. From the solar cycle variations of p- and f-modes [and we have now enough data from the SOHO spacecraft to make such a study] we now have an internally consistent picture of the origin of these frequency changes that implies a sun that is coolest at activity maximum when it is most irradiant. Now, how can that be? How can a cooler [overall, including the cooler sunspots, for instance, as the temperature of the non-magnetic areas of the sun didn’t change {see line 1 above}] sun radiate more? It can do that, if it is bigger!. Goode and Dziembowski (Sunshine, Earthshine and Climate Change I. Origin of, and Limits on Solar Variability, by Goode, Philip R. & Dziembowski, W. A., Journal of the Korean Astronomical Society, vol. 36, S1, pp. S75-S81, 2003) used the helioseismic data to determine the shape changes in the Sun with rising activity. They calculated the so-called shape asymmetries from the seismic data and found each coefficient was essentially zero at activity minimum and rose in precise spatial correlation with rising surface activity, as measured using Ca II K data from Big Bear Solar Observatory. From this one can conclude that there is a rising ‘corrugation’ of the solar surface due to rising activity, implying a sun, whose increased irradiance is totally due to activity induced corrugation. This interpretation has been recently observationally verified by Berger et al. (Berger, T.E., van der Voort, L., Rouppe, Loefdahl, M., Contrast analysis of Solar faculae and magnetic bright points. Astrophysical Journal, vol. 661, p.1272, 2007) using the new Swedish Solar Telescope. They have directly observed these corrugations. Goode & Dziembowski conclude that the Sun cannot have been any dimmer, on the time steps of solar evolution, than it is now at activity minimum.

Line 3:
Foukal et al. (Foukal, P., North, G., Wigley, T., A stellar view on solar variations and climate. Science, vol. 306, p. 68, 2004) point out the Sun’s web-like chromospheric magnetic network (an easily visible solar structure seen through a Ca II K filter) would have looked very different a century ago, if there had been a significant change in the magnetic field of the sun supposedly increasing TSI. However, there is a century of Mt. Wilson Solar Observatory Ca II K data which reveal that the early 20th century network is indistinguishable from that of today.

Line 4:
Svalgaard & Cliver have recently (A Floor in the Solar Wind Magnetic Field, by L. Svalgaard and E. W. Cliver, The Astrophysical Journal, vol. 661, L203�L206, 2007 June 1, 2007) shown that long-term (∼130 years) reconstruction of the interplanetary magnetic field (IMF) based on geomagnetic indices indicates that the solar wind magnetic field strength [and thus that of the sun itself, from which the IMF originates] has a ‘floor’, a baseline value in annual averages that it approaches at each 11 yr solar minimum. In the ecliptic plane at 1 AU [at the Earth], the IMF floor is ∼4.0 nT, a value substantiated by direct solar wind measurements and cosmogenic nuclei data. We identify the floor with a constant (over centuries) baseline open magnetic flux at 1 AU of 4×10^14 Weber. Solar cycle variations of the IMF strength ride on top of the floor. They point out that such a floor has implications for (1) the solar wind during grand minima: we are given a glimpse of Maunder minimum conditions at every 11 yr minimum; (2) current models of the solar wind: both source surface and MHD models are based on the assumption, invalidated by Ulysses, that the largest scale fields determine the magnitude of the IMF; consequently, these models are unable to reproduce the high-latitude observations; and (3) the use of geomagnetic input data for precursor-type predictions of the coming sunspot maximum; this common practice is rendered doubtful by the observed disconnect between solar polar field strength and heliospheric field strength [the wrong prediction by the NASA panel for cycle 23 was based on this, and the prediction {of a high cycle} by one half of panel for cycle 24 is also partly based on this]. The constancy of the IMF also has implications for the interpretation of the Galactic Cosmic Ray flux.

Line 5:
But maybe it is the Ultraviolet flux that varies and affects the stratospheric ozone concentration and thereby influences the climate. I have earlier in (Calibrating the Sunspot Number using the “Magnetic Needle”, L. Svalgaard; CAWSES News, 4(1), 6.5, 2007] pointed out that the amplitude of the diurnal variation of the geomagnetic Y-component is an excellent proxy for the F10.7 radio flux and thus also for the EUV flux (more precisely, the FUV, as the Sq current flows in the E layer). There is a weak trend in the amplitude of 10% since the 1840s that can be understood as being due to an increase of ionospheric conductance resulting from the 10% decrease of the Earth’s main field. Correcting for and removing this trend then leads to the conclusion that (as for the IMF) there seems to be a ‘floor’ in rY and hence in F10.7 and hence in the FUV flux, thus the geomagnetic evidence is that there has been no secular change in the background solar minimum EUV (FUV) flux in the past 165 years.

Line 6:
Careful analysis of the amplitude of the solar diurnal variation of the East-component of the geomagnetic field [we have accurate measurements back to the 1820s] allows us the obtain an independent measure of the FUV flux (and hence the sunspot number) back to then. The result is that the Wolf number before ~1945 should be increased by 20% and before ~1895 by another 20%. The Group Sunspot number in the 1840s is 40% too low compared to the official Wolf number. When all these adjustments are made we find that solar activity for cycles 11 and 10 were as high as for cycle 22 and 23. Thus there has been no secular increase in solar activity in the last ~165 years [a bit more precise than the 150 years I quoted earlier]. Of course, there has still been small and large cycles, but we are talking about the long-term trend here [or lack thereof].

Line 7:
Direct measurements (although beset by calibration problems) of the Total Solar Irradiance (TSI) from satellites have only been available for 30 years and indicate that solar irradiance increases with solar activity. Correlating mean annual TSI and sunspot numbers allows one to estimate the part of TSI that varies with the sunspot number. If TSI only depends linearly on the sunspot number then irradiance levels during the Maunder Minimum would be similar to the levels of current solar minima. But TSI is a delicate balance between sunspot darkening and facular brightening, and although both of these increase (in opposite directions) with increasing solar activity, it is not a given that there could not be secular variations in the relative importance of these competing effects. Several earlier reconstructions of TSI, reviewed in Froehlich, C. & J. Lean (Solar Radiative Output and its Variability; Evidence and Mechanisms, Astron..& Astrophys. Rev., 12(4), 273, 2004, Doi;10.1007/s00159-004-0024-1.[6] all postulate a source of long-term irradiance variability on centennial time scales. Each group of researchers have their own preferred additional source of changes of the ‘background’ TSI, such as evidence from geomagnetic activity, open magnetic flux, ephemeral region occurrence, umbral/penumbral ratios, and the like. The existence of ‘floors’ in IMF and FUV over ~1.6 centuries argues for a lack of secular variations of these parameters on that time scale. The six other lines of evidence discussed above suggest that the lack of such secular variation undermines the circumstantial evidence for a ‘hidden’ source of irradiance variability and that there therefore also might be a floor in TSI, such that TSI during Grand Minima would simply be that observed at current solar minima. At the recent SORCE meeting in Santa Fe [2008] Judith Lean discussed the various contributions to variations of TSI:
0.003% from 5-minute oscillations, 0.2% from solar rotation, 0.1% from 11-year solar cycle, and ended with: “longer-term variations not yet detectable – do they occur?”

I concluded that “So, if there is ’solar activity’ forcing, the sensitivity of the climate system to this must be much greater than generally assumed and understood. A simpler hypothesis is that there is no clear solar effects on the timescale of decades or centuries.”

Even the paper by Douglas and Christie conclude that solar influence explains but a fraction of the recent trend.

Se puede seguir el resto de la conversación, con otros intervinientes, en este enlace.