Expert: Philip Stahl - 4/5/2007

Question
I have been interested in the effects of variations of the suns radiance ever since I heard of gm when I was 11. It has reached a fevered pitch as of late. I'm not all that politically astute but there does some to be some uproar about this book " unstoppable global warming every 1500 years " I have been tirelessly searching the web the last few months to try and find an independent source on solar radiation output from astronomical sites and have been unsuccessful. Can you please tell me what if any solar variances do occur? and of this is to long to put in a single e-mail where I might look for such information.
   Take care Adam

Answer
Hello,

Unfortunately, when politics get injected into science everything suffers and reliable sources become harder to find. Often, this means having to go back to earlier research - especially that which has stood the test of time.

Perhaps the most definitive paper ever on the possible variations of the solar constant was: 'Solar constant - Constraints on possible variations derived from solar diameter measurements' Sofia, S.; Okeefe, J.; Lesh, J. R.; Endal, A. S. (Science vol. 204, June 22, 1979, p. 1306-1308.)

Their data, acquired for the interval between 1850 and 1937, set limits of variation on the solar radius to about 0.25 arc second. The Sun's actual aperture in seconds of arc is ~ 1920". So we are talking about 1.3 parts in ten thousand of change.

Generally (in terms of solar gas dynamics and with considerations of hydrostatic pressure balance)

one has

dP/dr = - GM(r) rho(r)/ r^2

e.g. the outward gas pressure gradient is balanced by the inward pull of gravity for a particular mass density at a given layer, at distance r from the center of the Sun

if the 0.25 arcsec change is a contraction of the diameter we can expect heating, while if it is an expansion we get cooling.

The authors' modeling of the Sun indicates that the solar constant did not vary by more than 0.3 percent during that time, either way.

More recent space-based observations appear to show a variation in solar irradiance of at least 0.15% over the standard 11-year solar cycle. (E.g. Parker, E.N., Nature, Vol. 399, p. 416). However, even with this higher percentage ascribed to solar changes, the heating effect is nowhere near comparable to that induced from man-made global warming. (See, e.g. Martin I. Hoffert et al, in Nature, Vol. 401, p. 764).

As the authors in the latter study point out, the heating component arising from greenhouse gas emissions from 1861-1990 amounted to anywhere from 2.0 to 2.8 watts per square meter. The solar variability component detected over the same period amounted to 0.1 to 0.5 watts per square meter. Thus, even the MAXIMUM solar variability amounted to only a fraction (25%) of the MINIMUM power input from human-induced greenhouse warming!

Soon and Baliunas, meanwhile, in a paper published four years ago, estimated a solar irradiance change of 0.4% which is hardly credible (to me) since no previous detailed research gets anywhere near that. (These two authors are global warming deniers, so it would fit that a higher solar irradiance is what would 'fit the bill' i.e. to blame warming miseries on ol' Sol)

My best instinct would be that Sofia et al's remain the best variance results.

John Eddy's radio-carbon data for the "little Ice Age" (cf. 'Historical and Arboreal Evidence for a Changing Sun' in The New Solar Physics, AAAS Selected Symposium, Westview Press, 1979, pp. 11-33) also falls well within the tolerance parameters and limits assigned by Sofia et al.

Moreover, their study has lots of heft because it is tied to actual physical *changes* in the Sun's diameter, as opposed to much more subjective (and questionable) methods.

Lastly, I would not personally put much stock in the book you read about 'Unstoppable Global Warming' etc. One of the authors, S. Fred Singer, is a well-known global warming skeptic who has also received funding from fossil fuel lobbies for his work. Writing as a solar physicist, there is also NO solid evidence that the Sun is the culprit as he and Avery - the co-author-  claim.

If it was true, that the Sun was really a variable star on the scale of 1500 yrs. it would be one of the most stupendous finds imaginable. Alas, it isn't. It is codswallop, pure and simple - but a great "excuse" for the naysayers to latch on to, in order to dodge the economic sacrifices we all know will have to be made if global warming is anthropogenic.

Solar physicist John Eddy, who made it is research specialty to study long-term solar variations connected to climate change, noted the period of 12th century warming in his book, ‘The New Solar Physics’, AAAS Selected Symposium, Westview Press, 1979, p. 17.

Eddy noted that this coincided with a period of higher solar activity (i.e. more sunspots) and possibly greater luminosity – on account of the fact that the irradiance is amplified around sunspots owing to redirection of convective heat flow. (Bear in mind the plasma in spots is at lower temperatures, by about 1500C, because of the powerful magnetic fields in them). The opposite effect occurred in the little ice age with few or no spots. But this certainly doesn't argue for a 1500 year period!

Bottom line is this: man-made global warming is real and is occurring even as I write this. The longer there is politically generated delay to avoid the economic fallout, the worse the fallout will eventually be. We diddle at our own peril, and it is now time to stop allowing politics and eonomics to trump the science.

And stop listening to or reading the neo-Luddites and flat earthers.

See also my other answer to a global warming query, in the 'previous answered questions'. (It has to do with whether the tilt of Earth's axis must be factored in)