Expert: Philip Stahl - 8/25/2007

Question
Climate-chnage skeptics point to the Sun to prove that it has more to do with climate-change than global warming via human- created CO2? What's your view? I've also heard that other planets in the Solar System(according to a science-journalist called Robert Matthews)  have cooled by a couple of degrees - is this prrof against global warming - or not?

Thanks
Geoff

Answer
Hello,

I’ve often been asked about a solar warming-global warming connection. I've always responded that no such connection has ever been verified, and hence all claims to such amount to so much speculative codswallop

Willie Soon and Sallie Baliunas claimed (in a 1996 paper in The Astrophysical Journal) to have found solar irradiance increases on the order of 0.4% or more – but that was long since shot down, and their methodology disclosed as flawed.

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).

During solar cycle 20 – when I also conducted investigations on solar flares and their effects- the then Solar Max satellite used an active cavity radiometer to measure temperature increases arising from higher activity – especially as generated by more convection at the periphery of large spots. The differential was something on the order of 0.1C at the Sun!  Since the radiant energy must now transit 150 million kilometers, and its intensity falls off as the inverse square, one can see this would translate into negligible increases at Earth.

What about longer period increases in solar luminosity associated with its possibly being a variable star – as opposed to sporadic sunspot outbursts?

The maximal magnitude of inherent solar -induced climate variability was probably first highlighted by Sabatino Sofia et al in their paper 'Solar Constant: Constraints on Possible Variations Derived from Solar Diameter Measurements', in Science, Vol. 204, 1306, 1979. Their estimate was a solar change in irradiance of roughly 0.1 % averaged over each solar cycle. (Irradiance is a measure of the energy per square meter received from the Sun).

Thus – if the solar irradiance effect at Earth (solar constant) is normally about 1360 watts/m^2, this would imply an increase of roughly 1.36 W/m^2.. The problem is that there is no observational evidence to support this in the warming period of the 12th century, or any time in the past century – when global warming spiked to serious levels. (Some like Sofia have argued that even if it had occurred, it would only engender a temp. increase contribution of perhaps one-fourth of one degree, or significantly less than what has been documented.

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).

Bottom line, climate change skeptics are merely grasping at straws - and mighty meager ones at that - if they are grasping at the Sun as the source of the warming! (And it's interesting that most of those making the claim are not solar physicists)

Re: other planets cooling, this neither proves nor disproves global warming on Earth. The reason is that the Earth is a distinct planet, with a distinct atmosphere, atmospheric density, composition etc. Hence, generalizations that might apply to Mars, say, would not apply to Earth. Unless one did a very careful analysis and showed he was taking into account the differences at ever level, and for every variable.

Let me also add, as an aside, the critics' carping about planetary "cloud cover"- and trying to insist that increase cloud cover means cooler temperatures - since more sunlight will be reflected.

In fact, though planetary albedo depends primarily on cloud cover, it is the least well studied climatic parameter. Clouds are very poorly parameterized in climate models as a whole. This has led to an ongoing debate over the past five years of whether in fact the sign of albedo change is positive or negative. (See e.g. ‘Can Earth’s Albedo and Surface Temperature Increase Together’ in EOS, Vol. 87, No. 4, Jan. 24, 2006, p. 37)

As the authors note, though there is some evidence that Earth’s albedo has increased from 2000 to 2004 this has NOT led to a reversal in global warming.  The authors cite the most up to date cloud data released in August, 2005 from the International Satellite Cloud Climatology Project (ISCCP). The data – from a range of meteorological satellites covering the entire Earth, discloses the most likely reason for the anomaly is primarily in the redistribution of the clouds.

Thus, as the authors point out (ibid.):

"whereas low clouds have decreased during the most recent years, high clouds have increased to a larger extent leading to both an increase in cloud amount AND *an increased trapping of infrared radiation* "

Prof. Gale Christianson in his book ‘Greenhouse’ (Penguin, 1999, p. 203)notes that “stratus clouds are gray, dense and low flying and have a net COOLING effect since their albedo is relatively high”.

BUT these are precisely the cloud type that has receded in incidence, as the ISCCP data show, from the EOS article! Now, what type has increased? Christianson again (ibid.)


“Conversely, wispy high flying cirrus are semi-transparent to incoming sunlight but block infrared radiation emitted by the Earth thus CONTRIBUTING to the Greenhouse Effect”


Meanwhile, towering cumulo-numbus clouds are neither here nor there and contribute minimally one way or the other.(Ibid).

Further note: critics and skeptics are also fond of dragging water vapor into the equation and insisting it is the biggest greenhouse gas and the atmosphere is always replete with it (e.g. in clouds). Again, this is oversimplified.  Even a tiny, minuscule amount of CO2 is vastly more efficient at blocking the re-radiation of energy than any equal amount of water vapor- at those bands. (See the NRC Report published ca. 2001 that gives the relative W/m^2 forcing contributions of each GH gas) Part of the misconception arose because early researchers, lacking the current technology of infrared spectroscopy, assumed that water vapor bands already blocked out most of what would (ordinarily) be taken by CO2. (Cf. ‘The Discovery of the Risk of Global Warming’, by Spencer Weart, in Physics Today, Jan. 1997, p. 34).

Lastly, a truly intelligent species would act on the basis of the "precautionary principle" and assume the worst case that can unfold and make preparations from now to minimize damage, e.g. from melting polar icecaps. In such a case, Homo Sapiens - if it were intelligent - would be well past the "debate" phase by now.

Hope this asnwer proves useful.