02 settembre 2008

Sole senza macchie, possibili effetti climatici?

Sembra che alla fine la NOAA abbia rovinato la festa di chi in questi giorni ha celebrato il trascorso mese di agosto come "primo mese senza macchie solari" in un secolo. Se davvero la superficie del sole non avesse mostrato neppura una piccola macchia, sarebbe stata la prima volta dal 1913. In realtà ieri la NOAA ha modificato la sua statistica relativa all'agosto 2008 attribuendo a una formazione osservata sul sole nei giorni 21 e 22 lo status di "mezza macchia". Insomma, un sole quasi illibato ma non del tutto.
La discussione sul minimo solare sta raggiungendo un massimo. Pochi si occupano della questione pensando alla propagazione dei segnali radio (bassa attività equivale a dire bassa probabilità di propagazione sulle frequenze alte delle onde corte, mentre chi si interessa di onde medie è invece molto contento). Le macchie solari vengono piuttosto considerate come possibile fattore nei modelli di previsione del clima e questo articolo di Michael Asher su Dailitech ne parla diffusamente, citando tra l'altro un libro di un climatologo svedese Henrik Svensmark in cui si ipotizza come l'elevata attività solare faccia da schermo alla radiazione cosmica contrastando così la naturale formazione di nuvole e provocando un innalzamento delle temperature (meno nuvole = più radiazione solare che riscalda il terreno). Il discorso viene approfondito su Watts Up With That, il blog di un meteorologo televisivo in pensione che parla di macchie solari e di indice geomagnetico. Ho trovato anche un interessante spazio di discussione sul ciclo solare 24, che proprio non si decide a decollare, frequentato da molti radioamatori.
Sun Makes History: First Spotless Month in a Century
Michael Asher - September 1, 2008

Drop in solar activity has potential effect for climate on earth.

The sun has reached a milestone not seen for nearly 100 years: an entire month has passed without a single visible sunspot being noted.
The event is significant as many climatologists now believe solar magnetic activity – which determines the number of sunspots -- is an influencing factor for climate on earth.
According to data from Mount Wilson Observatory, UCLA, more than an entire month has passed without a spot. The last time such an event occurred was June of 1913. Sunspot data has been collected since 1749.
When the sun is active, it's not uncommon to see sunspot numbers of 100 or more in a single month. Every 11 years, activity slows, and numbers briefly drop to near-zero. Normally sunspots return very quickly, as a new cycle begins.
But this year -- which corresponds to the start of Solar Cycle 24 -- has been extraordinarily long and quiet, with the first seven months averaging a sunspot number of only 3. August followed with none at all. The astonishing rapid drop of the past year has defied predictions, and caught nearly all astronomers by surprise.
In 2005, a pair of astronomers from the National Solar Observatory (NSO) in Tucson attempted to publish a paper in the journal Science. The pair looked at minute spectroscopic and magnetic changes in the sun. By extrapolating forward, they reached the startling result that, within 10 years, sunspots would vanish entirely. At the time, the sun was very active. Most of their peers laughed at what they considered an unsubstantiated conclusion.
The journal ultimately rejected the paper as being too controversial.
The paper's lead author, William Livingston, tells DailyTech that, while the refusal may have been justified at the time, recent data fits his theory well. He says he will be "secretly pleased" if his predictions come to pass.
But will the rest of us? In the past 1000 years, three previous such events -- the Dalton, Maunder, and Spörer Minimums, have all led to rapid cooling. One was large enough to be called a "mini ice age". For a society dependent on agriculture, cold is more damaging than heat. The growing season shortens, yields drop, and the occurrence of crop-destroying frosts increases.
Meteorologist Anthony Watts, who runs a climate data auditing site, tells DailyTech the sunspot numbers are another indication the "sun's dynamo" is idling. According to Watts, the effect of sunspots on TSI (total solar irradiance) is negligible, but the reduction in the solar magnetosphere affects cloud formation here on Earth, which in turn modulates climate.
This theory was originally proposed by physicist Henrik Svensmark, who has published a number of scientific papers on the subject. Last year Svensmark's "SKY" experiment claimed to have proven that galactic cosmic rays -- which the sun's magnetic field partially shields the Earth from -- increase the formation of molecular clusters that promote cloud growth. Svensmark, who recently published a book on the theory, says the relationship is a larger factor in climate change than greenhouse gases.
Solar physicist Ilya Usoskin of the University of Oulu, Finland, tells DailyTech the correlation between cosmic rays and terrestrial cloud cover is more complex than "more rays equals more clouds". Usoskin, who notes the sun has been more active since 1940 than at any point in the past 11 centuries, says the effects are most important at certain latitudes and altitudes which control climate. He says the relationship needs more study before we can understand it fully.
Other researchers have proposed solar effects on other terrestrial processes besides cloud formation. The sunspot cycle has strong effects on irradiance in certain wavelengths such as the far ultraviolet, which affects ozone production. Natural production of isotopes such as C-14 is also tied to solar activity. The overall effects on climate are still poorly understood.
What is incontrovertible, though, is that ice ages have occurred before. And no scientist, even the most skeptical, is prepared to say it won't happen again.

Article Update, Sep 1 2008. After this story was published, the NOAA reversed their previous decision on a tiny speck seen Aug 21, which gives their version of the August data a half-point. Other observation centers such as Mount Wilson Observatory are still reporting a spotless month. So depending on which center you believe, August was a record for either a full century, or only 50 years.

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