Presentation on theme: "“Adelaide May Have the Dubious Distinction of Being the First Industrialized City to Live in a Constant State of Water Shortage” (National Geographic Magazine,"— Presentation transcript:
“Adelaide May Have the Dubious Distinction of Being the First Industrialized City to Live in a Constant State of Water Shortage” (National Geographic Magazine, April, 2009)
Evidence on millennial Sun – climate effect (Bond et al.)
Does variable solar UV flux influence climate? Known: Fuv variation modulates stratospheric ozone concentration & temperature over the 11 yr sunspot cycle. Hypothesis: the resultant changes in stratospheric temperature influence propagation of planetary waves and latitudinal energy transport in troposphere. These changes affect climate - e.g. storm tracks, precipitation patterns, global temperature,Tg (Haigh,1996;Shindell et al., 2001). A simple test: Do variations of the “input” Fuv correlate with the “output”- i.e. with Tg variations on multi-decadal time scales?
Reconstruction of solar UV flux variation over past century Fuv variation (in wavelength range < 240 nm) arises from changing areas of magnetic plages and network These structures have been imaged since the 1880’s in the passband of the strongest solar spectral line Ca K (393.37nm) Recently digitized, daily Ca K images are now available back to 1907.
Comparison of 11- yr smoothed UV flux reconstructions, and Tg a: MWO (Pulkovo) b: MWO ( UCLA) c: Kodaikanal (Pulkovo) d: MWO + NSO (CRI) e: global temperature,Tg Conclusion: 20 th century Fuv and Tg correlation is relatively low (e.g. r 2 < 0.2 between 1915-1984).
Total Solar Irradiance and Luminosity Total solar irradiance (TSI) = flux of spectrally integrated solar radiation above the Earth’s atmosphere, at a mean distance of 1 A.U. TSI measured using cavity radiometers (equal response to UV, visible, IR) flown above the Earth’s atmosphere TSI variations arise primarily due to changes in globally integrated solar heat flux (i.e. solar luminosity)
TSI 1978 – 2008 (PMOD & ACRIM radiometry; by courtesy of C.Frohlich)
What photospheric structures cause measured TSI variation? On solar rotational to 11- yr time scales: >85% of TSI variance due to dark sunspots & bright faculae. Any residual TSI variations from slower changes in e.g. solar diameter or convective patterns? Procedure: reconstruct TSI variation from spot, facular imaging (need broad band), then subtract this from the radiometry.
Broad–band Imaging with the Solar Bolometric Imager First imaging with spectrally flat, 300 – 2600 nm response (like radiometers). 30 cm aperture telescope, 80,000 pixel bst detector, coated with gold black. Flights: Ft Sumner,TX 2003,07;McMurdo 06.
TSI measured and reconstructed from spots, faculae alone agree to <10% rms ( i.e. little evidence for additional TSI contributions)
How much dimmer might the Sun have been during the Maunder Minimum? Calculate ~0.04% (smoothed) dimming by spots, faculae alone – not climatically effective. Suggested ~ 0.2% additional dimming during Maunder Minimum by disappearance of all photospheric magnetism Not supported by imaging of Ca K network since1880’s,nor by stellar photometry of “solar analog” stars, nor by 10 Be data. Foukal, North and Wigley,Science,306,68 (2004)
How would TSI behave if Sun were more magnetically active? Luminosity variation increases with activity level ( Radick et al., 1998) More active stars get dimmer at peak of their cycle
The Sun at its highest observed activity level Giant spots of April 1947- amongst largest ever observed.
How “hyperactive” would the Sun need to get to achieve such greater TSI variability & dimming? The solar spot / facula area ratio increases at highest activity levels A solar activity level > cycle 19 could make the Sun’s irradiance variation: i) 3-5 x larger; ii) negatively correlated with activity (Foukal, Science,264,238 (1994))
Could extended hyperactivity have occurred during the Holocene? Unlikely ( Solanki et al., 2004) But that study does not consider C14 production by solar energetic particles (SEP’s) Total C14 = production by( GCR’s + SEP’s).(Oppositely correlated with activity level)
What does the 11- yr TSI variation tell us about likelihood of larger, slower TSI variations? Sunspot magnetic flux tubes inhibit convection, so produce local “thermal plug”, reduce local radiative output Faculae facilitate radiation from deeper, hotter layers and produce local “thermal leaks.” Why doesn’t heat flow quickly re-adjust around these local perturbations, leaving TSI constant?
Sun’s radiative relaxation time constrains climatically effective luminosity variations 100,000 yr radiative relaxation time of solar convection zone damps compensating brightness perturbations, so enables TSI variation caused by surface fields ( spots, faculae). But this same thermal inertia attenuates deeper thermal perturbations (e.g. dynamo - induced variations) by factor ~ 10 4. No evidence for non – (spot or facula) TSI variations from photometry,solar diameter measurements,helioseismology (but see recent radiometry of present anomalously low TSI)
Conclusions Low Fuv versus Tg correlation argues against major role of UV irradiance increase in global warming. This is consistent with more visible modelled Fuv effects on precipitation, storm tracks. No evidence for TSI increase since Maunder Minimum by more than spot, facular effect of ~ 0.04%. As Raper & Wigley pointed out in 1990, this is too small to drive climate. Spot – induced TSI variation of a hyper- active Sun could be (3 – 5) times larger, but negatively correlated with activity (i.e. dimmer sun) Sun’s enormous thermal inertia constrains, but does not rule out, mechanisms of slower TSI variation.