1. Solar Irradiance Variability and Climate
Claus Fröhlich1 and Judith Lean2
1) PMOD/WRC, Davos, Switzerland
2) Naval Research Laboratory, Washington DC
Observations
total irradiance since 1978
Empirical Models
sources and proxies of variability
modeled variations: present, past, future
Solar-Terrestrial Influence
Past Climate: Maunder Minimum
How much influence comes from the Sun

2. Total solar irradiance observations
UARS
SOHO
space era solar activity is historically high 20
Modern
Maximum
cycle 0 10
Maunder Minimum
Sunspot Number

3. Total solar irradiance database
Version: 24.00
composite_d24_00.asc
The dispersion of the original data is more than 7 times the solar cycle amplitude. The trend of the composite (difference between minima) is +7 ppm. Data and plots at:

4. Total solar irradiance database: Differences from composite
drifts in radiometer stability can reach fractions of the solar cycle amplitude
largest drifts tend to occur at start of mission
the most controversial changes in HF are the two glitches in late 1989

5. Total solar irradiance variability
% 27-day solar rotation
0.1% (1000 ppm) 11-year solar cycle
longer-term variations not yet reliably detected
composite total solar irradiance record: Fröhlich & Lean, GRL, 1998
solar irradiance increases when solar activity is high

6. Can magnetic fields explain irradiance variability directy?
TSI correlates poorly with global magnetic field

7. Magnetic sources of irradiance dimming
Sunspots:
Magnetic sources of irradiance dimming
Bolometric Sunspot Blocking:
PS= FS/FQ
= ASPOT[CS-1](3+2)/2
MDI 29 Mar 2001
FS irradiance change from spot
FQ quite Sun irradiance
 wavelength
ASPOT fractional disk area of spot
 heliocentric location
CS contrast (area-dependent) of spot
(3+2)/2 .. center-to-limb function
Hudson et al., 1982; Fröhlich et al.,1994; Brandt et al., 1994; Chapman et al., 1996

8. Rotation of sunspots causes large dips in total solar irradiance
ROME PSPT IMAGES
sunspots do not account for all variability during solar rotation:
PS uncertainties
other variability sources

9. Sunspots cannot account for the solar irradiance cycle varibility
sunspots cause net irradiance decrease of
 1 Wm2 during the solar cycle

10. Composite chromospheric irradiance index
BBSO Ca K
MgII index:
ratio of core-to-wing emission in Fraunhofer line near 280 nm
wing
wing
core
Lean et al., JGR, 106, 10645, 2001

11. Total solar irradiance brightness residuals track chromospheric index
Residual = F –FQ-FQxPs
highly correlated r=0.95
similar power distribution
Resid = ± 0.06
± 0.5ICH

12. Magnetic sources of irradiance brightening
Faculae
Magnetic sources of irradiance brightening
1. Empirical Relation with Chromospheric Index:
FF= a + bICH
2. Bolometric Facular Brightening:
PF= FF/FQ
= 5AFAC[CF-1]R(, )/2
FF irradiance change from faculae
FQ quite Sun irradiance
 wavelength
AFAC fractional disk area
 heliocentric location
CF facular contrast
R center-to-limb function
PSPT 29 Mar 2001

13. Total solar irradiance variability
model formulation
Irradiance =
Quiet Sun Irradiance
Sunspot Blocking
Facular Brightening + +
F(t) = FQ FS(t) + FF (t)
Approaches:
1. F(t) = a + bPs(t) + cICHst(t) + dICHlt(t)
2. F(t) = FQ(1+ Ps(t)) + [a + bICH (t)]
3. F(t) = FQ (1 + Ps(t) + PF (t))
Fröhlich & Lean, GRL, 1998
Foukal & Lean, ApJ, 1988
Lean et al., ApJ, 1998

14. Models of total irradiance variability based on PSI and MgII

15. Empirical models of total irradiance variability account for >85% of variance
Trend corresponds to -3.3 ppm/a. Compared to the 2s uncertainty of the composite of ±3 ppm/a this is barely significant.

16. Model accounts for observed total irradiance rotation and cycle

17. Sources of irradiance variability are wavelength dependent
Solar Active Region: BBSO Image
(Y. Unruh)
faculae
sunspots
Band Contribution to TSI
UV ~ 8%
VIS~44%
 IR ~48%
 EUV <0.0004%
(Y. Unruh)

18. Solar irradiance and the Earth’ climate

19. Temperature record of northern hemisphere
Maunder
minimum

20. Long-term solar activity
Solar activity proxies -- cosmogenic isotopes in tree-rings and ice-cores (below), geomagnetic activity, and the range of variability in Sun-like stars (right) -- suggest that long-term fluctuations in solar activity exceed the range of contemporary cycles.
Number
Solar Activity
Proxies
Ca Brightness of Sun-like Stars
Stuiver & Braziunas, 1993
Baliunas & Jastrow, 1990
DATA SOURCES:
Beer et al., 1988

21. Solar twins and sun-like stars in cluster M67
The solar-type stars in the open cluster M67 (constellation Cancer) have solar-age and solar-metallicity: 76 ‘solar-type’ stars (with unreddened colors in the range <= B-V <= +0.76) and 21 ‘solar-twins’ (+0.63 <= B-V <= +0.67) have been observed (Giampapa et al. 2000)

22. Solar-stellar connection and reconstruction of solar irradiance

23. Climate models forced by TSI variability

24. Future total solar irradiance and climate forcing
11-year cycles based on
Schatten et al., Hathaway et al., Thompson, 1993
background is ±0.04Wm-2/year
Lean, GRL, 2001
Anthropogenic Scenarios
IS92a
IPCC, 1995
Alternative
Hansen et al, 2000
Sun’s role in future climate change depends on irradiance cycles and trends relative to anthropogenic scenarios

25. Summary: TSI variability, solar-stellar connection and Earth’ climate
Long-term trend during last 23 years:
approx. 0.7 ± 3 ppm/a.
Variations are related to magnetic features:
sunspot darkening and faculae brightening
empirical models account for a large part (>90%) of the observed variations.
Long-term changes of TSI influence climate:
extrapolation to past still quite uncertain; the sun has probably not influenced our climate during the past years. Before, at most ½ of the climate change could be due to the sun.
changes of spectral distribution may be more important for sun-climate connection than just (energetic) changes of TSI.