The Impact of the Revised Sunspot Record on Solar Irradiance Reconstructions
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Reliable historical records of the total solar irradiance (TSI) are needed to assess the extent to which long-term variations in the Sun’s radiant energy that is incident upon Earth may exacerbate (or mitigate) the more dominant warming in recent centuries that is due to increasing concentrations of greenhouse gases. We investigate the effects that the new Sunspot Index and Long-term Solar Observations (SILSO) sunspot-number time series may have on model reconstructions of the TSI. In contemporary TSI records, variations on timescales longer than about a day are dominated by the opposing effects of sunspot darkening and facular brightening. These two surface magnetic features, retrieved either from direct observations or from solar-activity proxies, are combined in TSI models to reproduce the current TSI observational record. Indices that manifest solar-surface magnetic activity, in particular the sunspot-number record, then enable reconstructing historical TSI. Revisions of the sunspot-number record therefore affect the magnitude and temporal structure of TSI variability on centennial timescales according to the model reconstruction methods that are employed. We estimate the effects of the new SILSO record on two widely used TSI reconstructions, namely the NRLTSI2 and the SATIRE models. We find that the SILSO record has little effect on either model after 1885, but leads to solar-cycle fluctuations with greater amplitude in the TSI reconstructions prior. This suggests that many eighteenth- and nineteenth-century cycles could be similar in amplitude to those of the current Modern Maximum. TSI records based on the revised sunspot data do not suggest a significant change in Maunder Minimum TSI values, and from comparing this era to the present, we find only very small potential differences in the estimated solar contributions to the climate with this new sunspot record.
KeywordsTotal solar irradiance TSI Solar variability Climate change
We gratefully acknowledge the support of NASA’s SORCE (NAS5-97045) and SIST (NNX15AI51G) for this effort. J. Lean appreciates collaboration with Odele Coddington in developing the NOAA Solar Irradiance CDR. The authors also appreciate helpful suggestions from the referee. The SILSO data are courtesy of WDC-SILSO, Royal Observatory of Belgium, Brussels. Figure 1 includes data from www.ngdc.noaa.gov/stp/SOLAR/solar.html (NIMBUS7/ERB, ERBS/ERBE, NOAA9, and NOAA10); www.acrim.com (ACRIM1, ACRIM2, and ACRIM3); the VIRGO team via ftp://ftp.pmodwrc.ch ; lasp.colorado.edu/home/sorce/data/tsi-data/ (SORCE/TIM); the PICARD/PREMOS team (personal communication), A. Fehlmann, 2014; and lasp.colorado.edu/home/tcte/data/ (TCTE/TIM).
Disclosure of Potential Conflicts of Interest
The authors declare that they have no conflicts of interest.
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