[Paper Review] The Impact of the Revised Sunspot Record on Solar Irradiance Reconstructions

Greg Kopp, N. A. Krivova+2 more|Max Planck Digital Library|Jan 20, 2016

Solar and Space Plasma Dynamics3 citations

TL;DR

This study evaluates how the revised Sunspot Index and Long-term Solar Observations (SILSO) sunspot record affects reconstructions of total solar irradiance (TSI). Using the NRLTSI2 and SATIRE models, it finds that the new record increases amplitude of solar cycle fluctuations before 1885 but has minimal impact on Maunder Minimum TSI values, implying only minor changes to estimated solar contributions to climate change over the past few centuries.

ABSTRACT

Reliable historical records of total solar irradiance (TSI) are needed for climate change attribution and research to assess the extent to which long-term variations in the Sun's radiant energy incident on the Earth may exacerbate (or mitigate) the more dominant warming in recent centuries due to increasing concentrations of greenhouse gases. We investigate potential impacts of the new Sunspot Index and Long-term Solar Observations (SILSO) sunspot-number time series on model reconstructions of TSI. In contemporary TSI records, variations on time scales 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 the reconstruction of historical TSI. Revisions to the sunspot-number record therefore affect the magnitude and temporal structure of TSI variability on centennial time scales according to the model reconstruction methodologies. 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 greater amplitude solar-cycle fluctuations in TSI reconstructions prior, suggesting many 18th and 19th 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 comparing that era to the present we find only very small potential differences in estimated solar contributions to climate with this new sunspot record.

Motivation & Objective

• To assess the impact of the revised SILSO sunspot-number record on historical total solar irradiance (TSI) reconstructions.
• To evaluate whether updated sunspot data alters estimates of solar forcing during key climate periods such as the Maunder Minimum.
• To determine how changes in the sunspot record affect model-based reconstructions of long-term TSI variability.
• To compare the effects of the new sunspot record on two widely used TSI reconstruction models: NRLTSI2 and SATIRE.
• To quantify the implications for climate change attribution by assessing changes in solar irradiance magnitude and temporal structure over centuries.

Proposed method

• Utilizes the revised SILSO sunspot-number time series as a proxy for solar surface magnetic activity.
• Applies the NRLTSI2 and SATIRE models to reconstruct TSI using the updated sunspot data.
• Models TSI based on the opposing effects of sunspot darkening and facular brightening, both linked to magnetic activity.
• Compares TSI reconstructions from the revised sunspot record with those from previous versions, focusing on pre-1885 and modern periods.
• Analyzes temporal structure and amplitude of solar cycles in reconstructions to assess changes in variability over centennial timescales.
• Evaluates differences in TSI during the Maunder Minimum and modern maximum using both old and new sunspot records.

Experimental results

Research questions

• RQ1 How does the revised SILSO sunspot record affect the amplitude of reconstructed solar cycles before 1885?
• RQ2 What is the impact of the new sunspot data on TSI reconstructions during the Maunder Minimum?
• RQ3 How do the NRLTSI2 and SATIRE models respond differently to the revised sunspot-number series?
• RQ4 To what extent does the updated sunspot record alter estimates of solar contributions to climate change over the last few centuries?
• RQ5 Are there significant changes in the magnitude or temporal structure of long-term TSI variability due to the revised sunspot data?

Key findings

• The revised SILSO sunspot record leads to greater amplitude solar-cycle fluctuations in TSI reconstructions prior to 1885 compared to previous records.
• After 1885, the revised sunspot data has little effect on either the NRLTSI2 or SATIRE TSI reconstructions.
• Many 18th and 19th century solar cycles are now estimated to be similar in amplitude to those of the current Modern Maximum.
• TSI reconstructions based on the revised sunspot data do not suggest a significant change in TSI values during the Maunder Minimum.
• The comparison between the Maunder Minimum and present-day TSI levels shows only very small potential differences in estimated solar contributions to climate change.
• The updated sunspot record does not substantially alter the overall assessment of solar forcing in climate change attribution studies.