[Paper Review] Solar total and spectral irradiance reconstruction over the last 9000 years

Chi‐Ju Wu, N. A. Krivova+2 more|Springer Link (Chiba Institute of Technology)|Nov 8, 2018

Solar Radiation and Photovoltaics4 citations

TL;DR

This study presents the first physics-based reconstruction of solar total and spectral irradiance over the last 9,000 years using cosmogenic isotopes 14C and 10Be as proxies for solar activity. By applying updated SATIRE models to individual and composite isotope records, it reveals a millennial-scale TSI variability of ~0.11% (1.5 W/m²), with strong agreement between isotope-based reconstructions and direct observations post-Maunder Minimum.

ABSTRACT

Changes in solar irradiance and in its spectral distribution are among the main natural drivers of the climate on Earth. However, irradiance measurements are only available for less than four decades, while assessment of solar influence on Earth requires much longer records. The aim of this work is to provide the most up-to-date physics-based reconstruction of the solar total and spectral irradiance (TSI/SSI) over the last nine millennia. The concentrations of the cosmogenic isotopes 14C and 10Be in natural archives have been converted to decadally averaged sunspot numbers through a chain of physics-based models. TSI and SSI are reconstructed with an updated SATIRE model. Reconstructions are carried out for each isotope record separately, as well as for their composite. We present the first ever SSI reconstruction over the last 9000 years from the individual 14C and 10Be records as well as from their newest composite. The reconstruction employs physics-based models to describe the involved processes at each step of the procedure. Irradiance reconstructions based on two different cosmogenic isotope records, those of 14C and 10Be, agree well with each other in their long-term trends despite their different geochemical paths in the atmosphere of Earth. Over the last 9000 years, the reconstructed secular variability in TSI is of the order of 0.11%, or 1.5 W/m2. After the Maunder minimum, the reconstruction from the cosmogenic isotopes is consistent with that from the direct sunspot number observation. Furthermore, over the nineteenth century, the agreement of irradiance reconstructions using isotope records with the reconstruction from the sunspot number by Chatzistergos et al. (2017) is better than that with the reconstruction from the WDC-SILSO series (Clette et al. 2014), with a lower chi-square-value.

Motivation & Objective

• To reconstruct solar total and spectral irradiance (TSI/SSI) over the last 9,000 years using cosmogenic isotope proxies.
• To address the lack of long-term irradiance records beyond the 40-year satellite era for climate impact assessment.
• To improve upon previous reconstructions by using physics-based models rather than linear regressions.
• To assess the consistency and reliability of TSI/SSI reconstructions derived from individual 14C and 10Be records versus their composite.
• To provide a benchmark dataset for paleoclimatic modeling of solar forcing over the Holocene.

Proposed method

• Converted decadally averaged 14C and 10Be concentrations in terrestrial archives into sunspot numbers using physics-based models of atmospheric ionization and deposition.
• Employed the updated SATIRE-M model to reconstruct TSI and SSI based on the reconstructed sunspot number time series.
• Applied the SATIRE-T model with revised parameters for ephemeral region cycles, spatial distribution of faculae and sunspots, and reference irradiance contributions.
• Used independent input series: WDC-SILSO sunspot number for recent periods and isotope-derived sunspot numbers for earlier times.
• Validated reconstructions against direct satellite measurements (1978–present) and independent open magnetic flux reconstructions.
• Generated SSI reconstructions from UV to far-IR by modeling contributions from faculae, plages, and sunspots based on their magnetic field properties.

Experimental results

Research questions

• RQ1 How accurately can solar TSI and SSI be reconstructed over the last 9,000 years using cosmogenic isotope records?
• RQ2 To what extent do reconstructions based on 14C and 10Be isotopes agree with each other and with direct observations?
• RQ3 How does the use of a composite 14C–10Be record improve the reliability of long-term solar irradiance reconstructions?
• RQ4 What is the magnitude of secular TSI variability over the Holocene, and how does it compare to recent instrumental trends?
• RQ5 How well do physics-based models reproduce observed irradiance variability without free parameters?

Key findings

• The reconstructed millennial-scale TSI variability over the last 9,000 years is approximately 0.11% (1.5 W/m²), consistent across all isotope-based reconstructions.
• Reconstructions from individual 14C and 10Be records show strong agreement in long-term trends despite differing atmospheric geochemical pathways.
• Post-Maunder Minimum reconstructions from cosmogenic isotopes are consistent with direct sunspot number observations.
• Over the 19th century, the isotope-based reconstruction shows better agreement with the Chatzistergos et al. (2017) sunspot number reconstruction than with the WDC-SILSO series, as indicated by a lower χ²-value.
• This is the first SSI reconstruction over the Holocene using a fully physics-based model chain from isotope data to spectral irradiance.
• The composite 14C–10Be record (Wu et al., 2018) provides a robust and recommended dataset for long-term climate modeling applications.