Editing IPCC:AR6/WGI/Chapter-2 (section)

=== 2.2.1 Solar and Orbital Forcing ===

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The AR5 assessed solar variability over multiple time scales, concluding that total solar irradiance (TSI) multi-millennial fluctuations over the past 9 kyr were &lt;1 W m <sup>–2</sup> , but with no assessment of confidence provided. For multi-decadal to centennial variability over the last millennium, AR5 emphasized reconstructions of TSI that show little change (&lt;0.1%) since the Maunder Minimum (1645–1715) when solar activity was particularly low, again without providing a confidence level. The AR5 further concluded that the best estimate of radiative forcing due to TSI changes for the period 1750–2011 was 0.05–0.10 W m <sup>–2</sup> ( ''medium confidence'' ), and that TSI ''very likely'' changed by –0.04 [–0.08 to 0.00] W m <sup>–2</sup> between 1986 and 2008. Potential solar influences on climate due to feedbacks arising from interactions with galactic cosmic rays are assessed in Section 7.3.4.5.

Slow periodic changes in the Earth’s orbit around the Sun mainly cause variations in seasonal and latitudinal receipt of incoming solar radiation. Precise calculations of orbital variations are available for tens of millions of years ( [[#Berger--1991|Berger and Loutre, 1991]] ; [[#Laskar--2011|Laskar et al., 2011]] ). The range of insolation averaged over boreal summer at 65°N was about 83 W m <sup>−2</sup> during the past million years, and 3.2 W m <sup>−2</sup> during the past millennium, but there was no substantial effect upon global average radiative forcing (0.02 W m <sup>–2</sup> during the past millennium).

A new reconstruction of solar irradiance extends back 9 kyr based upon updated cosmogenic isotope datasets and improved models for production and deposition of cosmogenic nuclides ( [[#Poluianov--2016|Poluianov et al., 2016]] ), and shows that solar activity during the second half of the 20th century was in the upper decile of the range. TSI features millennial-scale changes with typical magnitudes of 1.5 [1.4 to 2.1] W m <sup>–2</sup> (C.-J. [[#Wu--2018|]] [[#Wu--2018|Wu et al., 2018]] ). Although stronger variations in the deeper past cannot be ruled out completely ( [[#Egorova--2018|Egorova et al., 2018]] ; [[#Reinhold--2019|Reinhold et al., 2019]] ), there is no indication of such changes having happened over the last 9 kyr.

Recent estimates of TSI and spectral solar irradiance (SSI) for the past millennium are based upon updated irradiance models (e.g., [[#Egorova--2018|Egorova et al., 2018]] ; C.-J. [[#Wu--2018|]] [[#Wu--2018|Wu et al., 2018]] ) and employ updated and revised direct sunspot observations over the last three centuries ( [[#Clette--2014|Clette et al., 2014]] ; [[#Chatzistergos--2017|Chatzistergos et al., 2017]] ) as well as records of sunspot numbers reconstructed from cosmogenic isotope data prior to this ( [[#Usoskin--2016|Usoskin et al., 2016]] ). These reconstructed TSI time series (Figure 2.2a) feature little variation in TSI averaged over the past millennium. The TSI between the Maunder Minimum (1645–1715) and second half of the 20th century increased by 0.7–2.7 W m <sup>–2</sup> ( [[#Jungclaus--2017|Jungclaus et al., 2017]] ; [[#Egorova--2018|Egorova et al., 2018]] ; [[#Lean--2018|Lean, 2018]] ; C.-J. [[#Wu--2018|]] [[#Wu--2018|Wu et al., 2018]] ; [[#Lockwood--2020|Lockwood and Ball, 2020]] ; [[#Yeo--2020|Yeo et al., 2020]] ). This TSI increase implies a change in ERF of 0.09–0.35 W m <sup>–2</sup> (Section 7.3.4.4).

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[[File:9c8adae7a832062e6d12e3e88dba7c14 IPCC_AR6_WGI_Figure_2_2.png]]

'''Figure''' '''2.2 |''' '''Time series of solar and volcanic forcing for the past 2500 years (a, c) and since 1850 (b, d). (a)''' Total solar irradiance (TSI) reconstruction (10-year running averages) recommended for CMIP6/PMIP4 millennial experiments based on the radiocarbon dataset before 1850 (blue) scaled to the CMIP6 historical forcing after 1850 (purple). '''(b)''' TSI time series (six-month running averages) from CMIP6 historical forcing as inferred from sunspot numbers (blue), compared to CMIP5 forcing based on (red) and an update to CMIP6 by a TSI composite (orange). '''(c)''' Volcanic forcing represented as reconstructed stratospheric aerosol optical depth (SAOD; as presented in Section 7.3.4.6) at 550 nm. Estimates covering 500 BCE to 1900 CE (green) and 1850–2015 (blue). '''(d)''' SAOD reconstruction from CMIP6 (v 4) (blue), compared to CMIP5 forcing (red). Note the change in y-axis range between panels (c) and (d). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Estimation of TSI changes since 1900 (Figure 2.2b) has further strengthened, and confirms a small (less than about 0.1 W m <sup>–2</sup> ) contribution to global climate forcing (Section 7.3.4.4). New reconstructions of TSI over the 20th century ( [[#Lean--2018|Lean, 2018]] ; [[#Wu--2018|]] [[#Wu--2018|C.-J. Wu et al., 2018]] ) support previous results that the TSI averaged over the solar cycle ''very likely'' increased during the first seven decades of the 20th century and decreased thereafter (Figure 2.2b). TSI did not change significantly between 1986 and 2019. Improved insights ( [[#Krivova--2006|Krivova et al., 2006]] ; [[#Yeo--2015|Yeo et al., 2015]] , 2017; [[#Coddington--2016|Coddington et al., 2016]] ) show that variability in the 200–400 nm UV range was greater than previously assumed. Building on these results, the forcing proposed by [[#Matthes--2017|Matthes et al. (2017)]] has a 16% stronger contribution to TSI variability in this wavelength range compared to the forcing used in the 5th Phase of the Coupled Model Intercomparison Project (CMIP5).

To conclude, solar activity since the late 19th century was relatively high but not exceptional in the context of the past 9 kyr ( ''high confidence'' ). The associated global mean ERF is in the range of –0.06 to +0.08 W m <sup>–2</sup> (Section 7.3.4.4).

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