Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.3.4.4 Solar ====

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Variations in the total solar irradiance (TSI) represent a natural external forcing agent. The dominant cycle is the solar 11-year activity cycle, which is superimposed on longer cycles ( [[IPCC:Wg1:Chapter:Chapter-2#2.2|Section 2.2]] ). Over the last three 11-year cycles, the peak-to-trough amplitude in TSI has differed by about 1 W m <sup>–2</sup> between solar maxima and minima (Figure 2.2).

The fractional variability in the solar irradiance, over the solar cycle and between solar cycles, is much greater at short wavelengths in the 200–400 nanometre (nm) band than for the broad visible/infrared band that dominates TSI ( [[#Krivova--2006|Krivova et al., 2006]] ). The IRF can be derived simply by Δ ''TSI'' × (1 – albedo)/4 irrespective of wavelength, where the best estimate of the planetary albedo is usually taken to be 0.29 and Δ ''TSI'' represents the change in total solar irradiance ( [[#Stephens--2015|Stephens et al., 2015]] ). (The factor 4 arises because TSI is per unit area of Earth cross section presented to the Sun and IRF is per unit area of Earth’s surface). The adjustments are expected to be wavelength dependent. [[#Gray--2009|Gray et al. (2009)]] determined a stratospheric temperature adjustment of –22% to spectrally resolved changes in the solar radiance over one solar cycle. This negative adjustment is due to stratospheric heating from increased absorption by ozone at the short wavelengths, increasing the outgoing longwave radiation to space. A multi-model comparison ( [[#Smith--2018b|Smith et al., 2018b]] ) calculated adjustments of –4% due to stratospheric temperatures and –6% due to tropospheric processes (mostly clouds), for a change in TSI across the spectrum (Figure 7.4). The smaller magnitude of the stratospheric temperature adjustment is consistent with the broad spectral change rather than the shorter wavelengths characteristic of solar variation. A single-model study also found an adjustment that acts to reduce the forcing ( [[#Modak--2016|Modak et al., 2016]] ). While there has not yet been a calculation based on the appropriate spectral change, the –6% tropospheric adjustment from [[#Smith--2018b|Smith et al. (2018b)]] is adopted along with the [[#Gray--2009|Gray et al. (2009)]] stratospheric temperature adjustment. The ERF due to solar variability over the historical period is therefore represented by 0.72 × Δ ''TSI'' × (1 – albedo)/4 using the TSI timeseries from ( [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] [[IPCC:Wg1:Chapter:Chapter-2#2.2.1|Section 2.2.1]] ).

The AR5 ( [[#Myhre--2013b|Myhre et al., 2013b]] ) assessed solar SARF from around 1750 to 2011 to be 0.05 [0.00 to 0.10] W m <sup>–2</sup> which was computed from the seven-year mean around the solar minima in 1745 (being closest to 1750) and 2008 (being the most recent solar minimum). The inclusion of tropospheric adjustments that reduce ERF (compared to SARF in AR5) has a negligible effect on the overall forcing. Prior to the satellite era, proxy records are used to reconstruct historical solar activity. In AR5, historical records were constructed using observations of solar magnetic features. In this assessment historical time series are constructed from radiogenic compounds in the biosphere and in ice cores that are formed from cosmic rays ( [[#Steinhilber--2012|Steinhilber et al., 2012]] ).

In this assessment the TSI from the Paleoclimate Model Intercomparison Project Phase 4 (PMIP4) reconstruction is used ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.1|Section 2.2.1]] ; [[#Jungclaus--2017|Jungclaus et al., 2017]] ). Proxies constructed from the <sup>14</sup> C and <sup>10</sup> Be radiogenic records for the SATIRE-M model ( [[#Vieira--2011|Vieira et al., 2011]] ) and <sup>14</sup> C record for the PMOD model ( [[#Shapiro--2011|Shapiro et al., 2011]] ) for the 1745 solar minimum provide ERFs for 1745–2008 of –0.01, –0.02 and 0.00 W m <sup>–2</sup> respectively. An independent dataset from the National Oceanic and Atmospheric Administration’s Climate Data Record ( [[#Coddington--2016|Coddington et al., 2016]] ; [[#Lean--2018|Lean, 2018]] ) provides an ERF for 1745–2008 of +0.03 W m <sup>–2</sup> . One substantially higher ERF estimate of +0.35 W m <sup>–2</sup> derived from TSI reconstructions is provided by [[#Egorova--2018|Egorova et al. (2018)]] . However, the estimate from [[#Egorova--2018|Egorova et al. (2018)]] hinges on assumptions about long-term changes in the quiet Sun for which there is no observed evidence. [[#Lockwood--2020|Lockwood and Ball (2020)]] analysed the relationship between observed changes in cosmic ray fluxes and recent, more accurate, TSI data and derived ERF between –0.01 and +0.02 W m <sup>–2</sup> , and [[#Yeo--2020|Yeo et al. (2020)]] modelling showed the maximum possible ERF to be 0.26 ± 0.09 W m <sup>–2</sup> . Hence the [[#Egorova--2018|Egorova et al. (2018)]] estimate is not explicitly taken into account in the assessment presented in this section.

In contrast to AR5, the solar ERF in this assessment uses full solar cycles rather than solar minima. The pre-industrial TSI is defined as the mean from all complete solar cycles from the start of the <sup>14</sup> C SATIRE-M proxy record in 6755 BCE to 1744 CE. The mean TSI from solar cycle 24 (2009–2019) is adopted as the assessment period for 2019. The best estimate solar ERF is assessed to be 0.01 W m <sup>–2</sup> , using the <sup>14</sup> C reconstruction from SATIRE-M, with a ''likely'' range of –0.06 to +0.08 W m <sup>–2</sup> ( ''medium confidence'' ). The uncertainty range is adopted from the evaluation of [[#Lockwood--2020|Lockwood and Ball (2020)]] using a Monte Carlo analysis of solar activity from the Maunder Minimum to 2019 from several datasets, leading to an ERF of –0.12 to +0.15 W m <sup>–2</sup> . The [[#Lockwood--2020|Lockwood and Ball (2020)]] full uncertainty range is halved as the period of reduced solar activity in the Maunder Minimum had ended by 1750 ( ''medium confidence'' ).

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