The Dimmest State of the Sun

Abstract

How the solar electromagnetic energy entering the Earth's atmosphere varied since preindustrial times is an important consideration in the climate change debate. Detrimental to this debate, estimates of the change in total solar irradiance (TSI) since the Maunder minimum, an extended period of weak solar activity preceding the industrial revolution, differ markedly, ranging from a drop of 0.75 W m −2 to a rise of 6.3 W m −2. Consequently, the exact contribution by solar forcing to the rise in global temperatures over the past centuries remains inconclusive. Adopting a novel approach based on state-of-the-art solar imagery and numerical simulations, we establish the TSI level of the Sun when it is in its least-active state to be 2.0 ± 0.7 W m −2 below the 2019 level. This means TSI could not have risen since the Maunder minimum by more than this amount, thus restricting the possible role of solar forcing in global warming. Plain Language Summary How the amount of energy the Earth receives from the Sun varied since preindustrial times is an important consideration in the climate change debate. Detrimental to this debate, it is not known whether the Sun grew brighter or dimmer since the 16th century, and by how much. As a consequence, the exact contribution by fluctuations in the brightness of the Sun to the rise in global temperatures over the past centuries remains controversial. It is established that the Sun was particularly inactive over much of the 16th century. Adopting a novel approach based on state-of-the-art solar imagery and computer models, we determined the brightness of the Sun when it is in its least-active state possible. This places a strict limit on how much the Sun could have grown brighter since the lull in solar activity over the 16th century, restricting the possible role the Sun could have played in global warming. Since the reliable measurement of solar irradiance only started in 1978 (Kopp, 2014), climate simulations rely on models of solar irradiance variability to provide the requisite historical solar forcing input. Except at RESEARCH LETTER

Modelling the evolution of the Sun’s open and total magnetic flux

“…The higher level of the magnetic flux during periods of very low solar activity (e.g., during the Maunder and the Dalton minima, see Fig. 4) will presumably lead to a weaker secular variation in the total solar irradiance (TSI) and in particular the rise in the TSI since the Maunder minimum (cf., Yeo et al 2020), an important parameter for understanding the influence of solar irradiance variability on long-term climate A70, page 7 of 8 trends (e.g., Gray et al 2010;Solanki et al 2013). The influence of the revised magnetic flux time series on TSI will be the subject of a future investigation.…”

Changes in the Total Solar Irradiance and climatic effects

“…In principle, these reconstructions have no lower limit for the TSI corresponding to minimal magnetic activity. However, in practice, because based on today's observational resolution, it is found that the magnetic content of the quiet Sun is found to be not much larger than about the amplitude to the maximum, the lowest limit to the dimmest state of the Sun has been derived to be about 2 W/m 2 below the 2019 minimum (Yeo et al, 2020). Thus, the reduction of a 31-year average TSI during the Maunder minimum relative to today's TSI would be about 2.5 W/m 2 .…”

Irradiance Variations of the Sun and Sun-Like Stars – Overview of Topical Collection

“…Recently, the MURaM simulations accounting for the effect of a small-scale dynamo became available (see, e.g., Rempel, 2018Rempel, , 2020, and the references therein). Utilising these simulations, Yeo et al (2020) found that the change of the TSI between 2019 and Maunder minimum could not be larger than 2 ± 0.7 W m −2 .…”

“…An improved understanding of the interactions between magnetic fields and matter in stellar atmospheres brought about by MHD simulations constitutes another important area of progress in modeling solar and stellar brightness variations. The MHD simulations of the solar atmosphere have already been used for reconstructing TSI variability and for constraining its secular component (Rempel, 2020;Yeo et al, 2020). In the absence of any direct observations of stellar magnetic features, there have been a number of attempts to modify solar semi-empirical models and apply them for modeling brightness variations of solar-like stars (see, e.g., Witzke et al, 2018Witzke et al, , 2020.…”

“…Empirical and semi-empirical solar-irradiance models can now reproduce the measurements of solar-irradiance variability in great detail (Ermolli et al, 2013;Solanki, Krivova, and Haigh, 2013;Coddington et al, 2019). Currently, a new generation of irradiance models based on realistic 3D magneto-hydrodynamic simulations and comprehensive radiative-transfer codes with improved atomic and molecular data are being developed (see, e.g., Yeo et al, 2020). These models are free from empirical adjustments and provide important constraints on historical solar-irradiance variability.…”