Sun-like Stars Shed Light on Solar Climate Forcing

Abstract: Recently published, precise stellar photometry of 72 Sun-like stars obtained at the Fairborn Observatory between 1993 and 2017 is used to set limits on the solar forcing of Earth's atmosphere of $\pm$ 4.5 W m$^{-2}$ since 1750. This compares with the +2.2 $\pm$ 1.1 W m$^{-2}$ IPCC estimate for anthropogenic forcing. Three critical assumptions are made. In decreasing order of importance they are: (a) most of the brightness variations occur within the average time-series length of $\approx$17 years; (b) the Sun seen from the ecliptic behaves as an ensemble of middle-aged solar-like stars; and (c) narrow-band photometry in the Str\"omgren $b$ and $y$ bands are linearly proportional to the total solar irradiance. Assumption (a) can best be relaxed and tested by obtaining more photometric data of Sun-like stars, especially those already observed. Eight stars with near-solar parameters have been observed from 1999, and two since 1993. Our work reveals the importance of continuing and expanding ground-based photometry, to complement expensive solar irradiance measurements from space.

• The paper estimates solar forcing of about -1.5 W m⁻² since 1750 using precise photometry of Sun-like stars.
• It rigorously selects a subset of 22 stars from 72 based on activity and luminosity to mirror solar behavior.
• The study highlights the need for extended observations to improve solar irradiance reconstructions and climate impact assessments.

Analysis of Stellar Photometry to Constrain Solar Climate Forcing

The paper, "Sun-like stars shed light on solar climate forcing," by Judge et al., investigates limits on the solar forcing of Earth's atmosphere since 1750 by utilizing precise stellar photometry of 72 Sun-like stars. The core objective is to improve understanding of solar irradiance's variability and its potential impact on climate change, using data obtained from the Fairborn Observatory between 1993 and 2017.

Research Framework and Assumptions

The study hinges on three key assumptions:

1. The brightness variations of solar analogs occur predominantly within the observed time-series length of approximately 17 years.
2. The Sun's behavior, as observed from Earth, resembles that of an ensemble of middle-aged Sun-like stars.
3. The narrow-band photometry in the Strömgren $b$ and $y$ bands is linearly proportional to the Total Solar Irradiance (TSI).

These assumptions are central to the methodology, as they provide a framework for extrapolating solar behavior from observed stellar data, particularly concerning long-term variability and its climatic effects.

Data Sources and Analysis

The research relies on two primary datasets:

• Photometric measurements of Sun-like stars from robotic telescopes at Fairborn Observatory covering up to 24 years.
• Chromospheric activity data converted to $R'_{\mathrm HK}$ metrics obtained from the Lowell Observatory.

The study's authors conducted rigorous vetting of these data, including the exclusion of outlier stars based on specific selection criteria that emphasized similarity to solar characteristics such as activity levels and luminosity class.

Numerical Results

An ensemble of 22 stars, selected based on the activity parameter $\log R^\prime_{HK} \le -4.8$, was identified as most representative of solar behavior. From this subset, an ensemble mean gradient in stellar brightness was calculated, leading to an estimate of solar forcing of about -1.5 W~m$^{-2}$ since 1750. This forcing estimate is crucial as it draws a contemporary comparison with the IPCC estimate of 2.2 ± 1.1 W~m$^{-2}$ for anthropogenic forcing.

Implications and Future Directions

The use of stellar photometry presents an alternative method to traditional TSI reconstructions. The methodology offers potential improvements over proxies and ad-hoc corrections commonly associated with extended solar irradiance datasets. The results underscore the need for continuous and extended observations to capture lower-frequency variance components, thus solidifying the constraints on long-term solar variability.

The research highlights two main pathways for future development:

1. Extending existing time series beyond 17 years to encompass more cycles, thereby reducing uncertainties and confirming or refuting the stability of solar variability assumptions.
2. Increasing the sample size of observed Sun-like stars, which would further diminish prediction uncertainties through enhanced statistical robustness.

Ultimately, the paper stresses the importance of continued investment in ground-based photometry to complement other expensive space-based irradiance measurements. These investments could sharpen the contrast between natural and anthropogenic climate forcings, offering stronger evidence to assess future solar impact on Earth's climate systems. Despite inherent challenges, the potential benefits of these constraints justify continued efforts in both data acquisition and methodological refinement.