Is the Sun Responsible for Recent Warming?

In a previous post, I pointed out that I find the notion of quantifying "consensus" to have limited value, but one value I admit it can have is to show areas of climate science about which practicing scientists no longer in actively debate in the literature. This allows us to understand what debates in popular discourse do not translate into debate among climate scientists. One of the most common objections to AGW I hear in popular circles is that warming is caused by the Sun, rather than human activity. The most common way I see this claim is with reference to solar variability in terms of total solar irradiance (TSI). Variability in TSI can be caused by two things: changes in solar output, like the 11-year solar cycle, or changes in the distance between the earth and the Sun. Variability in the Earth's orbit around the Sun, often referred to as orbital cycles, do not change the total amount of energy the Earth receives in a year, but they can change the distribution of energy, either between the equator and poles or throughout the year. Orbital cycles have known periodicities that are too long to be relevant to recent warming, so these can easily be excluded as an explanation for recent warming (but see below for a variation on this theme).

However, we now have precise measurements of solar variability, both as satellite TSI measurements in recent decades and reconstructions of TSI for hundreds of years. I've covered these reconstructions in other posts. The conclusive evidence we have is that the Sun is a stable star, varying by about 0.1% over time scales relevant to AGW. Scientists can quantify the forcings associated with solar variability and compare them to anthropogenic forcings from greenhouse gases and aerosol pollution. The comparisons between these forcings are accurate regardless of sensitivity, since a change in radiative forcing causes the same change in temperature regardless of causes of the change. Changes in solar forcing has been indisputably small compared to increasesbin greenhouse gas concentrations. 

There are at least two reasons why scientists have conclusively ruled out changes in TSI as an explanation for a significant fraction of global warming since 1950. First, over the last 60 years or so, TSI has been stable to decreasing. Second, the stratosphere is cooling while the troposphere warms. For the Sun to be a cause of current warming of the troposphere, we'd expect to see a warming in all the layers of the atmosphere, including the stratosphere, which isn't happening. The evidence is conclusive, and meaningful scientific debate among climate scientists is over. Here are two "exceptions" that prove the rule.

Connolly et al 2021

Connolly & Soon published a paper in a low impact journal in 2021 attempting to show that the the debate is not over on the role of the Sun in current warming. It was not well received, and I've posted rebuttals of it in other posts. The paper uses TSI reconstructions that are known to be wrong, and it assumes that urbanization is causing bias in NH temperature trends. But recently, a rebuttal paper was published demonstrating crucial errors in their calculations. "Should scientists rely on calculations we know are inaccurate? We strongly believe no: errors should be corrected. In our opinion, this is crucial not just for success in science, but for the credibility of science. Our position is that clearly the Connolly et al. approach is nonsense, there is no evidence for the paper’s main claim and it should be corrected or retracted."[1][2] I don't know if Connolly's paper will be retracted, but the paper is clearly flawed, and it has failed to generate any legitimate debate over the role of the Sun in global warming.

Correction to Connolly et al 2021

The above graph illustrates the correct magnitude of solar forcings and anthropogenic forcings plotted in units of Temperature. Solar forcings are indisputably small while greenhouse gas forcings (partially masked by aerosols) are indisputably large. The human forcings do a pretty good job of explaining the overall trend in global warming.

Zharkova et al 2019

Zharkova et al 2019 also attempted to blame global warming over the last two centuries on changes in the distance between the Earth and Sun. If you take all the mass of the solar system, most of it is in the Sun, but the masses of the larger planets, especially Jupiter, exert a large enough pull that their orbits actually affect the Sun's position relative to the barycentre of the Solar System. This paper suggested that this internal motion has resulted in reducing the average distance between the Earth and Sun, thus causing an increase in TSI that explains current warming.  That paper was published in a reputable journal, but was heavily criticized after publication, even by knowledgeable bloggers. Ken Rice's criticism was scathing: "However, in the case of the Zharkova et al. paper, the error is completely elementary. It’s something we teach our first-year students. There is no value in debating in the literature something that has been accepted by virtually all physicists/astronomers for a very long time. The community shouldn’t have to commit time and effort to correcting a basic error made by people who really should know better."[3][4][5] The paper was retracted because of its numerous flaws.[6]

Conclusion

Any honest accounting of anthropogenic vs solar forcings demonstrates that anthropogenic forcings are the cause of virtually all the warming above the 1850-1900 man. The cooling effect of aerosols have  to date largely canceled out the warming of greenhouse gases other than CO2, so if you plot CO2 forcings, they make a pretty good match with global temperatures, and the correlation between CO2 forcings and GMST is r^2 =0.872.

These papers are attempting to generate renewed debate about the role of the Sun in climate change, but they have only been able to do so by making mathematical and scientific blunders.[7] The result is that the papers either become retracted (Zharkova) or ignored/dismissed as irrelevant (Connolly). There is no longer any scientific argument that variability in TSI is responsible for a significant fraction of current warming.

References:

[1] Serious Mistakes Found in Recent Paper by Connolly. RealClimate.
https://www.realclimate.org/index.php/archives/2022/11/serious-mistakes-found-in-recent-paper-by-connolly-et-al/

[2] Mark T. Richardson and Rasmus E. Benestad 2022 Res. Astron. Astrophys. 22 125008
https://iopscience.iop.org/article/10.1088/1674-4527/ac981c

[3] Ken Rice. Retract. ...And Then There's Physics.
https://andthentheresphysics.wordpress.com/2019/07/25/retract/

[4] Ken Rice. Nature Scientific Reports. ...And Then There's Physics.
https://andthentheresphysics.wordpress.com/2019/07/07/nature-scientific-reports/

[5] Ken Rice. Zharkova et al An Update. ...And Then There's Physics.
https://andthentheresphysics.wordpress.com/2020/01/13/zharkova-et-al-an-update/

[6] The flaws lead to its retraction, though only one of the authors of the paper agreed with the retraction.
https://www.nature.com/articles/s41598-020-61020-3

[7] Why are so many Solar-Climate Papers Flawed? RealClimate.
https://www.realclimate.org/index.php/archives/2020/03/why-are-so-many-solar-climate-papers-flawed/