By Andy May

In May 2023, Clintel published a book (see figure 1) criticizing AR6 (IPCC, 2021), a publication that was supposed to summarize climate science research to date. We found that AR6 was biased in its reporting of recent developments in climate science, and they ignored published research contrary to their narrative that humans have caused all the warming since the Little Ice Age (the so called “preindustrial”), and that recent warming is somehow dangerous. Comments and reviews of the Clintel volume can be seen here and on Judith Curry’s website here.

Figure 1. The Clintel critique of the IPCC AR6 report. More details here or click on the image.

This post discusses a twitter debate about possible mistakes in the Clintel volume, specifically the Chapter 6 (written by Nicola Scafetta and Fritz Vahrenholt) discussion of the evidence that changes in the Sun affect Earth’s climate. We argue that recent evidence supports a role for the Sun in modern climate change, and the IPCC argues that the Sun has not contributed to recent (since 1750, see AR6, page 959, figure 7.6) warming or recent climate change.

We will see that Theodosios Chatzistergos, who also argues for no contribution from the Sun seems to confuse opinions with facts, and considers opinions different from his own as “mistakes.” This is a common problem with younger scientists, and undoubtably it is a product of poor scientific training in universities today. Opinions, regardless of who holds them, are not facts. Differing opinions, based on the same pool of evidence, are not mistakes, they are just different opinions. It is easy to see how “climate science” has devolved into “climate politics.”

Dr. Judith Curry praised the Clintel volume on twitter, which led to criticism from Dr. Theodosios Chatzistergos. Chatzistergos claims that Scafetta and Vahrenholt’s Chapter 6 had several errors, claims that I discuss in detail below.

Chatzistergos Point 1:

Chatzistergos points out that most TSI (total solar irradiance) composites agree with the IPCC favored PMOD composite and that all TSI composites show a declining trend since the mid-1990s. These points are questionable, because I would argue that RMIB (sometimes abbreviated IRMB) and NOAA composites are similar to ACRIM, see below and here, for more details on comparing the three composites. You can decide for yourself. All the composites are very similar, the differences are quite small and below the uncertainty in the data, see figures 5 and 6 here, and figure 2 below.

Figure 2. A comparison of the RMIB, PMOD, and ACRIM TSI composites with the uncertainty shown as gray shading. Notice the large reduction in uncertainty after 1996. The uncertainty assumes all TSI records are equally certain, allowing for the possibility that any of the composites could represent the longer-term secular trend. After­ (Coddington, et al., 2019).

Chatzistergos point is that the longer-term trend in solar activity can only be detected during solar cycle minima because solar cycle maxima are highly variable, yet the uncertainty in TSI does not drop enough to detect a possible trend until after 1996, all records more-or-less agree after that time. There are only two fully resolved solar cycle minima after 1996, including the most recent one. Two are not enough to resolve a trend with any confidence. Besides the critical difference in the longer-term trends occurs between 1985 and 1996 when the data during the ACRIM Gap (1989.5-1991.75) are very uncertain due to the trend difference between the Nimbus7 and ERBS data. See Scafetta et al. (2019) for a detailed discussion.

In any case, consensus, that is the majority of the TSI reconstructions, has little to do with science, and if more composites are similar to PMOD than ACRIM, that simply means there are more opinions that PMOD is preferred. This does not mean that the opinions expressed by Scafetta and Vahrenholt in the Clintel volume chapter 6 are mistaken. Nor do these opinions invalidate Connolly, et al., 2021 or Soon, Connoly, and Connolly, 2015. The truth is, the data we have on TSI is so poor prior to 1996, that any of the various TSI reconstructions could be correct, as Chatzistergos himself admits in his 2023 paper, quoted below:

Measurements of total solar irradiance (TSI) exist since 1978, but this is too short compared to climate-relevant time scales. Coming from a number of different instruments, these measurements require a cross-calibration, which is not straightforward, and thus several composite records have been created. All of them suggest a marginally decreasing trend since 1996. Most composites also feature a weak decrease over the entire period of observations, which is also seen in observations of the solar surface magnetic field and is further supported by Ca ii K data. Some inconsistencies, however, remain and overall the magnitude and even the presence of the long-term trend remain uncertain. Emphasis added.

(Chatzistergos, Krivova, & Yeo, 2023)

Chatzistergos Point 2:

Chatzistergos claims that the analysis of the NRLTSI2 (Coddington O. , Lean, Pilewskie, Snow, & Lindholm, 2016) and SATIRE (Krivova, Solanki, & Unruh, 2011) data performed by Nicola Scafetta is incorrect. For my discussion of Scafetta’s paper see here. Since 1996, the trends in all the TSI constructions match, the differences are in the period from 1978 to 1996 where the data is quite poor. Extrapolations of TSI into the past rely on solar models (such as SATIRE). As Scafetta and many others have pointed out, these models are based upon many speculative assumptions that are not consistent with the satellite data, particularly during the critical ACRIM data gap (see figure 3). Chatzistergos offers no evidence that Scafetta’s analysis is incorrect, just his opinion, which is contradicted by the quote above from Chatzistergos’ own 2023 paper.

Figure 3. The critical ACRIM gap is plotted in red. Plot (a) uses the original TSI calculated by the satellite teams and TSI is rising during the gap. The PMOD record is shown in plot (b), as modified with their solar model, and it is flat to declining. Source: (Scafetta, Willson, Lee, & Wu, 2019).

Figure 3 highlights the critical portion of the early TSI record. Figure 3a shows how the TSI satellite composite appears when the original TSI satellite records published by their original experimental teams are adopted (it looks more like ACRIM); Figure 3b shows how the TSI satellite composite appears when one adopts the TSI satellite modified by PMOD. Both figures were published by Dudok de Wit (Dudok de Wit, Kopp, Fröhlich, & Schöll, 2017) using the same composite methodology. The differences appear tiny, but when extrapolated back to the Little Ice Age Maunder Solar Grand Minimum, they make a big difference in the level of solar activity then versus now. Data does not exist at this time that can determine whether Chatzistergos or Scafetta are correct about the long-term trend in solar activity or how well it correlates with climate changes in the past.

Chatzistergos Point 3:

Chatzistergos claims that the following sentence in our book is incorrect.

“The main difference between the ACRIM and PMOD TSI satellite composites is that while the former uses the original raw satellite TSI records, the latter is based on TSI satellite records modified with a model.”

(Crok & May, 2023, Ch 6)

ACRIM uses the satellite data as interpreted by the respective satellite teams to compute TSI and prefers to bridge the ACRIM-gap using the Nimbus7 record because it is considered more accurate than the ERBS record from an experimental point of view, then the ACRIM team splices the data, as described here, and similar to the RMIB and Dudok de Wit reconstructions illustrated in figures 2 and 3a. One could nitpick, as Chatzistergos does, and claim that Dudok de Wit, the ACRIM team, and the RMIB team used a simple model to splice the satellite data together. But when we consider that the PMOD team changes the Nimbus7 satellite data to conform to their solar models, his nitpicking looks weak. The weak justification for the data changes made by the PMOD team is explained by Douglas Hoyt, the leader of the Nimbus 7 satellite team:

“[The NASA Nimbus7/ERB team] concluded there was no internal evidence in the [Nimbus7/ERB] records to warrant the correction that [PMOD] was proposing. Since the result was a null one, no publication was thought necessary. Thus, Fröhlich’s PMOD TSI composite is not consistent with the internal data or physics of the [Nimbus7/ERB] cavity radiometer.”

(Scafetta and Willson 2014, Appendix A)

One could quibble over the language of the contested statement from our book, but the bottom line is that the ACRIM adjustments can be justified by solid engineering data from the satellite teams, whereas the PMOD adjustments are not consistent with the satellite data according to the satellite teams. Our sentence, while possibly poorly worded, is correct.

Chatzistergos Point 4:

Chatzistergos complains that our book points out that the IPCC has progressively downgraded their estimate of the influence of the Sun, then admits that we are correct, but adds that the IPCC did nothing wrong. That is his opinion, ours is different. He claims again that “we” understand the Sun better today than in the 1980s and now “know” the Sun has little influence on climate change, nearly opposite of what he says in his own 2023 paper as quoted above. The truth is that a considerable amount of evidence exists that the Sun plays a role in recent climate change, but how the Sun accomplishes this is still debated and poorly understood. For a comprehensive discussion see here and here, Scafetta’s recent paper here, or Javier Vinós’ book (Vinós, 2022).

Chatzistergos Point 5&6:

Chatzistergos claims that the statement below, from Chapter 6 of our book, is incorrect:

“[The IPCC] TSI record is a combination of two TSI records (NRLTSI2 and SATIRE) that show a very small secular variability while many other TSI reconstructions show a much larger, up to about 10 times, larger secular variability and also slightly different patterns.”

(Crok & May, 2023, Ch 6)

Then confusingly, writes: “There are indeed many models reconstructing TSI in different ways…” He never explains how the statement from our book is incorrect, it just seems to be his opinion. On the face of it, the statement above is clearly accurate and well written.

Chatzistergos Point 7:

Here he claims that we listed the evidence that the Sun influences the number of cosmic rays that strike the Earth, which affects cloud cover and thus the climate, but that we ignored the evidence against this hypothesis. He did not read very carefully. The following is also from our book:

“During the period 1983-2002 global cloud cover developed synchronously with the eleven-year solar cycle (see Figure 3). After then, however, the relationship broke down, which led to criticism from Svensmark’s scientific opponents.”

(Crok & May, 2023, Ch 6, p 87)

The evidence against the hypothesis is the breakdown in the cloud/cosmic ray correlation during the 1990s, as described in our book, nothing was ignored.

Chatzistergos Point 8:

His opinion is that figure 2 in chapter 6 of our book is cherry picked and that the series shown in the figure are somehow inferior. Our opinion is different, and he does not present any evidence to support his opinion. The correlation between the long-term (century or more) trends in solar activity and the long-term trends in climate is clear and has been recognized by paleoclimatologists for centuries, see here and here. However, a proper explanation or model of the mechanics of the solar influence on climate eludes us.

Chatzistergos Point 9:

His point is that we “mislead with the grand solar maximum of the 20th century by conveniently failing to mention that solar activity peaked during late 50’s…” Here Chatzistergos makes the implicit assumption that solar changes affect climate in some linear and instantaneous way. If that were true, the mechanism would have been discovered long ago. The Modern Solar Maximum lasted from around 1935 to 2005, it was the longest solar maximum in at least 600 years, as described here. Figure 4 shows the Modern Solar Maximum, as reconstructed from sunspot records.

Figure 4. The Modern Solar Maximum, the longest solar maximum in 600 years. Source: (Vinós, 2022) and here.

Chatzistergos Point 10:

Chatzistergos’ 10th point is that we mislead when we state that “the increase in solar activity correlates well with the current global warming” referencing Connolly et al 2021. He claims this is incorrect, even though more than 50 paleoclimatologists have written that the solar modulation of climate is obvious in the data and that research should focus on finding out how it occurs, as reported by Vinós and myself here.


Chatzistergos inability to see the difference between opposing opinions and actual mistakes is not surprising given the appalling level of scientific training today and the politicization of climate science. That is why I took the time to write this post defending our book.

His tweets confuse facts with opinions. This is also commonly seen in supposed “fact checks” by Climate Feedback and other organizations of that ilk, as we discuss here. Clearly our universities are not training our young scientists very well, this is a real problem that should be addressed.

Download the bibliography here.