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ACRIM total solar irradiance satellite composite validation versus TSI proxy models

Abstract

The satellite total solar irradiance (TSI) database provides a valuable record for investigating models of solar variation used to interpret climate changes. The 35-year ACRIM total solar irradiance (TSI) satellite composite time series has been revised using algorithm updates based on 13 years of accumulated mission experience and corrections to ACRIMSAT/ACRIM3 results for scattering and diffraction derived from recent testing at the Laboratory for Atmospheric and Space Physics/Total solar irradiance Radiometer Facility (LASP/TRF). The net correction lowers the ACRIM3 scale by ∼3000 ppm, in closer agreement with the scale of SORCE/TIM results (average total solar irradiance ≈1361.5 W/m2). Differences between the ACRIM and PMOD TSI composites are investigated, particularly the decadal trending during solar cycles 21–22 and the Nimbus7/ERB and ERBS/ERBE results available to bridge the ACRIM Gap (1989–1992), are tested against a set of solar proxy models. Our findings confirm the following ACRIM TSI composite features: (1) The validity of the TSI peak in the originally published ERB results in early 1979 during solar cycle 21; (2) The correctness of originally published ACRIM1 results during the SMM spin mode (1981–1984); (3) The upward trend of originally published ERB results during the ACRIM Gap; (4) The occurrence of a significant upward TSI trend between the minima of solar cycles 21 and 22 and (5) a decreasing trend during solar cycles 22–23. The same analytical approach does not support some important features of the PMOD TSI composite: (1) The downward corrections applied to the originally published ERB and ACRIM1 results during solar cycle 21; (2) The step function sensitivity change in ERB results at the end-of-September 1989; (3) The downward trend of ERBE results during the ACRIM Gap and (4) the use of ERBE results to bridge the ACRIM Gap. Our analysis provides a first order validation of the ACRIM TSI composite approach and its 0.037 %/decade upward trend during solar cycles 21–22. The implications of increasing TSI during the global warming of the last two decades of the 20th century are that solar forcing of climate change may be a significantly larger factor than represented in the CMIP5 general circulation climate models.

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Acknowledgements

The National Aeronautics and Space Administration supported Dr. Willson under contracts NNG004HZ42C at Columbia University and Subcontracts 1345042 and 1405003 at the Jet Propulsion Laboratory.

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Correspondence to Nicola Scafetta.

Appendices

Appendix A: Hoyt’s statement about Nimbus7/ERB

In 2008 Scafetta asked Hoyt to comment the alterations of the ERB data implemented by Fröhlich to produce the PMOD composite. Hoyt returned by email the following statement where “N7” is for the Nimbus7/ERB TSI record prepared by Hoyt and collaborators:

September 16, 2008.

Dear Dr. Scafetta: Concerning the supposed increase in N7 sensitivity at the end of September 1989 and other matters as proposed by Fröhlich’s PMOD TSI composite:

  1. 1.

    There is no known physical change in the electrically calibrated N7 radiometer or its electronics that could have caused it to become more sensitive. At least neither Lee Kyle nor I could never imagine how such a thing could happen and no one else has ever come up with a physical theory for the instrument that could cause it to become more sensitive.

  2. 2.

    The N7 radiometer was calibrated electrically every 12 days. The calibrations before and after the September shutdown gave no indication of any change in the sensitivity of the radiometer. Thus, when Bob Lee of the ERBS team originally claimed there was a change in N7 sensitivity, we examined the issue and concluded there was no internal evidence in the N7 records to warrant the correction that he was proposing. Since the result was a null one, no publication was thought necessary.

  3. 3.

    Thus, Fröhlich’s PMOD TSI composite is not consistent with the internal data or physics of the N7 cavity radiometer.

  4. 4.

    The correction of the N7 TSI values for 1979–1980 proposed by Fröhlich is also puzzling. The raw data was run through the same algorithm for these early years and the subsequent years and there is no justification for Fröhlich’s adjustment in my opinion.

Sincerely, Douglas Hoyt

Appendix B: The importance of the TSI satellite debate for solar physics and climate change

The Sun is a variable star (Brekke 2012). However, the multi-decadal trending of solar activity is currently poorly modeled and numerous alternative proxy reconstructions have been proposed. Understanding the correct amplitude and dynamics of solar variability is important both for solar physics and climate change science.

The multi-decadal trending difference between the ACRIM (Willson and Mordvinov 2003) and PMOD TSI composites (Fröhlich and Lean 1998; Fröhlich 2006) shown in Fig. 2 is important for understanding the multi-decadal variation of solar dynamics and therefore for discriminating among solar models used also to interpret climate changes. Because the ACRIM TSI composite shows an evident upward pattern from 1980 to 2000 while PMOD shows a slight downward trend during the same period, the former would suggest a larger TSI low-frequency variability than the latter and different TSI multidecadal variation mechanisms. The origin of a slowly varying irradiance component may derive from changes in the solar faculae and/or in the background solar radiation from solar quiet regions. These mechanisms are currently poorly understood and modeled. However, if TSI increased from 1980 to 2000, total solar and heliospheric activity could have increased as well contributing significantly to the global warming observed from 1980 to 2000 (Scafetta and West 2005, 2007; Scafetta 2009, 2011, 2012a, 2013b,c).

The Coupled Model Intercomparison Project Phase 5 (CMIP5) used to study climate change (Scafetta 2013c) currently recommends the use of a solar forcing function deduced from the TSI proxy model proposed by Lean and collaborators (Wang et al. 2005; Kopp et al. 2007). Lean’s recent models show a small secular trend (about \(1~\mathrm{W/m}^{2}\)) from the Maunder minimum (1645–1715) to the present with a peak about 1960 and it is quasi stationary since. However, alternative TSI proxy reconstructions have been proposed and some of them present much larger secular variability and different decadal patterns. Figure 16A compares two alternative multisecular TSI proxy model: Lean’s TSI model and the TSI reconstruction proposed by Hoyt and Schatten (1993) rescaled at the ACRIM TSI scale. Figure 16A also shows in blue the annual mean ACRIM TSI satellite composite since 1981 (Willson and Mordvinov 2003).

Fig. 16
figure16

[A] Total solar irradiance (TSI) reconstruction by Hoyt and Schatten (1993) (red) rescaled on the ACRIM record (Willson and Mordvinov 2003) (since 1981) (blue) vs. the updated Lean model (Wang et al. 2005; Kopp et al. 2007) (green). [B] Comparison between the Central England Temperature (CET) record (black) Parker et al. (1992) and the TSI model by Hoyt and Schatten plus the ACRIM TSI record. The latter is linearly rescaled on the CET record using the formula T=0.1915∗TSI−251.05 that rescales the TSI record into a temperature record. Good correlation is observed at least since 1772. (Note CET is less certain before 1772). The Hoyt and Schatten (1993) reconstruction has been made by rescaling it on the ACRIM record from 1980 to 1992 using the formula HS93∗1361.267/1371.844, where 1371.844 is the 1981–1992 average of Hoyt and Schatten (1993)’s proxy reconstruction and 1361.267 is the 1981–1992 average of the ACRIM TSI composite. The value in 1980 in [B] was estimated as the average between the ACRIM mean and the rescaled Hoyt and Schatten (1993) reconstruction

Hoyt and Schatten (1993, Fig. 10) showed that their multi-proxy TSI model is highly correlated with an annual mean northern hemisphere temperature variation reconstruction since 1700. This correlation is confirmed (Fig. 16B) by comparing a Hoyt+ACRIM TSI combination model against the Central England Temperature (CET) record since 1700 (Parker et al. 1992). The comparison between the two records is made using a simple linear regression of the Hoyt+ACRIM TSI record against the CET record. The linear regression algorithm simplistically transforms the TSI curve into a temperature signal and only provides an approximate estimate of the climatic effect of the solar variability as described by the Hoyt+ACRIM TSI record.

The divergence observed during the last decades is likely due to (1) an additional anthropogenic warming component, which was quite significant during the last decades, and (2) to the necessity of using a more advanced model to obtain the temperature signature of the solar variability. This problem is better addressed in the literature interpreting global climate change (e.g.: Scafetta and West 2005, 2007; Scafetta 2009, 2010, 2011, 2012a, 2013b,c).

A good correlation between the same TSI proxy model and numerous climatic records for the 20th century including temperature records of the Arctic and of China, the sunshine duration record of Japan and the Equator-to-Pole (Arctic) temperature gradient record was demonstrated (Soon 2005; Soon et al. 2011; Soon and Legates 2013). Key features are a warming from 1910s to 1940s, a cooling from the 1940s to 1970s, a warming from the 1970s to 2000s and a steady-to-cooling temperature since ∼2000, all of which correlate much better with the Hoyt+ACRIM TSI composite than with Lean’s proxy model. The observed pattern is compatible with a quasi 60-year oscillation commonly observed in climate and solar records throughout the Holocene (e.g.: Chambers et al. 2012, Klyashtorin et al. 2009, Knudsen et al. 2011, Mazzarella and Scafetta 2012, Ogurtsov et al. 2002, Qian and Lu 2010, Scafetta 2010, 2012a,b, 2013a,b,c, Scafetta and Willson 2013a).

Recently, Liu et al. (2013, see also the supplementary information) used the ECHO-G model and showed that to reproduce the ∼0.7 C global cooling observed from the Medieval Warm Period (MWP: 900–1300) to the Little Ice Age (LIA: 1400–1800) according to recent paleoclimatic temperature reconstructions (e.g.: Ljungqvist 2010; Mann et al. 2008; Moberg et al. 2005), a TSI model with a secular variability ∼3.5 times larger than that shown by Lean’s TSI model would be required.

The IPCC (2007, Sect. 6.6.3.4 and its Fig. 6.14) reports that to obtain a cooling of about 0.7 C from the MWP to the LIA Maunder Minimum a corresponding TSI downward trend of −0.25 % is required. Lean’s TSI model shows a trend of only −0.08 % over this period (Wang et al. 2005). The same climate models rescaled using Lean’s TSI model predict a MWP-to-LIA Maunder Minimum cooling of only 0.25 C that is compatible only with the controversial hockey stick temperature reconstruction of Mann et al. (1999). It should be noted that the updated proxy temperature reconstructions by Mann et al. (2008) show a significantly warmer MWP than the Mann’s 1999 temperature reconstruction used by the IPCC in 2001. See the extended discussion in Scafetta (2013a).

Thus, recent paleoclimatic temperature reconstructions imply that the natural climate variability varied significantly more than predicted by the CMIP5 general circulation models, which use Lean’s low-variability TSI model (e.g.: Scafetta 2013a,b,c). The most likely explanation is that solar variations (TSI and other astronomical effects) are a more significant contributor to climate change than currently understood (see also: Liu et al. 2013; Scafetta 2013a). A stronger solar effect on the climate would also imply a significantly larger solar contribution to the 20th century global warming, as demonstrated in some works (Scafetta 2009, 2013a,b,c). Indeed, despite the IPCC (2007) claims the Sun has an almost negligible effect on climate, numerous authors found significant correlations between specific solar models and temperature records suggesting a strong climate sensitivity to solar variations (e.g.: Bond et al. 2001; Hoyt and Schatten 1993; Loehle and Scafetta 2011; Mazzarella and Scafetta 2012; Ogurtsov et al. 2002; Scafetta 2009, 2010, 2012b, 2013b; Schulz and Paul 2002; Soon 2005; Soon and Legates 2013; Steinhilber et al. 2012; Svensmark 2007; Thejll and Lassen 2000).

Shapiro et al. (2011) and Judge et al. (2012) proposed TSI models based a comparison between solar irradiance reconstructions and sun-like-stellar data that show a TSI secular variability at least 3-to-6 times greater than Lean’s TSI proxy. These new TSI models look similar to those proposed by Hoyt and Schatten (1993). The Shapiro model also predicts a small TSI increase between the solar minima of 1986 and 1996, that is more consistent with the ACRIM 1980–2000 upward TSI pattern and contradicts PMOD. This pattern derives from the fact that the cosmic ray flux record, which is inversely proportional to solar magnetic activity, presents a slight decrease from about 1970 to 2000 (Scafetta 2013c, Fig. 20).

It was recently speculated that long term changes in the solar interior due to planetary gravitational perturbations may produce gradual multi-decadal and secular irradiance changes (e.g.: Abreu et al. 2012; Charbonneau 2013; Scafetta 2012b,c; Scafetta and Willson 2013a,b,c). The planetary models proposed by Scafetta (2012b) and Scafetta and Willson (2013a) shows a quasi 60-year modulation of solar activity since 1850 with peaks in the 1880s, 1940s and 2000s. Thus, it shows good agreement with the ACRIM composite’s upward trending from about 1980 to 2000. Scafetta (2014) addresses the scientific background of the astronomical theory of solar and climate oscillations.

In conclusion, despite recent scientific climate change literature (e.g.: IPCC 2007) has favored the PMOD interpretation of the TSI experimental records we have provided experimental and theoretical reasons supporting the claim that the ACRIM TSI composite is a most likely interpretation of the current satellite TSI database. The dynamical pattern revealed by the ACRIM TSI composite appears to better agree with a number of new evidences that are emerging and, therefore, solving the TSI satellite controversies could be quite important for better understanding solar physics and climate change alike.

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Scafetta, N., Willson, R.C. ACRIM total solar irradiance satellite composite validation versus TSI proxy models. Astrophys Space Sci 350, 421–442 (2014). https://doi.org/10.1007/s10509-013-1775-9

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Keywords

  • Solar luminosity
  • Total solar irradiance (TSI)
  • Satellite experimental measurements
  • TSI satellite composites
  • TSI proxy model comparisons