We present a new analysis of the two-decade-old controversy over interpretation of satellite observations of total solar irradiance (TSI) since 1978 and the implications of our findings for TSI as a driver of climate change. Our approach compares the methods of constructing the two most commonly referenced TSI composites (ACRIM and PMOD) that relate successive observational databases and two others recently constructed using a novel statistical approach. Our primary focus is on the disparate decadal trending results of the ACRIM and PMOD TSI composite time series, namely, whether they indicate an increasing trend from 1980 to 2000 and a decreasing trend thereafter (ACRIM) or a continuously decreasing trend since 1980 (PMOD). Construction of the four-decade observational TSI composites from 1978 to the present requires the use of results from two less precise Earth Radiation Budget experiments (Nimbus7/ERB and ERBS/ERBE) during the so-called ACRIM-Gap (1989.5–1991.8), between the end of the ACRIM1 and the beginning of the ACRIM2 experiments. The ACRIM and PMOD composites used the ERB and ERBE results, respectively, to bridge the gap. The well-established paradigm of positive correlation between Solar Magnetic Field Strength (SMFS) and TSI supports the validity of the upward trend in the ERB results and the corresponding decadal upward trend of the ACRIM composite during solar cycles 21 and 22. The ERBE results have a sensor degradation caused downward gap trend, contrary to the SMFS/TSI paradigm, that biased the PMOD composite decadal trend downward during solar cycles 21 and 22. The different choice of gap bridging data is clearly the cause of the ACRIM and PMOD TSI trending difference, agreeing closely in both magnitude and direction. We also analyze two recently proposed statistical TSI composites. Unfortunately their methodology cannot account for the gap degradation of the ERBE experiment and their resulting uncertainties are too large to uniquely distinguish between the trending of the ACRIM and PMOD composites. Our analysis supports the ACRIM TSI increasing trend during the 1980 to 2000 period, followed by a long-term decreasing trend since.
Satellite total solar irradiance (TSI) composite databases, using observations from different satellites covering different segments of time since November 1978, have been constructed by several research teams (e.g., [
(a) ACRIM TSI composite. (b) PMOD (v. 1702) TSI composite [
ACRIM combines the published and archived NASA records collected and processed by the ACRIM science teams responsible for the Solar Maximum Mission/ACRIM1 (1980–1989), the Upper Atmosphere Research Satellite/ACRIM2 (1991–2001), and the ACRIMSAT/ACRIM3 (1999–2013) mission, together with the original ERB science team results from the Nimbus7/ERB (1978–1993) experiment prior to the launch of ACRIM1 (1878–1980) and during the about 2-year gap between ACRIM1 and ACRIM2 results (the so-called ACRIM-Gap from 1989.5 to 1991.8).
The PMOD composite uses their model-modified versions of the ACRIM1, ACRIM2, Nimbus7/ERB and Earth Radiation Budget Satellite/ERBE (1984–2003) records from late 1978 to 1996, together with the Solar and Heliospheric Observatory/VIRGO observational record (1996 to present).
Other TSI composites have been proposed. The RMIB [
The most significant difference between the ACRIM and PMOD composites is their multidecadal trending during solar cycles 21-24. This can be seen clearly in Table
Mean values of the TSI composite solar cycle activity during the year of their minima. The error bar of the annual mean values is less than
1986 | 1996 | 2009 | |
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ACRIM | 1360.62 | 1361.08 | 1360.78 |
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PMOD (v. 1702) | 1360.59 | 1360.54 | 1360.40 |
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de Wit - Unmodified | 1360.32 | 1360.66 | 1360.54 |
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de Wit - Modified | 1360.52 | 1360.68 | 1360.54 |
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Satire-T2 | 1365.63 | 1365.50 | missing |
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Satire-S | 1360.98 | 1360.75 | 1360.55 |
The different methodologies and components of the TSI records used to construct the ACRIM and PMOD composites cause subtle but important differences between them. The most significant of these, the opposite trends in TSI minima between 1986 and 1996, is caused by their different approaches to bridging the ACRIM-Gap (1989.5–1991.8):
The PMOD rationale for using models to alter the Nimbus7/ERB data was to compensate for the sparsity of the ERBS/ERBE data and conform their gap results more closely to the proxy predictions of solar emission line models of TSI behavior. In fact, the ERBS/ERBE record is too sparse and affected by uncalibrated degradation to provide a useful bridge of the gap between the ACRIM1 and ACRIM2 records using only its observational data.
The trending difference between the two composites has been the subject of a lengthy controversy. ACRIM contends the following:
(1) PMOD’s modifications of the published ACRIM and ERB TSI records are questionable because they are based on conforming satellite observational data to proxy model predictions rather than an original analysis of the ACRIM, ERB, and ERBE data [
(2) The PMOD trend during 1986 to 1996 is biased downward by scaling ERB results to the rapidly degrading ERBE results during the ACRIM-Gap using the questionable justification of agreement with some TSI proxy predictions first proposed by Lee III et al. [
(3) PMOD misinterpreted and erroneously corrected ERB results for an instrument power down event (Sep. 25-28, 1989) as an instrument “glitch” and sensitivity change and for a presumed drift (cf.: [
(4) The fabrication and endorsement of the PMOD composite by some might have been influenced by the fact that TSI proxy models popular at the time predicted a TSI decreasing trend similar to that in the PMOD composite (e.g., [
Regarding the Nimbus7/ERB data modification implemented by PMOD during the ACRIM-Gap, it is important to stress that Dr. Hoyt, who was the director of the Nimbus7/ERB mission, disregarded Fröhlich’s claims from an experimental perspective (see the supplement files published in [
The consistent downward trending of the PMOD TSI composite is negatively correlated with the global mean temperature anomaly during 1980–2000. This has been viewed with favor by those supporting the CO2 anthropogenic global warming (CAGW) hypothesis since it would minimize TSI variation as a competitive climate change driver to CO2, the featured driver of the hypothesis during the period (cf.: [
ACRIM composite trending is well correlated with the record of global mean temperature anomaly over the entire range of satellite observations (1980–2018) [
The paradigm of positive correlation between Solar Magnetic Field Strength (SMFS) and TSI, first established by ACRIM1 observations [
The above empirically based studies provide a strong indication that TSI variability resulting from solar magnetic activity variation is the main driver of the earth’s climate. Proxy TSI results, derived from the SMFS/TSI paradigm, correlate with the global mean temperature anomaly both during and prior to the satellite TSI observations [
It has been shown that the solar cycle amplitude from 1980 to 1989 and the trending from 1992 to 2002 of a proxy model represented as supporting the PMOD TSI composite [
More recently, a novel TSI composite has been constructed using a wavelet transform algorithm that simultaneously uses all available TSI records [
Composing a TSI database using a solely statistical methodology has a fatal flaw in that it fails to account for the physical limitations of observation, such as degradation of the TSI sensors. Such composites will have uncertainties so large that they have limited ability to uniquely discriminate between the ACRIM and PMOD TSI composites. The methodology proposed by Dudok de Wit et al. [
Moreover, it is important to clarify that the uncertainty produced by the TSI composites proposed by Dudok de Wit et al. [
Dudok de Wit et al. [
In the following we provide a detailed analysis of the alternative TSI composites recently proposed by Dudok de Wit et al. [
The ACRIM and the PMOD composites shown in Figure
The cause of the primary difference in trending between the ACRIM and PMOD during solar cycles 21–23 is shown in Figure
Comparison of the TSI results from the ACRIM1, Nimbus7/ERB, ERBS/ERBE experiments and the NSO/Kitt Peak Solar Magnetic Field Strength (SMFS) during the solar cycle 21-22 minimum and the upward trend to and through solar cycle 22 maximum. The effect of degradation for the ERBE sensors during the 1989–1992 maximum is seen in the downward trend of its results relative to the trends of the ERB results and the SMFS that is anticorrelated with the SMFS–TSI paradigm.
The TSI results and Solar Magnetic Field Strengths are all correlated except for the ACRIM-Gap where the ERBE results trend downward while the others trend up. This occurs during the increasing phase of solar magnetic activity leading to the peak of solar cycle 22 during 1990–1992. The most likely explanation is that the ERBE solar TSI detectors degraded from “bleaching” of their absorptive sensor coatings by the higher levels of short wavelength radiation and particle flux that occur during peaks of solar activity maxima. This effect had been observed in the ACRIM1 experiment during the high but descending SMFS phase of solar cycle 21 from its peak and was self-calibrated precisely using ACRIM1’s multisensor approach [
Sensor degradation caused by mission exposure to high SMFS solar fluxes has been observed in the performances of all satellite TSI experiments to date. Rapid detector degradation occurs during exposure to the enhanced solar short wavelengths and ionized particulate during peak levels of solar activity and reaches a saturation level, an asymptotic limit or a more slowly varying, more linear rate of degradation thereafter. The timing and shape of the degradation curve depends on the details of the solar sensor surfaces, geometries, and exposure rates [
Degradation of the ACRIMSAT/ACRIM3 sensors over the mission.
The ERB experiment exhibited rapid sensor degradation during the peak of solar cycle 21 but responded in correlation with the SMFS and, hence, the SMFS-TSI paradigm during the gap. This would be expected from the ACRIM1 degradation experience, since its initial sensor “saturation” degradation had occurred during the peak of solar cycle 21 and its subsequent rate of degradation would be slower. On the contrary, a rapid degradation of the ERBE observations during the ACRIM-Gap was likely caused by the highly energetic solar maximum fluxes it experienced for the first time during the gap since it was launched during the initial rising phase of SMFS for solar cycle 22.
Figure
TSI composites proposed by Dudok de Wit et al. [
The composite in Figure
Figure
(a) SATIRE-T2 TSI proxy reconstruction [
In Figure
Mean and standard deviation of the curves depicted in Figure
ACRIM | PMOD | SATIRE-S | |
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Original Unmodified TSI Results | |||
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1980-2013 | 0.38 ± 0.13 | −0.01 ± 0.24 | 0.21 ± 0.31 |
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1980-1990 | 0.40 ± 0.15 | 0.31 ± 0.18 | 0.01 ± 0.10 |
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1992-2013 | 0.37 ± 0.12 | −0.16 ± 0.05 | 0.01 ± 0.10 |
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PMOD Modified TSI Results | |||
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1980-2013 | 0.33 ± 0.19 | −0.07 ± 0.12 | 0.15 ± 0.21 |
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1980-1990 | 0.16 ± 0.16 | 0.07 ± 0.06 | 0.38 ± 0.14 |
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1992-2013 | 0.38 ± 0.13 | −0.15 ± 0.04 | 0.02 ± 0.11 |
(a) Variation between the ACRIM TSI composite and the unmodified and modified TSI by Dudok de Wit et al. [
Figure
The level of agreement between two records is measured by the standard deviation
The relative standard deviation of the deviation functions for the periods shown in Table
Comparing the intervals 1980-1990 and 1992-2003 using Table
Regarding the large error bars reported for the TSI composites proposed by Dudok de Wit et al. [
The TSI composite data.
ACRIM | |
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PMOD | |
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SORCE | |
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SATIRE-S | |
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SATIRE-T2 | |
Today there exists general agreement among various science teams that the mean TSI during solar cycle 23 is near 1361 W/m2 but differences have persisted about the decadal solar activity cycle-to-cycle trending of the ACRIM and PMOD composites. ACRIM contends that the original data from satellite measurements, as processed and published by the original science teams, are the best representation of the experimental results and demonstrate that the TSI increased from 1980 to 2000 and decreased afterwards. PMOD modifies the original science teams’ satellite results using proxy models causing the TSI to gradually decrease since 1980. Resolving this controversy has important implications for understanding climate change and assessing the usefulness of TSI proxy models.
We have shown that the average value of the statistical TSI composite models proposed by Dudok de Wit et al. [
The large divergence of the SATIRE TSI proxy models suggests they are inadequate to reproduce the cycle-by-cycle decadal TSI trending with useful precision, as discussed in a previous paper by Scafetta and Willson [
It is important to consider whether the satellite records require corrections not made by the original experiment teams. We label the second TSI composite proposed by Dudok de Wit et al. [
Analysis has disproved the validity of most of Fröhlich’s modifications to the satellite TSI records published by the original ACRIM and Nimbus7/ERB science teams he used in constructing the PMOD composite [
The use of unverified modified data has fundamentally flawed the PMOD TSI satellite composite construction. Composite TSI time series would have greater scientific credibility if the most flawed records, such as the Nimbus7/ERB before 1980 (cf. [
There is another important issue regarding the appropriateness of the algorithm proposed by Dudok de Wit et al. [
In general, modifying high quality records with those of lesser quality will not provide the most accurate representation of the data in particular when the low quality of a dataset has a physical rather than a statistical origin. This is certainly a concern with the TSI satellite databases. ACRIM 1, 2 & 3 made up to 720 30-second averaged, self-calibrated, shuttered measurements per day [
The difference between Nimbus7/ERB and ERBS/ERBE during the ACRIM-Gap is too large to be due to statistical fluctuation and so at least one of the two records erroneously represents the TSI variation trends during the ACRIM-Gap. In such a situation it is required to determine which of the two records is the most reliable (compare the various arguments proposed in [
The statistical issue of the variability of TSI composites resulting from the choice of the Nimbus7/ERB or ERBS/ERBE during the ACRIM-Gap was discussed in Scafetta [
Figure
Updates of the TSI composites proposed in Willson and Mordvinov [
These composites show a slight decrease between the TSI minima in 1996 and 2009 as do the ACRIM and PMOD. However, their variation between the 1986 and 1996 TSI minima depend on the specific record used to bridge ACRIM1 and ACRIM2 during the ACRIM-Gap period. As Table
The full maximum range of possible TSI composites suggests that the TSI minimum in 1996 was about 0.3 ± 0.4 W/m2 higher than that in 1986. Thus, once all available TSI records are used, the ACRIM upward 1986-1996 trending (0.46 ± 0.02 W/m2) is statistically favored above the downward trending of PMOD (-0.05 ± 0.02 W/m2) even if the ACRIM-Gap Nimbus7/ERB increased its sensitivity for some amount, e.g., for about 0.2 W/m2. The latter value falls within the observed divergence between Nimbus7/ERB and ACRIM1 between 1981 and 1989 on an annual time scale ([
The downward trending of PMOD between 1986 and 1996 would be acceptable only if it were experimentally demonstrated that ERBS/ERBE trending during the ACRIM-Gap was highly accurate. However, as Willson and Mordvinov [
The Dudok de Wit et al. [
Therefore, we contend that using a purely statistical methodology to compose TSI records that could contain a physically unreliable one is improper. Such an approach neither produces a more authentic composite nor improves our knowledge regarding a given phenomenon: it can only produce a composite affected by an anomalously large uncertainty that encloses all possibilities.
Improvement of the physical knowledge of TSI behavior since 1978 requires the determination of which of the Nimbus7/ERB and ERB/ERBE experiment results were least defective during the ACRIM-Gap (1989.5-1991.8). Then, our scientific knowledge could be improved by excluding the more flawed record from the composite. This was the logic applied by the ACRIM team. In point of fact PMOD failed to do this, instead selecting the ERBE results that were known to be degraded and sparse, because that made the solar cycle 21–22 trend agrees with TSI proxy models and the CAGW explanation of CO2 as the driver of the global warming trend of the late 20th century.
We note that the considerable evidence discussed by Scafetta and Willson [
Our summary conclusion is that the objective evidence produced by all of the independent TSI composites [
Note that the apparent agreement of some TSI proxy models with the PMOD trending between 1986 and 1996 solar cycle minima can be coincidental because these proxy models rely on proxy data that are mostly representative of the active regions of the sun such as photospheric sunspots and faculae ([ The solar cycle length model (e.g., [ A model of solar variability driven by planetary tidal harmonics [ The global surface temperature of the Earth increased from 1970 to 2000 and remained nearly stable from 2000 and 2018. This pattern is not reproduced by CO2 AGW climate models but correlates with a TSI evolution with the trending characteristics of the ACRIM TSI composite as explained in Scafetta [
All data can be downloaded from the websites listed in Table
The authors do not have any conflicts of interest to declare.
The National Aeronautics and Space Administration supported Richard C. Willson under contracts 1405003 at the Jet Propulsion Laboratory and ROSES 2016 Contract NNH15C0020.