From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
We begin this section of our review of the potential effects of solar activity on earth’s climate with the study of Karlén (1998), who examined proxy climate data related to changes in summer temperatures in Scandinavia over the past 10,000 years. This temperature record—derived from analyses of changes in the size of glaciers, changes in the altitude of the alpine tree-limit, and variations in the width of annual tree rings—was compared with contemporaneous solar irradiance data derived from 14C anomalies measured in tree rings. The former record revealed both long- and short-term temperature fluctuations; it was noted by Karlén that during warm periods the temperature was “about 2°C warmer than at present.” In addition, the temperature fluctuations were found to be “closely related” to the 14C-derived changes in solar irradiation, leading him to conclude that “the similarity between solar irradiation changes and climate indicate a solar influence on the Scandinavian and Greenland climates.” This association led him to further conclude that “the frequency and magnitude of changes in climate during the Holocene [i.e., the current interglacial] do not support the opinion that the climatic change of the last 100 years is unique.” He bluntly stated that “there is no evidence of a human influence so far.”
Also writing just before the turn of the century, Lockwood et al. (1999) analyzed measurements of the near-earth interplanetary magnetic field to determine the total magnetic flux leaving the sun since 1868. Based on their analysis, they were able to show that the total magnetic flux leaving the sun rose by a factor of 1.41 over the period 1964-1996, while surrogate measurements of the interplanetary magnetic field previous to this time indicated that this parameter had increased by a factor of 2.3 since 1901. These findings and others linking changes in solar magnetic activity with terrestrial climate change led the authors to state that “the variation [in the total solar magnetic flux] found here stresses the importance of understanding the connections between the sun’s output and its magnetic field and between terrestrial global cloud cover, cosmic ray fluxes and the heliospheric field.”
Parker (1999) noted that the number of sunspots also doubled over the same time period, and that one consequence of this phenomenon is a much more vigorous sun that is slightly brighter. Parker also drew attention to the fact that NASA spacecraft measurements had revealed that the brightness (B) of the sun varies by an amount “change in B/B = 0.15%, in step with the 11-year magnetic cycle.” He then pointed out that during times of much reduced activity of this sort (such as the Maunder Minimum of 1645-1715) and much increased activity (such as the twelfth century Mediaeval Maximum), brightness variations on the order of change in B/B = 0.5% typically occur, after which he indicated that the mean temperature (T) of the northern portion of the earth varied by 1 to 2°C in association with these variations in solar activity, stating finally that “we cannot help noting that change in T/T = change in B/B.”
Also in 1999, Chambers et al. noted that recent research findings in both palaeoecology and solar science “indicate a greater role for solar forcing in Holocene climate change than has previously been recognized,” which subject they then proceeded to review. In doing so, they found much evidence within the Holocene for solar-driven variations in earth-atmosphere processes over a range of timescales stretching from the 11-year solar cycle to century-scale events. They acknowledge that the absolute solar flux variations associated with these phenomena are rather small; but they identify a number of “multiplier effects” that can operate on solar rhythms in such a way that “minor variations in solar activity can be reflected in more significant variations within the earth’s atmosphere.”
The three researchers also noted, in this regard, that nonlinear responses to solar variability are inadequately represented (in fact, they are essentially ignored) in the global climate models used by the IPCC to predict future CO2-induced global warming, while at the same time other amplifier effects are used to model the hypothesized CO2-induced global warming of the future, where CO2 is only an initial perturber of the climate system which, according to the IPCC, sets other more powerful forces in motion that produce the bulk of the warming.
At the start of the new millennium, Bard et al. (2000) listed some of the many different types of information that have been used to reconstruct past solar variability, including “the envelope of the SSN [sunspot number] 11-year cycle (Reid, 1991), the length and decay rate of the solar cycle (Hoyt and Schatten, 1993), the structure and decay rate of individual sunspots (Hoyt and Schatten, 1993), the mean level of SSN (Hoyt and Schatten, 1993; Zhang et al., 1994; Reid, 1997), the solar rotation and the solar diameter (Nesme-Ribes et al., 1993), and the geomagnetic aa index (Cliver et al., 1998).” They also noted that “Lean et al. (1995) proposed that the irradiance record could be divided into 2 superimposed components: an 11-year cycle based on the parameterization of sunspot darkening and facular brightening (Lean et al., 1992), and a slowly varying background derived separately from studies of sun-like stars (Baliunas and Jastrow, 1990),” and that Solanki and Fligge (1998) had developed an even more convoluted technique. Bard et al., however, used an entirely different approach.
Rather than directly characterize some aspect of solar variability, they assessed certain consequences of that variability. Specifically, they noted that magnetic fields of the solar wind deflect portions of the primary flux of charged cosmic particles in the vicinity of the earth, leading to reductions in the creation of cosmogenic nuclides in earth’s atmosphere. Consequently, they reasoned that histories of the atmospheric concentrations of 14C and 10Be can be used as proxies for solar activity, as noted many years earlier by Lal and Peters (1967).
In employing this approach to the problem, the four researchers first created a 1,200-year history of cosmonuclide production in earth’s atmosphere from 10Be measurements of South Pole ice (Raisbeck et al., 1990) and the atmospheric 14C/12C record as measured in tree rings (Bard et al., 1997). This record was then converted to total solar irradiance (TSI) values by “applying a linear scaling using the TSI values published previously for the Maunder Minimum,” when cosmonuclide production was 30-50 percent above the modern value.
This approach resulted in an extended TSI record that suggests, in their words, that “solar output was significantly reduced between AD 1450 and 1850, but slightly higher or similar to the present value during a period centered around AD 1200.” “It could thus be argued,” they say, “that irradiance variations may have contributed to the so-called ‘little ice age’ and ‘medieval warm period’.’
In discussing this idea, Bard et al. downplay their own suggestion, because, as they report, “some researchers have concluded that the ‘little ice age’ and/or ‘medieval warm period’ [were] regional, rather than global events.” Noting the TSI variations they developed from their cosmonuclide data “would tend to force global effects,” they felt they could not associate this global impetus for climate change with what other people were calling regional climatic anomalies. With respect to these thoughts, we refer the reader to Section 3.2 of this report, where it is demonstrated that the Little Ice Age and Medieval Warm Period were truly global in extent.
Rozelot (2001) conducted a series of analyses designed to determine whether phenomena related to variations in the radius of the sun may have influenced earth’s climate over the past four centuries. The results of these analyses revealed, in the words of the researcher, that “at least over the last four centuries, warm periods on the earth correlate well with smaller apparent diameter of the Sun and colder ones with a bigger sun.” Although the results of this study were correlative and did not identify a physical mechanism capable of inducing significant climate change on earth, Rozelot reports that the changes in the sun’s radius are “of such magnitude that significant effects on the earth’s climate are possible.”
Rigozo et al. (2001) created a history of sunspot numbers for the last 1,000 years “using a sum of sine waves derived from spectral analysis of the time series of sunspot number RZ for the period 1700-1999,” and from this record they derived the strengths of a number of parameters related to various aspects of solar variability. In describing their results, the researchers say that “the 1000-year reconstructed sunspot number reproduces well the great maximums and minimums in solar activity, identified in cosmonuclides variation records, and, specifically, the epochs of the Oort, Wolf, Sporer, Maunder, and Dalton Minimums, as well [as] the Medieval and Modern Maximums,” the latter of which they describe as “starting near 1900.” The mean sunspot number for the Wolf, Sporer, and Maunder Minimums was 1.36. For the Oort and Dalton Minimums it was 25.05; for the Medieval Maximum it was 53.00; and for the Modern Maximum it was 57.54. Compared to the average of the Wolf, Sporer, and Maunder Minimums, therefore, the mean sunspot number of the Oort and Dalton Minimums was 18.42 times greater; that of the Medieval Maximum was 38.97 times greater; and that of the Modern Maximum was 42.31 times greater. Similar strength ratios for the solar radio flux were 1.41, 1.89, and 1.97, respectively. For the solar wind velocity the corresponding ratios were 1.05, 1.10, and 1.11; while for the southward component of the interplanetary magnetic field they were 1.70, 2.54, and 2.67. In comparing these numbers, both the Medieval and Modern Maximums in sunspot number and solar variability parameters stand out above all other periods of the past thousand years, with the Modern Maximum slightly besting the Medieval Maximum.
Noting that a number of different spacecraft have monitored total solar irradiance (TSI) for the past 23 years, with at least two of them operating simultaneously at all times, and that TSI measurements made from balloons and rockets supplement the satellite data, Frohlich and Lean (2002) compared the composite TSI record with an empirical model of TSI variations based on known magnetic sources of irradiance variability, such as sunspot darkening and brightening, after which they described how “the TSI record may be extrapolated back to the seventeenth century Maunder Minimum of anomalously lower solar activity, which coincided with the coldest period of the Little Ice Age.” This exercise, as they have described it, “enables an assessment of the extent of post-industrial climate change that may be attributable to a varying sun, and how much the sun might influence future climate change.”
In reporting their results, Frolich and Lean state that “warming since 1650 due to the solar change is close to 0.4°C, with pre-industrial fluctuations of 0.2°C that are seen also to be present in the temperature reconstructions.” From this study, therefore, it would appear that solar variability can explain a significant portion of the warming experienced by the earth in recovering from the global chill of the Little Ice Age, with a modicum of positive feedback accounting for the rest. With respect to the future, however, the two solar scientists say that “solar forcing is unlikely to compensate for the expected forcing due to the increase of anthropogenic greenhouse gases which are projected to be about a factor of 3-6 larger.” The magnitude of that anthropogenic forcing, however, has been computed by many different approaches to be much smaller than the value employed by Frohlich and Lean in making this comparison (Idso, 1998).
Contemporaneously, Douglass and Clader (2002) used multiple regression analysis to separate surface and atmospheric temperature responses to solar irradiance variations over the past two-and-a-half solar cycles (1979-2001) from temperature responses produced by variations in ENSO and volcanic activity. Based on the satellite-derived lower tropospheric temperature record, they evaluated the sensitivity (k) of temperature (T) to solar irradiance (I), where temperature sensitivity to solar irradiance is defined as k = ΔT/ΔI, obtaining the result of k = 0.11 ± 0.02°C/(W/m2). Similar analyses based on the radiosonde temperature record of Parker et al. (1997) and the surface air temperature records of Jones et al. (2001) and Hansen and Lebedeff (1987, with updates) produced k values of 0.13, 0.09, and 0.11°C/(W/m2), respectively, with the identical standard error of ± 0.02°C/(W/m2). In addition, they reported that White et al. (1997) derived a decadal timescale solar sensitivity of 0.10 ± 0.02°C/(W/m2) from a study of upper ocean temperatures over the period 1955-1994 and that Lean and Rind (1998) derived a value of 0.12 ± 0.02°C/(W/m2) from a paleo-reconstructed temperature record spanning the period 1610-1800. They concluded that “the close agreement of these various independent values with our value of 0.11 ± 0.02 [°C/(W/m2)] suggests that the sensitivity k is the same for both decadal and centennial time scales and for both ocean and lower tropospheric temperatures.” They further suggest that if these values of k hold true for centennial time scales, which appears to be the case, their high-end value implies a surface warming of 0.2°C over the last 100 years in response to the 1.5 W/m2 increase in solar irradiance inferred by Lean (2000) for this period. This warming represents approximately one-third of the total increase in global surface air temperature estimated by Parker et al. (1997), 0.55°C, and Hansen et al. (1999), 0.65°C, for the same period. It does not, however, include potential indirect effects of more esoteric solar climate-affecting phenomena, such as those discussed in Section 5.1 of this report, that could also have been operative over this period.
Foukal (2002) analyzed the findings of space-borne radiometry and reported that “variations in total solar irradiance, S, measured over the past 22 years, are found to be closely proportional to the difference in projected areas of dark sunspots, AS, and of bright magnetic plage elements, APN, in active regions and in enhanced network,” plus the finding that “this difference varies from cycle to cycle and is not simply related to cycle amplitude itself,” which facts suggest there is “little reason to expect that S will track any of the familiar indices of solar activity.” On the other hand, he notes that “empirical modeling of spectro-radiometric observations indicates that the variability of solar ultraviolet flux, FUV, at wavelengths shorter than approximately 250 nm, is determined mainly by APN alone.”
Building upon this conceptual foundation, and using daily data from the Mt. Wilson Observatory that covered the period 1905-1984, plus partially overlapping data from the Sacramento Peak Observatory that extended through 1999, Foukal derived time series of both total solar and UV irradiances between 1915 and 1999, which he then compared with global temperature data for the same time period. This work revealed, in his words, that “correlation of our time series of UV irradiance with global temperature, T, accounts for only 20% of the global temperature variance during the 20th century,” but that “correlation of our total irradiance time series with T accounts statistically for 80% of the variance in global temperature over that period.”
The UV findings of Foukal were not impressive, but the results of his total solar irradiance analysis were, leading him to state that “the possibility of significant driving of twentieth century climate by total irradiance variation cannot be dismissed.” Although the magnitude of the total solar effect was determined to be “a factor 3-5 lower than expected to produce a significant global warming contribution based on present-day climate model sensitivities,” what Foukal calls the “high correlation between S and T” strongly suggests that changes in S largely determine changes in T, the confirmation of which suggestion likely merely awaits what he refers to as an “improved understanding of possible climate sensitivity to relatively small total irradiance variation.”
In the following year, Willson and Mordvinov (2003) analyzed total solar irradiance (TSI) data obtained from different satellite platforms over the period 1978-2002, attempting to resolve various small but important inconsistencies among them. In doing so, they came to the realization that “construction of TSI composite databases will not be without its controversies for the foreseeable future.” Nevertheless, their most interesting result, in the estimation of the two researchers, was their confirmation of a +0.05%/decade trend between the minima separating solar cycles 21-22 and 22-23, which they say “appears to be significant.”
Willson and Mordvinov say the finding of the 0.05 percent/decade minimum-to-minimum trend “means that TSI variability can be caused by unknown mechanisms other than the solar magnetic activity cycle,” which means that “much longer time scales for TSI variations are therefore a possibility,” which they say “has obvious implications for solar forcing of climate.” Specifically, it means there could be undiscovered long-term variations in total solar irradiance of a magnitude that could possibly explain centennial-scale climate variability, which Bond et al. (2001) have already demonstrated to be related to solar activity, as well as the millennial-scale climatic oscillation that pervades both glacial and interglacial periods for essentially as far back in time as paleoclimatologists can see (Oppo et al., 1998; Raymo et al., 1998).
Like Willson and Mordvinov, Foukal (2003) acknowledged that “recent evidence from ocean and ice cores suggests that a significant fraction of the variability in northern hemisphere climate since the last Ice Age correlates with solar activity (Bond et al., 2001),” while additionally noting that “a recent reconstruction of S [total solar irradiance] from archival images of spots and faculae obtained daily from the Mt. Wilson Observatory in California since 1915 shows remarkable agreement with smoothed global temperature in the 20th century,” citing his own work of 2002. However, he was forced to acknowledge that the observed variations in S between 1978 and 2002 were not large enough to explain the observed temperature changes on earth within the context of normal radiative forcing. Hence, he proceeded to review the status of research into various subjects that might possibly be able to explain this dichotomy. Specifically, he presented an overview of then-current knowledge relative to the idea that “the solar impact on climate might be driven by other variable solar outputs of ultraviolet radiation or plasmas and fields via more complex mechanisms than direct forcing of tropospheric temperature.” As could have been expected, the article contained no grand revelations; when all was said and done, Foukal returned pretty much to where he had started, concluding that “we cannot rule out multi-decadal variations in S sufficiently large to influence climate, yet overlooked so far through limited sensitivity and time span of our present observational techniques.”
The following year, Damon and Laut (2004) reported what they described as errors made by Friis-Christensen and Lassen (1991), Svensmark and Friis-Christensen (1997), Svensmark (1998) and Lassen and Friis-Christensen (2000) in their presentation of solar activity data, correlated with terrestrial temperature data. The Danish scientists’ error, in the words of Damon and Laut, was “adding to a heavily smoothed (‘filtered’) curve, four additional points covering the period of global warming, which were only partially filtered or not filtered at all.” This in turn led to an apparent dramatic increase in solar activity over the last quarter of the twentieth century that closely matched the equally dramatic rise in temperature manifest by the Northern Hemispheric temperature reconstruction of Mann et al. (1998, 1999) over the same period. With the acquisition of additional solar activity data in subsequent years, however, and with what Damon and Laut called the proper handling of the numbers, the late twentieth century dramatic increase in solar activity totally disappears.
This new result, to quote Damon and Laut, means that “the sensational agreement with the recent global warming, which drew worldwide attention, has totally disappeared.” In reality, however, it is only the agreement with the last quarter-century of the discredited Mann et al. “hockey stick” temperature history that has disappeared. This new disagreement is most welcome, for the Mann et al. temperature reconstruction is likely in error over this stretch of time. (See Section 3.2.)
Using a nonlinear non-stationary time series technique called empirical mode decomposition, Coughlin and Tung (2004) analyzed monthly mean geopotential heights and temperatures—obtained from Kalnay et al. (1996)—from 1000 hPa to 10 hPa over the period January 1958 to December 2003. This work revealed the existence of five oscillations and a trend in both datasets. The fourth of these oscillations has an average period of 11 years and indicates enhanced warming during times of maximum solar radiation. As the two researchers describe it, “the solar flux is positively correlated with the fourth modes in temperature and geopotential height almost everywhere [and] the overwhelming picture is that of a positive correlation between the solar flux and this mode throughout the troposphere.”
Coughlin and Tung concluded that “the atmosphere warms during the solar maximum almost everywhere over the globe.” And the unfailing omnipresent impact of this small forcing (a 0.1 percent change in the total energy output of the sun from cycle minimum to maximum) suggests that any longer-period oscillations of the solar inferno could well be causing the even greater centennial- and millennial-scale oscillations of temperature that are observed in paleotemperature data from various places around the world.
Additional light on the subject has been provided by widespread measurements of the flux of solar radiation received at the surface of the earth that have been made since the late 1950s. Nearly all of these measurements reveal a sizeable decline in the surface receipt of solar radiation that was not reversed until the mid-1980s, as noted by Wild et al. (2005). During this time, there was also a noticeable dip in earth’s surface air temperature, after which temperatures rose at a rate and to a level of warmth that the IPCC claims were both without precedent over the past one to two millennia, which phenomena they attribute to similarly unprecedented increases in greenhouse gas concentrations, the most notable, of course, being CO2.
This reversal of the decline in the amount of solar radiation incident upon the earth’s surface, in the words of Wild et al., “is reconcilable with changes in cloudiness and atmospheric transmission and may substantially affect surface climate.” They say, for example, that “whereas the decline in solar energy could have counterbalanced the increase in down-welling longwave energy from the enhanced greenhouse effect before the 1980s, the masking of the greenhouse effect and related impacts may no longer have been effective thereafter, enabling the greenhouse signals to become more evident during the 1990s.” Qualitatively, this scenario sounds reasonable; but when the magnitude of the increase in the surface-received flux of solar radiation over the 1990s is considered, the statement is seen to be rather disingenuous.
Over the range of years for which high-quality data were available to them (1992-2002), Wild et al. determined that the mean worldwide increase in clear-sky insolation averaged 0.68 Wm-2 per year, which increase they found to be “comparable to the increase under all-sky conditions.” Consequently, for that specific 10-year period, these real-world data suggest that the total increase in solar radiation received at the surface of the earth should have been something on the order of 6.8 Wm-2, which is not significantly different from what is implied by the satellite and “earthshine” data of Palle et al. (2004), although the satellite data of Pinker et al. (2005) suggest an increase only about a third as large for this period.
Putting these numbers in perspective, Charlson et al. (2005) report that the longwave radiative forcing provided by all greenhouse gas increases since the beginning of the industrial era has amounted to only 2.4 Wm-2, citing the work of Anderson et al. (2003), while Palle et al. say that “the latest IPCC report argues for a 2.4 Wm-2 increase in CO2 longwave forcing since 1850.” Consequently, it can be readily appreciated that the longwave forcing of greenhouse gases over the 1990s would have been but a fraction of a fraction of the observed increase in the contemporary receipt of solar radiation at the surface of the earth. To thus suggest, as Wild et al. do, that the increase in insolation experienced at the surface of the earth over the 1990s may have enabled anthropogenic greenhouse gas signals of that period to become more evident, seems incongruous, as their suggestion implies that the bulk of the warming of that period was due to increases in greenhouse gas concentrations, when the solar component of the temperature forcing was clearly much greater. And this incongruity is made all the worse by the fact that methane concentrations rose ever more slowly over this period, apparently actually stabilizing near its end (see Section 2.6. Methane). Consequently, a much more logical conclusion would be that the primary driver of the global warming of the 1990s was the large increase in global surface-level insolation.
A final paper of note from 2005 was that of Soon (2005), who explored the question of which variable was the dominant driver of twentieth century temperature change in the Arctic—rising atmospheric CO2 concentrations or variations in solar irradiance—by examining what roles the two variables may have played in decadal, multi-decadal, and longer-term variations in surface air temperature (SAT). He performed a number of statistical analyses on (1) a composite Arctic-wide SAT record constructed by Polyakov et al. (2003), (2) global CO2 concentrations taken from estimates given by the NASA GISS climate modeling group, and (3) a total solar irradiance (TSI) record developed by Hoyt and Schatten (1993, updated by Hoyt in 2005) over the period 1875-2000.
The results of these analyses indicated a much stronger statistical relationship between SATs and TSI, as opposed to SATs and CO2. Solar forcing generally explained well over 75 percent of the variance in decadal-smoothed seasonal and annual Arctic SATs, while CO2 forcing explained only between 8 and 22 percent of the variance. Wavelet analysis further supported the case for solar forcing of the SAT record, revealing similar time-frequency characteristics for annual and seasonally averaged temperatures at decadal and multi-decadal time scales. By contrast, wavelet analysis gave little to no indication of a CO2 forcing of Arctic SSTs. Based on these data and analyses, therefore, it would appear that the sun, not atmospheric CO2, has been the driving force for temperature change in the Arctic.
Lastovicka (2006) summarized recent advancements in the field, saying “new results from various space and ground-based experiments monitoring the radiative and particle emissions of the sun, together with their terrestrial impact, have opened an exciting new era in both solar and atmospheric physics,” stating that “these studies clearly show that the variable solar radiative and particle output affects the earth’s atmosphere and climate in many fundamental ways.” That same year, Bard and Frank (2006) examined “changes on different time scales, from the last million years up to recent decades,” and in doing so critically assessed recent claims that “the variability of the sun has had a significant impact on global climate.” “Overall,” in the judgment of the two researchers, the role of solar activity in causing climate change “remains unproven.” However, as they state in the concluding sentence of their abstract, “the weight of evidence suggests that solar changes have contributed to small climate oscillations occurring on time scales of a few centuries, similar in type to the fluctuations classically described for the last millennium: the so-called Medieval Warm Period (AD 900-1400) followed on by the Little Ice Age (AD 1500-1800).”
In another study from 2006, which also reviewed the scientific literature, Beer et al. (2006) explored what we know about solar variability and its possible effects on earth’s climate, focusing on two types of variability in the flux of solar radiation incident on the earth. The first type, in their words, “is due to changes in the orbital parameters of the earth’s position relative to the sun induced by the other planets,” which arises from gravitational perturbations that “induce changes with characteristic time scales in the eccentricity (~100,000 years), the obliquity (angle between the equator and the orbital plane, ~40,000 years) and the precession of the earth’s axis (~20,000 years),” while the second type is due to variability within the sun itself.
With respect to the latter category, the three researchers report that direct observations of total solar irradiance above the earth’s atmosphere have been made only over the past quarter-century, while observations of sunspots have been made and recorded for approximately four centuries. In between the time scales of these two types of measurements fall neutron count rates and aurora counts. Therefore, 10Be and other cosmogenic radionuclides (such as 14C) —stored in ice, sediment cores, and tree rings—currently provide our only means of inferring solar irradiance variability on a millennial time scale. These cosmogenic nuclides “clearly reveal that the sun varies significantly on millennial time scales and most likely plays an important role in climate change,” especially within this particular time domain. In reference to their 10Be-based derivation of a 9,000-year record of solar modulation, Beer et al. note that its “comparison with paleoclimatic data provides strong evidence for a causal relationship between solar variability and climate change.”
We have now reached the work of Nicola Scafetta, a research scientist in the Duke University physics department, and Bruce West, chief scientist in the mathematical and information science directorate of the U.S. Army Research Office in Research Triangle Park, North Carolina. To better follow the arc of their work, we’ll temporarily abandon our chronological ordering of this literature review.
Scafetta and West (2006a) developed “two distinct TSI reconstructions made by merging in 1980 the annual mean TSI proxy reconstruction of Lean et al. (1995) for the period 1900-1980 and two alternative TSI satellite composites, ACRIM (Willson and Mordvinov, 2003), and PMOD (Frohlich and Lean, 1998), for the period 1980-2000,” and then used a climate sensitivity transfer function to create twentieth century temperature histories. The results suggested that the sun contributed some 46 to 49 percent of the 1900-2000 global warming of the earth. Considering that there may have been uncertainties of 20 to 30 percent in their sensitivity parameters, the two researchers suggested the sun may have been responsible for as much as 60 percent of the twentieth century temperature rise.
Scafetta and West say the role of the sun in twentieth century global warming has been significantly underestimated by the climate modeling community, with various energy balance models producing estimates of solar-induced warming over this period that are “two to ten times lower” than what they found. The two researchers say “the models might be inadequate because of the difficulty of modeling climate in general and a lack of knowledge of climate sensitivity to solar variations in particular.” They also note that “theoretical models usually acknowledge as solar forcing only the direct TSI forcing,” thereby ignoring “possible additional climate effects linked to solar magnetic field, UV radiation, solar flares and cosmic ray intensity modulations.” In this regard, we additionally note that some of these phenomena may to some degree be independent of, and thereby add to, the simple TSI forcing Scafetta and West employed, which suggests that the totality of solar activity effects on climate may be even greater than what they calculated.
In a second study published in the same year, Scafetta and West (2006b) begin by noting that nearly all attribution studies begin with pre-determined forcing and feedback mechanisms in the models they employ. “One difficulty with this approach,” according to Scafetta and West, “is that the feedback mechanisms and alternative solar effects on climate, since they are only partially known, might be poorly or not modeled at all.” Consequently, “to circumvent the lack of knowledge in climate physics,” they adopt “an alternative approach that attempts to evaluate the total direct plus indirect effect of solar changes on climate by comparing patterns in the secular temperature and TSI reconstructions,” where “a TSI reconstruction is not used as a radiative forcing, but as a proxy [for] the entire solar dynamics.” They then proceed on the assumption that “the secular climate sensitivity to solar change can be phenomenologically estimated by comparing … solar and temperature records during the pre-industrial era, when, reasonably, only a negligible amount of anthropogenic-added climate forcing was present,” and when “the sun was the only realistic force affecting climate on a secular scale.”
Scafetta and West used the Northern Hemispheric temperature reconstruction of Moberg et al. (2005), three alternative TSI proxy reconstructions developed by Lean et al. (1995), Lean (2000), and Wang et al. (2005), and a scale-by-scale transfer model of climate sensitivity to solar activity changes created by themselves (Scafetta and West, 2005, 2006a) and found what they called a “good correspondence between global temperature and solar induced temperature curves during the pre-industrial period, such as the cooling periods occurring during the Maunder Minimum (1645-1715) and the Dalton Minimum (1795-1825).” In addition, they note that since the time of the seventeenth century solar minimum, “the sun has induced a warming of ΔT ~ 0.7 K,” and that “this warming is of the same magnitude [as] the cooling of ΔT ~ 0.7 K from the medieval maximum to the 17th century minimum,” which finding, in their words, “suggests the presence of a millenarian solar cycle, with … medieval and contemporary maxima, driving the climate of the last millennium,” as was first suggested fully three decades ago by Eddy (1976) in his seminal study of the Maunder Minimum.
Scafetta and West say their work provides substantive evidence for the likelihood that “solar change effects are greater than what can be explained by several climate models,” citing Stevens and North (1996), the Intergovernmental Panel on Climate Change (2001), Hansen et al. (2002), and Foukal et al. (2004); and they note that a solar change “might trigger several climate feedbacks and alter the greenhouse gas (H2O, CO2, CH4, etc.) concentrations, as 420,000 years of Antarctic ice core data would also suggest (Petit et al., 1999),” once again reiterating that “most of the sun-climate coupling mechanisms are probably still unknown,” and that “they might strongly amplify the effects of small solar activity increase.” That being said, however, the researchers note that in the twentieth century there was “a clear surplus warming” above and beyond what is suggested by their solar-based temperature reconstruction, such that something in addition to the sun may have been responsible for approximately 50 percent of the total global warming since 1900. This anomalous increase in temperature, it could be argued, was due to anthropogenic greenhouse gas emissions. Scafetta and West say the temperature difference since 1975, where the most noticeable part of the discrepancy occurred, may have been due to “spurious non-climatic contamination of the surface observations such as heat-island and land-use effects (Pielke et al., 2002; Kalnay and Cai, 2003),” which they say is also suggested by “an anomalous warming behavior of the global average land temperature vs. the marine temperature since 1975 (Brohan et al., 2006).”
In their next paper, Scafetta and West (2007) reconstructed a phenomenological solar signature (PSS) of climate for the Northern Hemisphere for the last four centuries that matches relatively well the instrumental temperature record since 1850 and the paleoclimate temperature proxy reconstruction of Moberg (2005). The period from 1950 to 2010 showed excellent agreement between 11- and 22-year PSS cycles when compared to smoothed average global temperature data and the global cooling that occurred since 2002. Describing their research in an opinion essay in Physics Today (published by the American Institute of Physics), they say “this cooling seems to have been induced by decreased solar activity from the 2001 maximum to the 2007 minimum as depicted in two distinct TSI reconstructions” and “the same patterns are poorly reproduced by present-day GCMs and are dismissively interpreted as internal variability (noise) of climate. The nonequilibrium thermodynamic models we used suggest that the Sun is influencing climate significantly more than the IPCC report claims” (Scafetta and West, 2008).
In 2009, Scafetta and a new coauthor, Richard C. Willson, senior research scientist at Columbia’s Center for Climate Systems Research, addressed the issue of whether or not TSI increased from 1980 to 2002. The IPCC assumed there was no increase by adopting the TSI satellite composite produced by the Physikalisch-Meteorologisches Observatorium Davos (PMOD) (see Frohlich, 2006). PMOD assumed that the NIMBUS7 TSI satellite record artificially increased its sensitivity during the ACRIM-gap (1999.5-1991.75), and therefore it reduced the NIMBUS7 record by 0.86 W/m2 during the ACRIM-gap period, and consequently the TSI results changed little since 1980. However, this PMOD adjustment of NIMBUS7 TSI satellite data was never acknowledged by the experimental teams (Willson and Mordvinov, 2003; supporting material in Scafetta and Willson, 2009).
Scafetta and Willson (2009) proposed to solve the ACRIM-gap calibration controversy by developing a TSI model using a proxy model based on variations of the surface distribution of solar magnetic flux designed by Krivova et al. (2007) to bridge the two-year gap between ACRIM1 and ACRIM2. They use this to bridge “mixed” versions of ACRIM and PMOD TSI before and after the ACRIM-gap. Both “mixed” models show, in the authors’ words, “a significant TSI increase of 0.033%/decade between the solar activity minima of 1986 and 1996, comparable to the 0.037% found in the TSI satellite ACRIM composite.” They conclude that “increasing TSI between 1980 and 2000 could have contributed significantly to global warming during the last three decades. Current climate models have assumed that TSI did not vary significantly during the last 30 years and have, therefore, underestimated the solar contribution and overestimated the anthropogenic contribution to global warming.”
Backing up now to 2007, Krivova et al. (2007) note there is “strong interest” in the subject of long-term variations of total solar irradiance or TSI “due to its potential influence on global climate,” and that “only a reconstruction of solar irradiance for the pre-satellite period with the help of models can aid in gaining further insight into the nature of this influence,” which is what they set about to achieve in their paper. They developed a history of TSI “from the end of the Maunder minimum [about AD 1700] to the present based on variations of the surface distribution of the solar magnetic field,” which was “calculated from the historical record of the sunspot number using a simple but consistent physical model,” i.e., that of Solanki et al. (2000, 2002).
Krivova et al. report that their model “successfully reproduces three independent datasets: total solar irradiance measurements available since 1978, total photospheric magnetic flux since 1974 and the open magnetic flux since 1868,” which was “empirically reconstructed using the geomagnetic aa-index.” Based on this model, they calculated an increase in TSI since the Maunder minimum somewhere in the range of 0.9-1.5 Wm-2, which encompasses the results of several independent reconstructions that have been derived over the past few years. In the final sentence of their paper, however, they also note that “all the values we obtain are significantly below the ΔTSI values deduced from stellar data and used in older TSI reconstructions,” the results of which range from 2 to 16 Wm-2.
Shaviv (2008) has attempted to quantify solar radiative forcing using oceans as a calorimeter. He evaluated three independent measures of net ocean heat flux over five decades, sea level change rate from twentieth century tide gauge records, and sea surface temperature. He found a “very clear correlation between solar activity and sea level” including the 11-year solar periodicity and phase, with a correlation coefficient of r=0.55. He also found “that the total radiative forcing associated with solar cycles variations is about 5 to 7 times larger than those associated with the TSI variations, thus implying the necessary existence of an amplification mechanism, though without pointing to which one.” Shaviv claims “the sheer size of the heat flux, and the lack of any phase lag between the flux and the driving force further implies that it cannot be part of an atmospheric feedback and very unlikely to be part of a coupled atmosphere-ocean oscillation mode. It must therefore be the manifestation of real variations in the global radiative forcing.” This provides “very strong support for the notion that an amplification mechanism exists. Given that the CRF [Cosmic Ray Flux]/climate links predicts the correct radiation imbalance observed in the cloud cover variations, it is a favorable candidate.” These results, Shaviv says, “imply that the climate sensitivity required to explain historic temperature variations is smaller than often concluded.”
Pallé et al. (2009) re-analyzed the overall reflectance of sunlight from Earth (“earthshine”) and re-calibrated the CERES satellite data to obtain consistent results for Earth’s solar reflectance. According to the authors, “Earthshine and FD [flux data] analyses show contemporaneous and climatologically significant increases in the Earth’s reflectance from the outset of our earthshine measurements beginning in late 1998 roughly until mid-2000.After that and to date, all three show a roughly constant terrestrial albedo, except for the FD data in the most recent years. Using satellite cloud data and Earth reflectance models, we also show that the decadal-scale changes in Earth’s reflectance measured by earthshine are reliable and are caused by changes in the properties of clouds rather than any spurious signal, such as changes in the Sun-Earth-Moon geometry.”
Ohmura (2009) reviewed surface solar irradiance at 400 sites globally to 2005. They found a brightening phase from the 1920s to 1960s followed by a 20-year dimming phase from 1960 to 1980. Then there is another 15-year brightening phase from 1990 to 2005. Ohmura finds “aerosol direct and indirect effects played about an equal weight in changing global solar radiation. The temperature sensitivity due to radiation change is estimated at 0.05 to 0.06 K/(W m-2).”
Long et al. (2009) analyzed “all-sky and clear-sky surface downwelling shortwave radiation and bulk cloud properties” from 1995 through 2007. They “show that widespread brightening has occurred over the continental United States … averaging about 8 W m-2/decade for all-sky shortwave and 5 W m-2/decade for the clear-sky shortwave. This all-sky increase is substantially greater than the (global) 2 W m-2/decade previously reported …” Their “results show that changes in dry aerosols and/or direct aerosol effects alone cannot explain the observed changes in surface shortwave (SW) radiation, but it is likely that changes in cloudiness play a significant role.”
These observations by Shaviv, Pallé, Ohmura, and Long et al. point to major variations in earth’s radiative budget caused by changes both in aerosols and clouds. Both are affected by natural and anthropogenic causes, including aircraft, power plants, cars, cooking, forest fires, and volcanoes. However, solar activity and cosmic rays also modulate clouds. When GCMs ignore or underestimate causes or modulation by solar cycles, magnetic fields and/or cosmic rays, they overestimate climate sensitivity and anthropogenic impacts.
Although there thus is still significant uncertainty about the true magnitude of the TSI change experienced since the end of the Maunder Minimum, the wide range of possible values suggests that long-term TSI variability cannot be rejected as a plausible cause of the majority of the global warming that has fueled earth’s transition from the chilling depths of the Little Ice Age to the much milder weather of the Current Warm Period. Indeed, the results of many of the studies reviewed in this summary argue strongly for this scenario, while others suggest it is the only explanation that fits all the data.
The measured judgment of Bard and Frank (2006) seems to us to be right on the mark. The role of solar activity in causing climate change is so complex that most theories of solar forcing of climate change must be considered to be as yet “unproven.” It would also be appropriate for climate scientists to admit the same about the role of rising atmospheric CO2 concentrations in driving recent global warming. If it is fairly certain that the sun was responsible for creating multi-centennial cold and warm periods, it is clear the sun could easily be responsible for the majority or even the entirety of the global warming of the past century or so.
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