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Solar Influence on Climate (Irradiance Measurements) -- Summary
We begin our review of the potential effects of solar activity on earth's climate with the study of Karln (1998), who examined proxy climate data related to changes in summer temperatures in Scandinavia over the last 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; and it was noted by Karln that during warm periods the temperature was "about 2C 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." And 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." In fact, 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."

Ruminating on Lockwood et al.'s findings, 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 2C 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 not the major direct cause of the predicted temperature increase but is instead 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 1200-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% 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." Hence, they said "it could thus be argued that irradiance variations may have contributed to the so-called 'little ice age' and 'medieval warm period'.'

In discussing this idea, Bard et al. amazingly 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 that 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 note that "some researchers" have indeed been working overtime to rewrite this aspect of earth's climatic history, claiming that both the Little Ice Age and Medieval Warm Period were restricted to lands bordering on the North Atlantic Ocean. However, as may readily be seen by perusing the many materials we have filed under the headings of Little Ice Age and Medieval Warm Period in our Subject Index, these phenomena were truly global in extent. Hence, when Bard et al. say their TSI variations "would tend to force global effects," they truly hit the nail on the head, as their global forcing function meshes well with the global climatic response of the earth.

Stepping one more year into the future, 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 precise 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." Surely, such findings demand that we explore them thoroughly before accepting some other phenomenon as the cause of global climate change, especially when that factor may be totally benign, or even beneficial, as in the case of atmospheric CO2 enrichment, which significantly enhances the growth of nearly all plants on the face of the earth (and in the ocean, as well).

In a second study from 2001, Rigozo et al. created a history of sunspot numbers for the last 1000 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; while 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; while 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 head and shoulders above all other periods of the past thousand years, with the Modern Maximum slightly besting the Medieval Maximum. Due to the many empirical evidences for climate modulation by solar variability, therefore, it is only to be expected that current temperatures may well be higher than at any other time during the past millennium. Since other factors come into play too, however, and since the Medieval and Modern Maximums were not all that different, this conclusion may not hold that precisely. In any event, the observations of this study suggest no need whatsoever for invoking variations in the air's CO2 content as a cause of temperature variations during any period of the past thousand years.

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, Frolich 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 0.4C, with pre-industrial fluctuations of 0.2C 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). Likewise, the anticipated rise in the air's CO2 content may also be much smaller than what is specified by the set of scenarios employed by them, due to simultaneous CO2-induced increases in biospheric carbon sequestration (Idso, 1991a,b). Hence, while past temperature changes seem reasonably well explained by solar radiation variations, the future -- as always -- is a much more murky matter.

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 0.02C/(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 0.11C/(W/m2), respectively, with the identical standard error of 0.02C/(W/m2). In addition, they reported that White et al. (1997) derived a decadal timescale solar sensitivity of 0.10 0.02C/(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 0.02C/(W/m2) from a paleo-reconstructed temperature record spanning the period 1610-1800. Hence, 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." And 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 0.2C 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), 0.55C, and Hansen et al. (1999), 0.65C, for the same period. It does not, however, include potential indirect effects of more esoteric solar climate-affecting phenomena that could also have been operative over this period.

Rounding out 2002, Foukal based his contribution to the quest for a solar-climate connection on the finding from analyses of space-borne radiometry 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 based upon 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."

Clearly, the UV findings of Foukal were not incredibly impressive; but the results of his total solar irradiance analysis were, leading him to emphatically state that "the possibility of significant driving of 20th 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 [our italics]," 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, Wilson 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."

Wilson and Mordvinov then went on to state that the finding of the 0.05%/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 well 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 the eye of proxy-climate science can see (Oppo et al., 1998; Raymo et al., 1998).

Like Wilson and Mordvinov, Foukal (2003) also 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; and 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."

In the year that followed, 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, which they correlated with terrestrial temperature data in a number of papers that appeared to explain most of the temperature variability of the earth over the past 140 years as arising from solar variability. The Danish scientists' error, in the words of Damon and Laut, of "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," led to an apparent dramatic increase in solar activity over the last quarter of the 20th 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 call the proper handling of the numbers, the late 20th-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 Mann et al. hockeystick temperature history that has disappeared; and this new disagreement is most welcome, for the Mann et al. temperature reconstruction is likely vastly in error over this stretch of time. Our website, for example, is replete with numerous reviews of studies that have found the late 1930s and early 1940s to have been the warmest period of the past century throughout the part of the planet claimed by climate-alarmists to be the most sensitive to CO2-induced global warming, i.e., the Arctic (see Arctic Temperatures (Variability - Late Holocene) in our Subject Index); and these temperature reconstructions now take the place of the Mann et al. temperature history in displaying "sensational agreement" with the new-and-improved solar activity history produced by Damon and Laut, which also peaks in the late 1930s and early 1940s.

Also in 2004, using a nonlinear non-stationary time series technique called empirical mode decomposition, Coughlin and Tung 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 data sets. The fourth of these oscillations has an average period of eleven 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."

Yes, north to south, east to west, top to bottom, 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% 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 climate alarmists claim 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 W m-2 per year, which increase they found to be "comparable to the increase under all-sky conditions." Consequently, for that specific ten-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 W m-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 W m-2, citing the work of Anderson et al. (2003), while Palle et al. say that "the latest IPCC report argues for a 2.4 W m-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 -- i.e., 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 just a tad 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 Methane (Atmospheric Concentrations) in our Subject Index. 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, who explored the question of which variable was the dominant driver of 20th-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). More specifically, 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. In fact, solar forcing generally explained well over 75% of the variance in decadal-smoothed seasonal and annual Arctic SATs, while CO2 forcing only explained between 8 and 22% 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. In 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, and not atmospheric CO2, has been the driving force for temperature change in the Arctic.

Moving on to another year, 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 (Wilson and Mordvinov, 2003), and PMOD (Frolich and Lean, 1998), for the period 1980-2000," after which they used what they deemed to be appropriate climate sensitivity transfer functions to transform the TSI histories they developed into 20th-century temperature histories. The results of these several procedures suggested that the sun contributed some 46-49% of the 1900-2000 global warming of the earth; and considering that there may have been uncertainties of 20-30% in their sensitivity parameters, the two researchers suggested that the sun may possibly have been responsible for as much as 60% of the 20th-century temperature rise.

In discussing their findings, Scafetta and West suggest that the role of the sun in 20th-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. Why is this so? The two researchers say that "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) 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) -- together with the scale-by-scale transfer model of climate sensitivity to solar activity changes created by themselves (Scafetta and West, 2005, 2006a) to make a strong case for the proposition that most of the major temperature fluctuations of the prior millennium were driven by changes in solar activity.

The two researchers began by noting that in nearly all attribution studies, where attempts had been made to determine what was responsible for orchestrating the course of earth's past temperature history, the approach used by the Intergovernmental Panel on Climate Change and most climate modelers had been to use 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," as they describe it, they adopted "an alternative approach that attempts to evaluate the total direct plus indirect effect of solar changes on climate by comparing patterns [our italics] 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," parts of which, we again emphasize, may be only partially understood or even totally unknown.

Based on this underlying philosophy, Scafetta and West proceeded 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."

In pursuing this course of action, the two scientists 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 17th 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.

In discussing their findings, Scafetta and West assert that their work provides substantive evidence for the likelihood that "solar change effects are greater than what can be explained by several climate models," citing, in this regard, the models of Stevens and North (1996), the Intergovernmental Panel on Climate Change (2001), Hansen et al. (2002) and Foukal et al. (2004); and in further explaining how this may be, 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," as Scafetta and West's findings clearly indicate they do, although by what singular or multiple means remains to be clarified.

We agree with this assessment of the subject; for it is the story told by real-world data, as revealed by the logical way in which Scafetta and West conducted their insightful but straightforward analysis. The sun's fingerprints on earth's climatic history have been laid bare by them for all to see. Without even a need to squint, there is simply no mistaking them.

That being said, however, the researchers note that in the 20th 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% of the total global warming since 1900; and this anomalous increase in temperature could be argued to be due to anthropogenic greenhouse gas emissions.

On the other hand, 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)."

The take-home message of the Scafetta and West paper would thus appear to be that the sun alone was responsible for most of the temperature variability of earth's Northern Hemisphere over all but perhaps the final 25 years of the past four centuries, as well as over much of the prior 600 years (which includes a good portion the Medieval Warm Period), while it is yet to be conclusively determined if the non-solar-induced portion of the warming of the last quarter of the 20th century was due to (1) anthropogenic greenhouse gas emissions, (2) spurious non-climatic contamination of the temperature record or (3) some mix of the two factors.

In introducing another of the 2006 studies we have reviewed, we noted that throughout the 1980s and 90s, Idso (1998) had published several papers wherein he had analyzed a number of what he called "natural experiments" in a multifaceted quest to quantify earth's near-surface air temperature response to perturbations of the planet's surface radiative heat balance, after which Nir J. Shaviv of the Hebrew University of Jerusalem's Racah Institute of Physics took up the identical challenge, similarly deriving a number of pertinent results (Shaviv, 2005).

Knowing that variations in solar activity correlate closely with climatic variations, but that climatic changes attributable to changes in solar activity are much larger than can be explained by changes in solar irradiance, Shaviv realized that an amplifier of some sort must be involved in the solar/climate relationship. What he and many other researchers have suggested, in this regard, is that when solar activity increases and the weak magnetic field that is carried by the solar wind intensifies (providing more shielding of the earth from low-energy galactic cosmic rays), there is a reduction in cosmic ray-induced ion production in the lower atmosphere that results in the creation of fewer condensation nuclei there and, hence, less low-level cloud cover, which allows more solar radiation to impinge upon the earth, increasing surface air temperature (and vice versa throughout).

Shaviv next identified six periods in earth's history (the entire Phanerozoic, the Cretaceous, the Eocene, the Last Glacial Maximum, the 20th century, and the eleven-year solar cycle as manifest over the last three centuries) for which he was able to derive reasonably sound estimates of different time-scale changes in radiative forcing, temperature and cosmic ray flux. From these sets of data he derived probability distribution functions of whole-earth temperature sensitivity to radiative forcing for each of the six time periods and combined them to obtain a mean planetary temperature sensitivity to radiative forcing of 0.28C per Wm-2. Then, noting that the IPCC (2001) had suggested that the increase in anthropogenic radiative forcing over the 20th century was about 0.5 Wm-2, Shaviv calculated that the anthropogenic-induced warming of the globe over this period was approximately 0.14C (0.5 Wm-2 x 0.28C per Wm-2). This result harmonizes perfectly with the temperature increase (0.10C) that was calculated by Idso (1998) to be due solely to the 20th-century increase in the air's CO2 concentration (75 ppm), which would have been essentially indistinguishable from Shaviv's result if the warming contributions of the 20th-century concentration increases of all greenhouse gases had been included in the calculation.

Next, based on information that indicated a solar activity-induced increase in radiative forcing of 1.3 Wm-2 over the 20th century (by way of cosmic ray flux reduction), plus the work of others (Hoyt and Schatten, 1993; Lean et al., 1995; Solanki and Fligge, 1998) that indicated a globally-averaged solar luminosity increase of approximately 0.4 Wm-2 over the same period, Shaviv calculated an overall solar activity-induced warming of 0.47C (1.7 Wm-2 x 0.28C per Wm-2) over the 20th century. Added to the 0.14C of anthropogenic-induced warming, the calculated total warming of the 20th century thus came to 0.61C, which was noted by Shaviv to be very close to the 0.57C temperature increase that was said by the IPCC to have been observed over that 100-year time period. Consequently, both Shaviv's and Idso's analyses, which mesh well with real-world data of both the recent and distant past, suggest that only 15-20% (0.10C/0.57C) of the observed warming of the 20th-century can be attributed to the concomitant rise in the air's CO2 content.

In a further development in the same year, Lastovicka (2006), in broadly summarizing recent advancements in the field, wrote that "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." Consequently, in a review of this broad area of research, 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)."

The measured judgment of Bard and Frank seems to us to be right on the mark. The subject they treat is so complex that most theories of solar forcing of climate change must be considered to be as yet "unproven." It would also be well for climate alarmists to admit the same about the role of rising atmospheric CO2 concentrations in driving recent global warming, especially in light of Bard and Frank's conclusion that "solar fluctuations were involved in causing widespread but limited climatic changes, such as the Little Ice Age (AD 1500-1800) that followed the Medieval Warm Period (AD 900-1400)," for if it is fairly certain that the sun was responsible for creating these multi-centennial cold and warm periods, it is clear it could easily be responsible for the vast majority of the global warming of the past century or so, which has yet to return the earth to the level of warmth experienced during the Medieval Warm Period, when there was 100 ppm less CO2 in the air than there is today, which CO2 deficit -- according to the climate-alarmist way of thinking -- should have made it even more difficult to sustain the higher-than-current temperatures of that earlier warm period.

In one final study from 2006, in which they too reviewed the scientific literature, Beer et al. 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 only been made 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; and as reported by Beer et al., who have studied the subject in depth, 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, for example, Beer et al. note that its "comparison with paleoclimatic data provides strong evidence [our italics] for a causal relationship between solar variability and climate change.

Rounding out this summary of work conducted on effects of solar irradiance on climate, we have the paper of Krivova et al. (2007), who 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. And achieve it they did, developing 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 data sets: 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.

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 out-of-hand 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 other studies reviewed in this summary argue strongly for this scenario, while others suggest it is the only explanation that fits all the data.

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Last updated 18 March 2009