Supporters of solar-effects theories counter this argument by contending that various positive feedback mechanisms may amplify minor solar perturbations to the extent that significant changes in climate do indeed result from changes in solar activity. In this summary, we highlight the findings of some of the scientific studies that describe such solar-climate linkages and that provide support for the emerging belief that small changes in solar activity were truly responsible for bringing about the coldest period of the past millennium (and perhaps of the entire Holocene), i.e., the Little Ice Age.

It has long been known that periods of higher solar activity result in a lower production of atmospheric ¹⁴C (Perry and Hsu, 2000). Therefore, as plants remove carbon from the air via photosynthesis and sequester it in their tissues -- and as they do so generation after generation -- they create and compile a continuous history of solar activity; and because of this fact, the history of ¹⁴C contained in tree-ring chronologies and other repositories of plant materials has been frequently employed as a proxy for solar activity and compared with various climatic indices.

A good example of this type of work was the study of Hong et al. (2000), who developed a high-resolution ð¹⁸O (proxy temperature) record from plant cellulose deposited in a peat bog in the Jilin Province of China, from which they inferred that the most dramatic cold events of the Little Ice Age were centered at roughly AD 1550, 1650 and 1750. In comparing this record with an atmospheric ¹⁴C record derived from tree rings, they found what they called a "remarkable, nearly one to one, correspondence," which led them to conclude that the temperature history of the region was "forced mainly by solar variability."

Two years later, this same sentiment was echoed by Xu et al. (2002), who had studied ð¹⁸O variations in cores retrieved from peat deposits at the northeastern edge of the Qinghai-Tibetan Plateau of China. On the basis of power spectrum analyses they conducted, they too concluded that the "main driving force" of the changing climate that produced the Little Ice Age's three ultra-cold episodes was obtained "from solar activities."

Working with a marine sediment core retrieved from the southern Norwegian continental margin, Berstad et al. (2003) established its chronology over the past 600 years by means of ²¹⁰Pb and ¹⁴C measurements, while they reconstructed sea surface temperatures from ð¹⁸O data derived from the remains of the planktonic foraminifera Neogloboquadrina pachyderma (summer temperatures) and Globigerina bulloides (spring temperatures). This work indicated, in their words, "that the summer water temperatures in the Norwegian Current were 1-2°C colder than at present most of the time between ca. AD 1400 and 1920." They also state that their data "suggest that the spring water temperature along the southern Norwegian continental margin was 1-3°C colder than at present most of the time between AD 1400 and 1700." In addition, they note that "the cold interval between ca. AD 1400 and 1700/1920 is coincident to the Little Ice Age," and within this interval they report that the two coldest periods were coincident with "the solar minima of 'Maunder' and 'Sporer'." Last of all, they indicate that "the youngest 70 years of the proxy record show the warmest temperature of the entire record, and are coincident to the 'Modern maximum' in solar irradiation."

In another study that found linkages between the coldest periods of the Little Ice Age and well-known solar minima, Mauquoy et al. (2002) extracted peat monoliths from ombrotrophic mires at Lille Vildmose (LVM), Denmark, and Walton Moss (WLM), UK, some 800 miles distant. From these monoliths, vegetative macrofossils were extracted at 1-cm intervals and examined by means of light microscopy. Where increases in the abundances of S. tenellum and S. cuspidatum were found, a closely spaced series of ¹⁴C AMS-dated samples preceding and following each of these "wet-shifts" was used to "wiggle-match" date them (van Geel and Mook, 1989), thereby enabling comparison of the wet-shifts with the history of ¹⁴C production during the Holocene.

This protocol revealed the existence of wet-shift climatic deteriorations that began in the mid-1400s and early 1600s at both the LVM and WLM sites, while data from the WLM monolith additionally revealed a wet-shift that started about 1215. These three climatic deteriorations mark the beginnings of periods of cool and wet conditions that correspond closely in time with the Wolf, Sporer and Maunder Minima of solar activity, as manifest in contemporary ¹⁴C data; and the researchers note that "these time intervals correspond to periods of peak cooling in 1000-year Northern Hemisphere climate records," leading them to state that their work adds to the "increasing body of evidence" that "variations in solar activity may well have been an important factor driving Holocene climate change."

Many other researchers have also made discoveries that suggest the Little Ice Age was likely produced by a state of unusually low solar activity. In the Asian subarctic, for example, Vaganov et al. (2000) discovered a significant correlation between solar activity and temperature during the Little Ice Age; and in North America, an analysis of more than 700 pollen diagrams by Viau et al. (2002) revealed a vegetation transition that "culminat[ed] in the Little Ice Age, with maximum cooling 300 years ago." In contemplating the reason for this transition, the latter researchers concluded that "although several mechanisms for such natural forcing have been advanced, recent evidence points to a potential solar forcing associated with ocean-atmosphere feedbacks acting as global teleconnections agents."

In the study that provided the backbone for this latter perspective, Bond et al. (2001) examined deep-sea sediment cores in the North Atlantic and cosmogenic nuclides sequestered in the Greenland ice cap (¹⁰Be) and Northern Hemispheric tree rings (¹⁴C), concluding that "it seems almost certain that the well-documented connection between the Maunder solar minimum and the coldest decades of the Little Ice Age could not have been a coincidence," and that the Little Ice Age "may have been partly or entirely linked to changes in solar irradiance."

Two model-based studies also point to a significant role for the sun in producing earth's Little Ice Age climate. Using a version of the Goddard Institute for Space Studies GCM, Shindell et al. (2001) estimated climatic differences between the period of the Maunder Minimum in solar irradiance (mid-1600s to early 1700s) and a century later, when solar output was relatively high for several decades. The results of their analysis led them to conclude that "colder winter temperatures over the Northern Hemispheric continents during portions of the 15th through the 17th centuries (sometimes called the Little Ice Age) ... may have been influenced by long-term solar variations."

In the second of the two model studies, Perry and Hsu (2000) developed a simple solar-luminosity model and used it to estimate total solar-output variations throughout the Holocene. The model output was well correlated with the amount of ¹⁴C in well-dated tree rings during and prior to the Little Ice Age, which finding, in their words, "supports the hypothesis that the sun is varying its energy production in a manner that is consistent with the superposition of harmonic cycles of solar activity."

So what changes in solar activity were responsible for producing earth's Little Ice Age? Several researchers have targeted the approximate 11-year sunspot cycle as a primary suspect, but recent work by Rozelot (2001) -- who noted that "warm periods on Earth correlate well with smaller apparent diameter of the Sun and colder ones with a bigger Sun" -- has added variations in the sun's radius to the mix.

With respect to the 11-year sunspot cycle, Dean et al. (2002) examined a 1500-year varve thickness time series they derived from a lake sediment core in Minnesota, USA; and they report that the signal of this oscillation was strongest between the 14th and 19th centuries, during the Little Ice Age. In addition, Parker (1999), Solanki et al. (2000) and Rigozo et al. (2001) all reported relative minima in the mean number of annual sunspots during the Little Ice Age, with sunspot numbers during this cold period registering more than 40 times fewer than recently (Rigozo et al., 2001). Similarly, analyses of other solar parameters by Rigozo et al. indicated that the strengths of the solar radio flux, the solar wind velocity and the southward component of the interplanetary magnetic field were 1.97, 1.11 and 2.67 times weaker during the Little Ice Age than they are presently.

How do these small changes in solar activity bring about significant and pervasive shifts in earth's global climate, such as the Medieval Warm Period to Little Ice Age to Current Warm Period transitions? In answer to this question, which has long plagued proponents of a solar-climate link, Bond et al. (2001) described a scenario in which solar-induced changes high in the stratosphere propagate downward through the atmosphere to the earth's surface, where they provoke changes in North Atlantic Deep Water formation that alter the global Thermohaline Circulation. Viewed from this perspective, Bond and his co-workers suggested that "solar signals thus may have been transmitted through the deep ocean as well as through the atmosphere, further contributing to their amplification and global imprint;" and in concluding their landmark paper, they wrote that the results of their study "demonstrate that the earth's climate system is highly sensitive to extremely weak perturbations in the sun's energy output," noting that their work "supports the presumption that solar variability will continue to influence climate in the future."

Work conducted subsequent to Bond et al.'s landmark study has pointed to the same conclusion. Frohlich and Lean (2002), for example, compared the composite total solar irradiance (TSI) record derived from 23 years of spacecraft-acquired data, augmented by balloon- and rocket-acquired data, 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, according to them, "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."

So what did the two solar scientists find? They report 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 it would thus appear that solar irradiance variability alone can explain a significant portion of the warming experienced by the earth in recovering from the global chill of the Little Ice Age, leaving not a whole lot more to be attributed to other solar-related phenomena.

In another enlightening study, Pang and Yau (2002) assembled and analyzed a vast amount of data pertaining to phenomena that have been reliably linked to variations in solar activity, including frequencies of sunspot and aurora sightings, the abundance of ¹⁴C in the rings of long-lived trees, and the amount of ¹⁰Be in the annual layers of polar ice cores. In the case of sunspot sightings, they say they used a catalogue of 235 Chinese, Korean and Japanese records compiled by Yau (1988), a catalogue of 270 Chinese records compiled by Zhuang and Wang (1988), and a time chart of 139 records developed by Clark and Stephenson (1979), as well as a number of later catalogues that made the overall record more complete.

Based on the histories they developed for the past 1800 years, the two researchers identified "some nine cycles of solar brightness change," including the well-known Oort, Wolf, Sporer, Maunder and Dalton Minima. With respect to the Maunder Minimum -- which occurred between 1645 and 1715 and is widely acknowledged to have been responsible for some of the coldest weather of the Little Ice Age -- they report that the temperatures of that period "were about one-half of a degree Celsius lower than the mean for the 1970s, consistent with the decrease in the decadal average solar irradiance." Then, from 1795 to 1825, came the Dalton Minimum, along with another dip in Northern Hemispheric temperatures. Since that time, however, the authors say "the sun has gradually brightened" and "we are now in the Modern Maximum," which we and many others feel is largely responsible for the warmth of the Current Warm Period.

Pan and Yau go on to say that although the long-term variations in solar brightness they identified "account for less than 1% of the total irradiance, there is clear evidence that they affect the earth's climate." Their dual plot of total solar irradiance and Northern Hemispheric temperature from 1620 to the present, for example, indicates that the former parameter (when appropriately scaled, but without reference to any specific climate-change mechanism) can account for essentially all of the net change experienced by the latter parameter up to about 1980. After that time, the IPCC surface air temperature record rises dramatically, but radiosonde and satellite temperature histories largely match what would be predicted from the solar irradiance record.

In another study of solar irradiance variations, this one covering the past 1200 years, Bard et al. (2000) list some of the many different types of information that have been used to reconstruct past solar variability, including "the envelope of the 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 sunspot number (Hoyt and Schatten, 1993; Zhang et al., 1994; Reid, 1997), the solar rotation and diameter (Nesme-Ribes et al., 1993), and the geomagnetic aa index (Cliver et al., 1998)." They also note 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 they report that Solanki and Fligge (1998) developed an even more complex technique.

In their own research, Bard et al. used an entirely different approach. Rather than directly characterizing some aspect of solar variability, certain consequences of that variability were assessed. Noting that magnetic fields of the solar wind deflect portions of the flux of charged cosmic particles in the vicinity of the earth, leading to reductions in the creation of cosmogenic nuclides in earth's atmosphere, they worked with histories of atmospheric ¹⁴C and ¹⁰Be concentrations that can be used as proxies for solar activity, as noted many years earlier by Lal and Peters (1967).

In applying this approach to the subject, Bard et al. 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 record contained in tree rings (Bard et al., 1997). This cosmonuclide history was then converted to a Total Solar Irradiance (TSI) history 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 protocol resulted in an extended TSI record that suggests, in the words of Bard et al., 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." These results led the researchers to say "it could thus be argued that irradiance variations may have contributed to the so-called 'little ice age' and 'medieval warm period'," noting further that the TSI variations they identified "would tend to force global effects."

In a more recent review paper dealing with the temporal variability of various solar phenomena, Lean (2005) of the Naval Research Laboratory's E.O. Hulburt Center for Space Research in Washington, DC, USA makes the following important but disturbing point about climate models and the sun-climate connection: "a major enigma is that general circulation climate models predict an immutable climate in response to decadal solar variability, whereas surface temperatures, cloud cover, drought, rainfall, tropical cyclones, and forest fires show a definite correlation with solar activity (Haigh, 2001; Rind, 2002)."

So what's going on here? The answer is that a vast repository of empirical findings from an array of scientific disciplines is being ignored by a small coterie of climate scientists who are focused almost exclusively on developing computer models of how they believe earth's climate system operates. Any observation that fails to harmonize with that belief system is generally ignored by its adherents, while those who champion their approach to the subject often question the judgment and/or motives of scientists who place greater confidence in real-world observations.

So just how real is the sun-climate connection that ranks so low on the climate modelers' scale of significance?

Lean begins her foray into this highly-charged subject by noting that the beginning of the Little Ice Age "coincided with anomalously low solar activity (the so-called Sporer and Maunder minima)," and that "the latter part coincided with both low solar activity (the Dalton minimum) and volcanic eruptions." Then, after discussing the complexities and implications of these facts, she muses about an alternative thought -- "might the Little Ice Age be simply the most recent cool episode of millennial climate-oscillation cycles?" -- which, we hasten to add, might well be driven by a millennial-scale cycle of solar activity.

Lean also cites evidence for the sensitivity of drought and rainfall to solar variability, stating that climate models are unable to reproduce the plethora (her word) of sun-climate connections. In addition, she notes that simulations with climate models yield decadal and centennial variability even in the absence of external forcing, stating that "arguably, this very sensitivity of the climate system to unforced oscillation and stochastic noise predisposes it to nonlinear responses to small forcings such as by the sun," which argument pretty much invalidates the climate modelers' claim that solar forcing is too weak to produce the degree of warming and cooling that is often ascribed to it by scientists who are not fettered by the constraints of the climate modeling enterprise.

In further buttressing her position on the issue, Lean accurately reports that "various high-resolution paleoclimate records in ice cores, tree rings, lake and ocean sediment cores, and corals suggest that changes in the energy output of the sun itself may have contributed to sun-earth system variability," citing the work of Verschuren et al. (2000), Hodell et al. (2001), and Bond et al. (2001). Indeed, she notes that "many geographically diverse records of past climate are coherent over time, with periods near 2400, 208 and 90 years that are also present in the ¹⁴C and ¹⁰Be archives," which isotopes (produced at the end of a complex chain of interactions that are initiated by galactic cosmic rays) contain information about various aspects of solar activity (Bard et al., 1997). As a result, Lean rhetorically wonders in her concluding paragraph "How much of earth's recent surface warming is induced by solar rather than anthropogenic forcings?" We likewise wonder, suspecting that solar forcing may well be the dominant driver of 20th-century global warming.

In light of these several empirical findings, it is clear there is ample evidence for defending the proposition that the global warming of the past century or so may well have been nothing more than the natural recovery of the earth from the global chill of the solar-induced Little Ice Age. Viewed from this perspective, the Current Warm Period is seen to be the normal state of Holocene or current interglacial climate, with the Little Ice Age being the aberration, which was likely caused by a less-common state of significantly reduced solar activity.

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