Many solar-climate studies utilize tree-ring records of ¹⁴C as a measure of solar activity, because solar activity (including variations in the number of sunspots and the brightness of the sun) influences the production of atmospheric ¹⁴C, such that periods of higher solar activity yield a lower production and atmospheric burden of ¹⁴C (Perry and Hsu, 2000). This being the case, it can be appreciated that as trees remove carbon from the air and sequester it in their tissues, they are recording a history of solar activity that could be influencing earth's atmosphere-ocean system. Thus, the history of ¹⁴C contained in tree rings has been examined by a number of authors as a proxy indicator of solar activity and compared with various indices of climate.

As a specific example of this type of work, Neff et al. (2001) investigated the relationship between a ¹⁴C tree-ring record and a proxy record of monsoon rainfall intensity recorded in calcite δ¹⁸O data obtained from a stalagmite in northern Oman for the period 9,600-6,100 years ago. In doing so, they found what they called an "extremely strong" relationship between the two data sets; and the presence of this strong correlation, coupled with the fact that a spectral analysis yielded similar periodicities in both data sets, led them to conclude there is "solid evidence" that both the ¹⁴C and δ¹⁸O signals are responding to solar forcing.

In another tree-ring study, this time from northeastern Mongolia, Pederson et al. (2001) also report "possible evidence for solar influences." For the period 1651-1995, they reconstructed annual precipitation and streamflow histories for this region from tree-ring chronologies. Then, they subjected their data to spectral analysis, which revealed significant periodicities around 12 and 20-24 years that are believed to be solar-induced.

Moving to equatorial east Africa, Verschuren et al. (2000) developed a decadal-scale history of rainfall and drought for the past thousand years based on lake-level and salinity fluctuations of a small crater-lake basin in Kenya, after which they compared this history with an equally long record of atmospheric ¹⁴C production. The results of their analysis showed that a relatively wet period from AD 1270 to 1850 was interrupted by three periods of prolonged dryness: 1390-1420, 1560-1625 and 1760-1840, all three of which episodes were "broadly coeval with phases of high solar radiation," while "the intervening periods of increased moisture were coeval with phases of low solar radiation."

In Europe, a review of the relationship of extreme weather events to climate during the Holocene implicates solar forcing as the factor responsible for above-average rainfall during the Little Ice Age. There, according to Starkel (2002), continuous rains and high-intensity downpours that coincided with periods of reduced solar activity were major problems that often led to severe flooding.

Although a realistic physical mechanism for a solar-induced precipitation effect has been difficult to identify, numerous studies have suggested that the increased (decreased) cosmic ray flux at the solar minimum (maximum) causes increased (decreased) ice-nucleation, precipitation and precipitation efficiency at high geomagnetic latitudes and decreased (increased) ice-nucleation, precipitation and precipitation efficiency at low geomagnetic latitudes. Hence, using (1) cosmic ray data recorded by ground-based neutron monitors, (2) global precipitation data from the Climate Predictions Center Merged Analysis of Precipitation (CMAP) project, and (3) estimates of monthly global moisture from the National Centers for Environmental Prediction (NCEP) reanalysis project, Kniveton and Todd (2001) set out to determine whether there is any empirical evidence to support the hypothesis that solar variability (determined by changes in cosmic ray flux) is linked to climate change (manifested by changes in precipitation and precipitation efficiency) over the period 1979-1999.

What the two scientists found was "evidence of a statistically strong relationship between cosmic ray flux, precipitation and precipitation efficiency over ocean surfaces at mid to high latitudes," as variations in both precipitation and precipitation efficiency for mid to high latitudes showed a close relationship in both phase and magnitude with variations in cosmic ray flux, varying 7-9% during the solar cycle of the 1980s. Other potential factors that might explain the trends in precipitation and precipitation efficiency were ruled out due to poorer statistical relationships between them and the precipitation parameters investigated.

The study of Kniveton and Todd suggests that small changes in solar output can indeed produce significant changes in earth's climate. Consequently, with empirical evidence mounting for a solar-induced effect on precipitation, and given the fact that the total magnetic flux leaving the sun has risen by a factor of 1.41 over the period 1964-1996 and by a factor of 2.3 since 1901 (Lockwood et al., 1999), climate modelers should be paying more attention to these phenomena and incorporating them into their general circulation models of the atmosphere; for it could well be that much, if not all, of the warming of the past century had its origins in solar variability and not the historical rise in the air's CO2 concentration. Not surprisingly, however, the Chambers et al. (1999) review of the climate models used by the Intergovernmental Panel on Climate Change to predict future greenhouse gas-induced global warming revealed such solar-induced processes to be inadequately represented and even ignored.

So what has been learned over the last couple of years?

Several researchers have studied the precipitation histories of regions along the Danube River in western Europe and their effects on river discharge, with some suggesting that an anthropogenic signal is present in the latter decades of the 20th century and that it is responsible for that period's drier conditions. Wondering about the veracity of these claims, Ducic (2005) examined them by analyzing observed and reconstructed river discharge rates near Orsova, Serbia over the period 1731-1990. This work revealed that the lowest five-year discharge value in the pre-instrumental era (period of occurrence: 1831-1835) was practically equal to the lowest five-year discharge value in the instrumental era (period of occurrence: 1946-1950), and that the driest decade of the entire 260-year period was 1831-1840. Similarly, the highest five-year discharge value for the pre-instrumental era (period of occurrence: 1736-1740) was nearly equal to the five-year maximum discharge value for the instrumental era (period of occurrence: 1876-1880), differing by only 0.7%. What is more, the discharge rate for the last decade of the record (1981-1990), which prior researchers had claimed was anthropogenically-influenced, was found to be "completely inside the limits of the whole series," in Ducic's words, and only slightly (38 m³s^-1 or 0.7%) less than the 260-year mean of 5356 m³s^-1. Consequently, Ducic concluded, and rightly so, that "modern discharge fluctuations do not point to [a] dominant anthropogenic influence." In fact, Ducic's correlative analysis suggests that the detected cyclicity in the record could "point to the domination of the influence of solar activity."

Wang et al. (2005), who had previously reported on a Holocene record of the Asian Monsoon (AM) that shows that shifts in stalagmite δ¹⁸O largely reflect changes in precipitation that relate to changes in AM strength (Yuan et al., 2004; Dykoski et al., 2005), extended that earlier work with a 9000-year-long higher resolution (4.5-year) absolute-dated δ¹⁸O record of a stalagmite recovered from Dongge Cave (25°17'N, 108°5'E) in southern China, which they compared with the atmospheric ¹⁴C record (a proxy for solar activity) and climate records from lands surrounding the North Atlantic Ocean. This work indicated that their record broadly follows summer insolation but is punctuated by eight significantly weaker monsoon periods lasting from one to five centuries, most of which correlate with North Atlantic ice-rafting events. In addition, they find that "cross-correlation of the decadal- to centennial-scale monsoon record with the atmospheric carbon-14 record shows that some, but not all, of the monsoon variability at these frequencies results from changes in solar output," similar to "the relation observed in the record from a southern Oman stalagmite (Fleitmann et al., 2003)."

In a News item (Kerr, 2005) that accompanied the report of Wang et al., one of the report's authors (Hai Cheng of the University of Minnesota) says their study clearly suggests that "the intensity of the summer [East Asian] monsoon is affected by solar activity." Dominik Fleitman, who worked with the Oman stalagmite, also says "the correlation is very strong," stating that it is probably the best monsoon record he has seen, calling it "even better than ours." Last of all, Gerald North of Texas A & M University, who Kerr calls a "longtime doubter," says he finds the monsoon's solar connection "very hard to refute," although he says that "the big mystery is that the solar signal should be too small to trigger anything." Clearly, it is time to put more effort into solving that "big mystery," for as is readily apparent from the wealth of material archived in the Solar Effects section of our Subject Index, there are a host of climatic phenomena that owe their existence to some type of solar-climate connection.

Also studying the Asian monsoon were Ji et al. (2005), who used reflectance spectroscopy on a sediment core taken from Qinghai Lake, located in the northeastern part of the Qinghai-Tibet Plateau, to obtain a continuous high-resolution proxy record of the Asian monsoon over the past 18,000 years. Their efforts indicated that monsoonal moisture since the late glacial period was subject to "continual and cyclic variations," among which was a "very abrupt onset and termination" of a 2000-year dry spell that started about 4200 yr BP and ended around 2300 yr BP. Other variations included the well-known centennial-scale cold and dry spells of the Dark Ages Cold Period (DACP) and Little Ice Age (LIA), which lasted from 2100 yr BP to 1800 yr BP and 780 yr BP to 400 yr BP, respectively. Sandwiched between them was the warmer and wetter Medieval Warm Period, while preceding the DACP was the Roman Warm Period.

Time series analysis of the sediment record revealed statistically significant periodicities (above the 95% level) of 123, 163, 200 and 293 years, the 200-year periodicity of which corresponds well with the de Vries or Suess solar cycle and implies that change in solar activity is an important trigger for some of the cyclic environmental changes at Qinghai Lake. Once again, therefore, we have another study that indicates that large and abrupt fluctuations in the Asian monsoon have occurred with great regularity throughout the Holocene, and that the sun is an important factor driving some of these changes.

At this point in time (five years into the 21st century), in a review of the temporal variability of various solar phenomena, Lean (2005) made 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?

What's going on is that a vast repository of empirical findings from an array of scientific disciplines is being ignored by a small coterie of climate scientists that is 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 practitioners, 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, be they based on instrumental or proxy data.

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

Lean begins her foray into this aspect of her review 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 of this potential relationship, she muses about another alternative: "Or might the Little Ice Age be simply the most recent cool episode of millennial climate-oscillation cycles?" ... which, we hasten to add, may be driven by a similar-scale cycle of solar activity.

Lean also cites evidence that reveals 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).

In her concluding paragraph, Lean thus rhetorically wonders "How much of earth's recent surface warming is induced by solar rather than anthropogenic forcings?" We likewise wonder the same, suspecting that solar forcing may well be the dominant driver of 20th-century global warming, with anthropogenic CO2 emissions playing a much more minor role.

Echoing the sentiments of Lean, Hughes et al. (2006) write that, over the last decade, "evidence suggesting a link between solar irradiance and sub-Milankovitch-scale palaeoclimatic change has mounted," and they say that this "solar hypothesis, as an explanation for Holocene climate change, is now gaining wider acceptance." Pursuing it still further, they present "a multi-proxy palaeoclimate record from a coastal plateau bog in Newfoundland called Nordan's Pond Bog," based upon "analyses of plant macrofossils, testate amoebae and the degree of peat humification," which enabled them to create "a single composite reconstruction of bog surface wetness (BSW)" that they compare with "records of cosmogenic isotope flux."

The seven researchers' study demonstrates that "at least 14 distinctive phases of increased BSW may be inferred from the Nordan's Pond Bog record," commencing at 8270 cal. years BP, and that "comparisons of the BSW reconstruction with records of cosmogenic isotope flux ... suggest a persistent link between reduced solar irradiance and increased BSW during the Holocene." As a result, they conclude that the "strong correlation between increased ¹⁴C production [which accompanies reduced solar activity] and phases of maximum BSW supports the role of solar forcing as a persistent driver of changes to the atmospheric moisture balance throughout the Holocene," which finding further suggests that the sun likely orchestrated the Little Ice Age to Current Warm Period transition, which altered precipitation regimes around the globe. Consequently, variable solar activity is likely also largely responsible for the temperature change associated with that transition, which leaves little room for increasing greenhouse gas concentrations to have had much of an impact on the planet's temperature over this period.

Last of all, and most recently, we come to the study of Asmerom et al. (2007), who developed what they call "the first complete high-resolution [17-year] climate proxy for the southwest [United States] in the form of δ¹⁸O variations in a speleothem covering the entire Holocene," which they derived from a 14-cm-long stalagmite found in Pink Panther Cave in the Guadalupe Mountains of New Mexico. A spectral analysis performed on the raw δ¹⁸O data revealed significant peaks that the researchers say "closely match previously reported periodicities in the ¹⁴C content of the atmosphere, which have been attributed to periodicities in the solar cycle (Stuiver and Braziunas, 1993)." More specifically, they say that cross spectral analysis of the Δ¹⁴C and δ¹⁸O data confirms that the two records have matching periodicities at 1533 years (the Bond cycle), 444 years, 170 years, 146 years, and 88 years (the Gleissberg cycle). In addition, they report that periods of increased solar radiation correlate with periods of decreased rainfall in the southwestern United States (via changes in the North American monsoon), and that this behavior is just the opposite of what is observed with the Asian monsoon. These observations thus lead them to suggest that the proposed solar link to Holocene climate operates "through changes in the Walker circulation and the Pacific Decadal Oscillation and El Niño-Southern Oscillation systems of the tropical Pacific Ocean."

In conclusion, since the warming of the 20th century appears to represent the most recent rising phase of the Bond cycle, which in its previous rising phase produced the Medieval Warm Period (see Bond et al., 2001), and since we could still be imbedded in that rising temperature phase, which could well continue for some time to come, there is a reasonable probability that the desert southwest of the United States could experience an intensification of aridity in the not too distant future, and that wetter conditions could be expected in the monsoon regions of Asia, even without any further increases in atmospheric greenhouse gases, which realization makes planning for these contingencies a much higher priority -- and a much more logical endeavor -- than fooling ourselves into thinking we can prevent such climate changes by cutting back on the burning of fossil fuels.

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