Monsoons and solar variability
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From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
The IPCC’s computer models fail to predict variability in monsoon weather. One reason is because they underestimate the sun’s role.
Contents |
Tree Rings
For the period 9,600-6,100 years before present, Neff et al. (2001) investigated the relationship between a 14C tree-ring record and a δ18O proxy record of monsoon rainfall intensity as recorded in calcite δ18O data obtained from a stalagmite in northern Oman. According to the authors, the correlation between the two datasets was reported to be “extremely strong,” and a spectral analysis of the data revealed statistically significant periodicities centered on 779, 205, 134, and 87 years for the δ18O record and periodicities of 206, 148, 126, 89, 26, and 10.4 years for the 14C record.
Because variations in 14C tree-ring records are generally attributed to variations in solar activity and intensity, and because of this particular 14C record’s strong correlation with the δ18O record, as well as the closely corresponding results of the spectral analyses, the authors conclude there is “solid evidence” that both signals (the 14C and δ18O records) are responding to solar forcing.
Sediment Cores
Similar findings were reported by Lim et al. (2005), who examined the eolian quartz content (EQC) of a high-resolution sedimentary core taken from Cheju Island, Korea, creating a 6,500-year proxy record of major Asian dust events produced by northwesterly winter monsoonal winds that carry dust from the inner part of China all the way to Korea and the East China Sea. The Asian dust time series was found to contain both millennial- and centennial-scale periodicities; cross-spectral analysis between the EQC and a solar activity record showed significant coherent cycles at 700, 280, 210, and 137 years with nearly the same phase changes, leading the researchers to conclude that centennial-scale periodicities in the EQC could be ascribed primarily to fluctuations in solar activity.
In another study, Ji et al. (2005) used reflectance spectroscopy on a sediment core taken from Qinghai Lake in the northeastern part of the Qinghai-Tibet Plateau to construct a continuous high-resolution proxy record of the Asian monsoon over the past 18,000 years. As a result of this effort, monsoonal moisture since the late glacial period was shown to be subject to “continual and cyclic variations,” including the well-known centennial-scale cold and dry spells of the Dark Ages Cold Period (DACP) and Little Ice Age, which lasted from 2,100 to 1,800 yr BP and 780 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. Also, time series analysis of the sediment record revealed statistically significant periodicities (above the 95 percent level) of 123, 163, 200, and 293 years. The third of these periodicities corresponds well with the de Vries or Suess solar cycle, which suggests cyclical changes in solar activity are important triggers for some of the cyclical changes in monsoon moisture at Qinghai Lake.
Fossil Records
Citing studies that suggest the Indian summer monsoon may be sensitive to changes in solar forcing of as little as 0.25 percent (Overpeck et al., 1996; Neff et al., 2001; Fleitmann et al., 2003), Gupta et al. (2005) set out to test this hypothesis by comparing trends in the Indian summer monsoon with trends in solar activity across the Holocene. In this endeavor, temporal trends in the Indian summer monsoon were inferred from relative abundances of fossil shells of the planktic foraminifer Globigerina bulloides in sediments of the Oman margin, while temporal trends in solar variability were inferred from relative abundances of 14C, 10Be and haematite-stained grains.
Spectral analyses of the various datasets revealed statistically significant periodicities in the G. bulloides time series centered at 1550, 152, 137, 114, 101, 89, 83, and 79 years, all but the first of which periodicities closely matched periodicities of sunspot numbers centered at 150, 132, 117, 104, 87, 82, and 75 years. This close correspondence, in the words of Gupta et al., provides strong evidence for a “century-scale relation between solar and summer monsoon variability.” In addition, they report that intervals of monsoon minima correspond to intervals of low sunspot numbers, increased production rates of the cosmogenic nuclides 14C and 10Be, and increased advection of drift ice in the North Atlantic, such that over the past 11,100 years “almost every multi-decadal to centennial scale decrease in summer monsoon strength is tied to a distinct interval of reduced solar output,” and nearly every increase “coincides with elevated solar output,” including a stronger monsoon (high solar activity) during the Medieval Warm Period and a weaker monsoon (low solar activity) during the Little Ice Age.
As for the presence of the 1,550-year cycle in the Indian monsoon data, Gupta et al. consider it to be “remarkable,” since this cycle has been identified in numerous climate records of both the Holocene and the last glacial epoch (including Dansgaard/Oeschger cycles in the North Atlantic), strengthening the case for a sun-monsoon-North Atlantic link. Given the remarkable findings of this study, it is no wonder the researchers who conducted it say they are “convinced” there is a direct solar influence on the Indian summer monsoon in which small changes in solar output bring about pronounced changes in tropical climate.
In still another study, Khare and Nigam (2006) examined variations in angular-asymmetrical forms of benthic foraminifera and planktonic foraminiferal populations in a shallow-water sediment core obtained just off Kawar (14°49’43”N, 73°59’37”E) on the central west coast of India, which receives heavy river discharge during the southwest monsoon season (June to September) from the Kali and Gangavali rivers.
Down-core plots of the data showed three major troughs separated by intervening peaks; “since angular-asymmetrical forms and planktonic foraminiferal population are directly proportional to salinity fluctuations,” according to Khare and Nigam, “the troughs … suggest low salinity (increased river discharge and thus more rainfall),” and that “these wet phases are alternated by dry conditions.” They further report that the dry episodes of higher salinity occurred from AD 1320-1355, 1445-1535, and 1625-1660, and that the wet phases were centered at approximately AD 1410, 1590, and 1750, close to the ending of the sunspot minima of the Wolf Minima (AD 1280-1340), the Sporer Minima (AD 1420-1540), and the Maunder Minima (AD 1650-1710), respectively.
Although Khare and Nigam say that “providing a causal mechanism is beyond the scope of the present study,” they note that “the occurrence of periods of enhanced monsoonal precipitation slightly after the termination of the Wolf, Sporer and Maunder minima periods (less sun activity) and concomitant temperature changes could be a matter of further intense research.” The correspondences seem to be more than merely coincidental, especially when the inferences of the two researchers are said by them to be “in agreement with the findings of earlier workers, who reported high lake levels from Mono Lake and Chad Lake in the vicinity of solar minima,” as well as the Nile river in Africa, which “witnessed high level at around AD 1750 and AD 1575.”
Nearby in the Arabian Sea, Tiwari et al. (2005) conducted a high-resolution (~50 years) oxygen isotope analysis of three species of planktonic foraminifera (Globigerinoides ruber, Gs. sacculifer and Globarotalia menardii) contained in a sediment core extracted from the eastern continental margin (12.6°N, 74.3°E) that covered the past 13,000 years. Data for the final 1,200 years of this period were compared with the reconstructed total solar irradiance (TSI) record developed by Bard et al. (2000), which is based on fluctuations of 14C and 10Be production rates obtained from tree rings and polar ice sheets.
Results of the analysis showed that the Asian SouthWest Monsoon (SWM) “follows a dominant quasi periodicity of ~200 years, which is similar to that of the 200-year Suess solar cycle (Usokin et al., 2003).” This finding indicates, in their words, “that SWM intensity on a centennial scale is governed by variation in TSI,” which “reinforces the earlier findings of Agnihotri et al. (2002) from elsewhere in the Arabian Sea.” However, in considering the SWM/TSI relationship, the five researchers note that “variations in TSI (~0.2%) seem to be too small to perturb the SWM, unless assisted by some internal amplification mechanism with positive feedback.” In this regard, they discuss two possible mechanisms. The first, in their words, “involves heating of the earth’s stratosphere by increased absorption of solar ultraviolet (UV) radiation by ozone during periods of enhanced solar activity (Schneider, 2005).” According to this scenario, more UV reception leads to more ozone production in the stratosphere, which leads to more heat being transferred to the troposphere, which leads to enhanced evaporation from the oceans, which finally enhances monsoon winds and precipitation. The second mechanism, as they describe it, is that “during periods of higher solar activity, the flux of galactic cosmic rays to the earth is reduced, providing less cloud condensation nuclei, resulting in less cloudiness (Schneider, 2005; Friis-Christensen and Svensmark, 1997),” which then allows for “extra heating of the troposphere” that “increases the evaporation from the oceans.”
Further Geological Research
In another study, Dykoski et al. (2005) obtained high-resolution records of stable oxygen and carbon isotope ratios from a stalagmite recovered from Dongge Cave in southern China and utilized them to develop a proxy history of Asian monsoon variability over the last 16,000 years. In doing so, they discovered numerous centennial- and multi-decadal-scale oscillations in the record that were up to half the amplitude of interstadial events of the last glacial age, indicating that “significant climate variability characterizes the Holocene.” As to what causes this variability, spectral analysis of δ14C data revealed significant peaks at solar periodicities of 208, 86, and 11 years, which they say is “clear evidence that some of the variability in the monsoon can be explained by solar variability.”
Building upon this work, as well as that of Yuan et al. (2004), Wang et al. (2005) developed a shorter (9,000-year) but higher-resolution (4.5-year) absolute-dated δ18O monsoon record for the same location, which they compared with atmospheric 14C data and climate records from lands surrounding the North Atlantic Ocean. This work indicated their monsoon record broadly followed summer insolation but was punctuated by eight significantly weaker monsoon periods lasting from one to five centuries, most of which coincided with North Atlantic ice-rafting events. In addition, they found that “cross-correlation of the decadal- to centennial-scale monsoon record with the atmospheric 14C 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 by Kerr (2005) accompanying the report of Wang et al., one of the report’s authors (Hai Cheng of the University of Minnesota) was quoted as saying their study suggests that “the intensity of the summer [East Asian] monsoon is affected by solar activity.” Dominik Fleitman, who worked with the Oman stalagmite, also said that “the correlation is very strong,” stating that it is probably the best monsoon record he had seen, calling it “even better than ours.” Lastly, Gerald North of Texas A & M University, who Kerr described as a “longtime doubter,” admitted that he found the monsoon’s solar connection “very hard to refute,” although he stated that “the big mystery is that the solar signal should be too small to trigger anything.”
Next, Porter and Weijian (2006) used 18 radiocarbon-dated aeolian and paleosol profiles (some obtained by the authors and some by others) within a 1,500-km-long belt along the arid to semi-arid transition zone of north-central China to determine variations in the extent and strength of the East Asian summer monsoon throughout the Holocene.
In the words of the authors, the dated paleosols and peat layers “represent intervals when the zone was dominated by a mild, moist summer monsoon climate that favored pedogenesis and peat accumulation,” while “brief intervals of enhanced aeolian activity that resulted in the deposition of loess and aeolian sand were times when strengthened winter monsoon conditions produced a colder, drier climate.” The most recent of the episodic cold periods, which they identify as the Little Ice Age, began about AD 1370, while the preceding cold period ended somewhere in the vicinity of AD 810. Consequently, their work implies the existence of a Medieval Warm Period that began some time after AD 810 and ended some time before AD 1370. They also report that the climatic variations they discovered “correlate closely with variations in North Atlantic drift-ice tracers that represent episodic advection of drift ice and cold polar surface water southward and eastward into warmer subpolar water,” which correlation implies solar forcing (see Bond et al., 2001) as the most likely cause of the alternating multi-century mild/moist and cold/dry periods of North-Central China. As a result, Porter and Weijian’s work helps to establish the global extent of the Medieval Warm Period, as well as its likely solar origin.
We end with a study of the North American monsoon by Asmerom et al. (2007), who developed a high-resolution climate proxy for the southwest United States in the form of δ18O variations in a stalagmite found in Pink Panther Cave in the Guadalupe Mountains of New Mexico.
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 1,533 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.”
Conclusion
In conclusion, research conducted in countries around the world has found a significant effect of solar variability on monsoons. This necessarily implies a small role, or no role, for anthropogenic causes.
References
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Bard, E., Raisbeck, G., Yiou, F. and Jouzel, J. 2000. Solar irradiance during the last 1200 years based on cosmogenic nuclides. Tellus B 52: 985-992.
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Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change. http://www.nipccreport.org/archive/archive.html
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