Droughts and solar variability
From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
The IPCC claims earth’s climate is becoming more variable and extreme as a result of CO2-induced global warming, and it forecasts increasing length and severity of drought as one of the consequences. There is evidence that modern drought frequency and severity fall well within the range of natural variability. The analysis in this section is limited to the issue of attribution, specifically investigating the natural influence of the sun on drought.
North American Research
According to Cook et al. (2007), recent advances in the reconstruction of past drought over North America “have revealed the occurrence of a number of unprecedented megadroughts over the past millennium that clearly exceed any found in the instrumental records.” Indeed, they state that “these past megadroughts dwarf the famous droughts of the twentieth century, such as the Dust Bowl drought of the 1930s, the southern Great Plains drought of the 1950s, and the current one in the West that began in 1999,” all of which dramatic droughts pale when compared to “an epoch of significantly elevated aridity that persisted for almost 400 years over the AD 900-1300 period.”
Of central importance to North American drought formation, in the words of the four researchers, “is the development of cool ‘La Niña-like’ SSTs in the eastern tropical Pacific.” Paradoxically, as they describe the situation, “warmer conditions over the tropical Pacific region lead to the development of cool La Niña-like SSTs there, which is drought inducing over North America.” In further explaining the mechanics of this phenomenon, on which both “model and data agree,” Cook et al. state that “if there is a heating over the entire tropics then the Pacific will warm more in the west than in the east because the strong upwelling and surface divergence in the east moves some of the heat poleward,” with the result that “the east-west temperature gradient will strengthen, so the winds will also strengthen, so the temperature gradient will increase further … leading to a more La Niña-like state.” They add that “La Niña-like conditions were apparently the norm during much of the Medieval period when the West was in a protracted period of elevated aridity and solar irradiance was unusually high.”
Shedding some more light on the subject, Yu and Ito (1999) studied a sediment core from a closed-basin lake in the northern Great Plains of North America, producing a 2,100-year record that revealed four dominant periodicities of drought that matched “in surprising detail” similar periodicities of various solar indices. The correspondence was so close, in fact, that they say “this spectral similarity forces us to consider solar variability as the major cause of century-scale drought frequency in the northern Great Plains.”
One year later, Dean and Schwalb (2000) derived a similar-length record of drought from sediment cores extracted from Pickerel Lake, South Dakota, which also exhibited recurring incidences of major drought on the northern Great Plains. They too reported that the cyclical behavior appeared to be in synchrony with similar variations in solar irradiance. After making a case for “a direct connection between solar irradiance and weather and climate,” they thus concluded that “it seems reasonable that the cycles in aridity and eolian activity over the past several thousand years recorded in the sediments of lakes in the northern Great Plains might also have a solar connection.”
Moving to east-central North America, Springer et al. (2008) derived a multi-decadal-scale record of Holocene drought based on Sr/Ca ratios and δ13C data obtained from stalagmite BCC-002 from Buckeye Creek Cave (BCC), West Virginia (USA) that “grew continuously from ~7000 years B.P. to ~800 years B.P.” and then again “from ~800 years B.P. until its collection in 2002.”
Results of their study indicated the presence of seven significant Mid- to Late-Holocene droughts, six of which “correlate with cooling of the Atlantic and Pacific Oceans as part of the North Atlantic Ocean ice-rafted debris [IRD] cycle, which has been linked to the solar irradiance cycle,” as per Bond et al. (2001). In addition, they determined that the Sr/Ca and δ13C time series “display periodicities of ~200 and ~500 years and are coherent in those frequency bands.” They also say “the ~200-year periodicity is consistent with the de Vries (Suess) solar irradiance cycle,” and they “interpret the ~500-year periodicity to be a harmonic of the IRD oscillations.” Noting further that “cross-spectral analysis of the Sr/Ca and IRD time series yields statistically significant coherencies at periodicities of 455 and 715 years,” they go on to note that “these latter values are very similar to the second (725-years) and third (480-years) harmonics of the 1450 ± 500-years IRD periodicity.” As a result of these observations, the five researchers conclude their report by saying their findings “corroborate works indicating that millennial-scale solar-forcing is responsible for droughts and ecosystem changes in central and eastern North America (Viau et al., 2002; Willard et al., 2005; Denniston et al., 2007),” adding that their high-resolution time series now provide even stronger evidence “in favor of solar-forcing of North American drought by yielding unambiguous spectral analysis results.”
In Nevada, Mensing et al. (2004) analyzed a set of sediment cores extracted from Pyramid Lake for pollen and algal microfossils deposited there over the past 7,630 years that allowed them to infer the hydrological history of the area over that time period. According to the authors, “sometime after 3430 but before 2750 cal yr B.P., climate became cool and wet,” but “the past 2500 yr have been marked by recurring persistent droughts.” The longest of these droughts, according to them, “occurred between 2500 and 2000 cal yr B.P.,” while others occurred “between 1500 and 1250, 800 and 725, and 600 and 450 cal yr B.P.” They also note that “the timing and magnitude of droughts identified in the pollen record compares favorably with previously published δ18O data from Pyramid Lake” and with “the ages of submerged rooted stumps in the Eastern Sierra Nevada and woodrat midden data from central Nevada.” When they compared the pollen record of droughts from Pyramid Lake with the stacked petrologic record of North Atlantic drift ice of Bond et al. (2001), like other researchers they too found “nearly every occurrence of a shift from ice maxima (reduced solar output) to ice minima (increased solar output) corresponded with a period of prolonged drought in the Pyramid Lake record.” Mensing et al. conclude that “changes in solar irradiance may be a possible mechanism influencing century-scale drought in the western Great Basin [of the United States].” Indeed, it would appear that variable solar activity is the major factor in determining the hydrological state of the region and all of North America.
Moving slightly south geographically, Asmerom et al. (2007) developed a high-resolution climate proxy for the southwest United States from 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 monsoons. 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.”
Making our way to Mexico, Hodell et al. (2001) analyzed sediment cores obtained from Lake Chichancanab on the Yucatan Peninsula, reconstructing the climatic history of this region over the past 2,600 years. Long episodes of drought were noted throughout the entire record, and spectral analysis revealed a significant periodicity that matched well with a cosmic ray-produced 14C record preserved in tree rings that is believed to reflect variations in solar activity. Hence, they too concluded that “a significant component of century-scale variability in Yucatan droughts is explained by solar forcing.”
Expanding the geographical scope of such studies, Black et al. (1999) found evidence of substantial decadal and centennial climate variability in a study of ocean sediments in the southern Caribbean that were deposited over the past 825 years. Their data suggested that climate regime shifts are a natural aspect of Atlantic variability; in relating these features to records of terrestrial climate, they concluded that “these shifts may play a role in triggering changes in the frequency and persistence of drought over North America.” In addition, because there was a strong correspondence between these phenomena and similar changes in 14C production rate, they further concluded that “small changes in solar output may influence Atlantic variability on centennial time scales.”
Verschuren et al. (2000) conducted a similar study in a small lake in Kenya, documenting the existence of three periods of prolonged dryness during the Little Ice Age that were, in their words, “more severe than any recorded drought of the twentieth century.” In addition, they discovered that all three of these severe drought events “were broadly coeval with phases of high solar radiation”—as inferred from 14C production data—”and the intervening periods of increased moisture were coeval with phases of low solar radiation.” They thus concluded that variations in solar activity “may have contributed to decade-scale rainfall variability in equatorial east Africa.”
Also in Africa, working with three sediment cores extracted from Lake Edward (0°N, 30°E), Russell and Johnson (2005) developed a continuous 5,400-year record of Mg concentration and isotopic composition of authigenic inorganic calcite as proxies for the lake’s water balance, which is itself a proxy for regional drought conditions in equatorial Africa. They found “the geochemical record from Lake Edward demonstrates a consistent pattern of equatorial drought during both cold and warm phases of the North Atlantic’s ‘1500-year cycle’ during the late Holocene,” noting that similar “725-year climate cycles” are found in several records from the Indian and western Pacific Oceans and the South China Sea, citing as authority for the latter statement the studies of von Rad et al. (1999), Wang et al. (1999), Russell et al. (2003) and Staubwasser et al. (2003). In light of these findings, the two scientists say their results “show that millennial-scale high-latitude climate events are linked to changes in equatorial terrestrial climate … during the late Holocene,” or as they phrase it in another place, that their results “suggest a spatial footprint in the tropics for the ‘1500-year cycle’ that may help to provide clues to discern the cycle’s origin,” noting there is already reason to believe that it may be solar-induced.
Lastly, Garcin et al. (2007) explored hydrologic change using late-Holocene paleoenvironmental data derived from several undisturbed sediment cores retrieved from the deepest central part of Lake Masoko (9°20.0’S, 33°45.3’E), which occupies a maar crater in the Rungwe volcanic highlands of the western branch of Africa’s Rift Valley, where it is situated approximately 35 km north of Lake Malawi.
According to the 10 researchers who conducted the work, “magnetic, organic carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550-1850), confirming that the LIA in eastern Africa resulted in marked and synchronous hydrological changes,” although “the direction of response varies between different African lakes.” In this regard, for example, they report that “to the south (9.5-14.5°S), sediment cores from Lake Malawi have revealed similar climatic conditions (Owen et al., 1990; Johnson et al., 2001; Brown and Johnson, 2005)” that are “correlated with the dry period of Lakes Chilwa and Chiuta (Owen and Crossley, 1990),” and they say that “lowstands have been also observed during the LIA at Lake Tanganyika … from AD 1500 until AD 1580, and from ca. AD 1650 until the end of the 17th century, where the lowest lake-levels are inferred (Cohen et al., 1997; Alin and Cohen, 2003).” By contrast, however, they report that “further north, evidence from Lakes Naivasha (0.7°S) and Victoria (2.5°S-0.5°N) indicates relatively wet conditions with high lake-levels during the LIA, interrupted by short drought periods (Verschuren et al., 2000; Verschuren, 2004; Stager et al., 2005).” Lastly, Garcin et al. state that “inferred changes of the Masoko hydrology are positively correlated with the solar activity proxies.”
In discussing their findings, the African and French scientists note the Little Ice Age in Africa appears to have had a greater thermal amplitude than it did in the Northern Hemisphere, citing in support of this statement the paleoclimate studies of Bonnefille and Mohammed (1994), Karlén et al. (1999), Holmgren et al. (2001), and Thompson et al. (2002). Nevertheless, the more common defining parameter of the Little Ice Age in Africa was the moisture status of the continent, which appears to have manifested opposite directional trends in different latitudinal bands. In addition, the group of scientists emphasizes that the positive correlation of Lake Masoko hydrology with various solar activity proxies “implies a forcing of solar activity on the atmospheric circulation and thus on the regional climate of this part of East Africa.”
There seems to be little question but what variations in solar activity have been responsible for much of the drought variability of the Holocene in many parts of the world.
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