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Drought (Solar-Induced) -- Summary
What causes droughts? There is clearly no single, simple answer to this question. Much recent research, however, points to solar activity as one of the primary behind-the-scenes phenomena that determine the broad outlines of drought severity and frequency, as is suggested by the findings of the various research projects described below.

Yu and Ito (1999) studied a sediment core retrieved from a closed-basin lake in the northern Great Plains of North America, producing a 2100-year record that revealed four dominant periodicities of drought that matched "in surprising detail," as they describe it, 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."

Dean and Schwalb (2000) derived a similar-length record of drought from sediment cores extracted from Pickerel Lake, South Dakota (USA), 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 concluded "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."

Hodell et al. (2001) analyzed sediment cores obtained from Lake Chichancanab on the Yucatan Peninsula of Mexico, reconstructing the climatic history of that region over the past 2600 years. This work revealed the presence of long episodes of drought 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, which is believed to reflect variations in solar activity. Hence, they concluded that "a significant component of century-scale variability in Yucatan droughts is explained by solar forcing."

Expanding the geographical scope of such studies still more, Black et al. (1999) found evidence for 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; and 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 that "the intervening periods of increased moisture were coeval with phases of low solar radiation." As a result, they too concluded that variations in solar activity "may have contributed to decade-scale rainfall variability in equatorial east Africa."

Mensing et al. (2004) analyzed a set of sediment cores extracted from Pyramid Lake, Nevada (USA) for pollen and algal microfossils deposited there over the prior 7630 years, which allowed them to infer the area's hydrologic history. This data set indicated that "sometime after 3430 but before 2750 cal yr B.P., climate became cool and wet," but that "the past 2500 years have been marked by recurring persistent droughts," the longest of which 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 discovered that "the timing and magnitude of droughts identified in the pollen record compares favorably with previously published δ18O data from Pyramid Lake," as well as with "the ages of submerged rooted stumps in the Eastern Sierra Nevada and woodrat midden data from central Nevada." Last of all, they say Bond et al. (2001) "found that over the past 12,000 years, decreases in [North Atlantic] drift ice abundance corresponded to increased solar output," and that when they "compared the pollen record of droughts from Pyramid Lake with the stacked petrologic record of North Atlantic drift ice ... 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." Based upon these findings, they concluded 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 may well have been the major factor determining the hydrologic state of the region.

Working with three sediment cores extracted from Lake Edward in Africa, Russell and Johnson (2005) developed a continuous 5400-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 the equatorial portion of the continent. The fruits of this enterprise revealed, in their words, that "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," and they report 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 this statement the studies of von Rad et al. (1999), Wang et al. (1999), Russell et al. (2003) and Staubwasser et al. (2003). Hence, 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," and that these observations "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 good reason to believe the cycle may be solar-induced.

Asmerom et al. (2007) developed "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. Spectral analysis performed on the 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)." Specifically, they say 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 led 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."

Garcin et al. (2007) obtained late-Holocene paleoenvironmental data from several undisturbed sediment cores retrieved from the deepest central part of Lake Masoko in the Rungwe volcanic highlands of the western branch of Africa's Rift Valley, approximately 35 km north of Lake Malawi; and based on "magnetic, organic carbon, geochemical proxies and pollen assemblages," they inferred the existence of "a dry climate during the 'Little Ice Age' (AD 1550-1850), confirming that the Little Ice Age in eastern Africa resulted in marked and synchronous hydrological changes," although they indicate that "the direction of response varies between different African lakes." In this regard, for example, they report that "to the south, 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 "lowstands have been also observed during the Little Ice Age 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)." In contrast, they report that "further north, evidence from Lakes Naivasha and Victoria indicates relatively wet conditions with high lake-levels during the Little Ice Age, interrupted by short drought periods (Verschuren et al., 2000; Verschuren, 2004; Stager et al., 2005)." Last of all, they 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 that 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), Karlen 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 researchers emphasize 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."

On another front, Cook et al. (2007) -- in a comprehensive review article dealing with droughts of North America -- report that recent advances in the reconstruction of past drought "have revealed the occurrence of a number of unprecedented megadroughts over the past millennium that clearly exceed any found in the instrumental records." In fact, they say "these past megadroughts dwarf [italics added] the famous droughts of the 20th 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 fade into almost total insignificance when compared to the granddaddy of them all, which they describe as "an epoch of significantly elevated aridity that persisted for almost 400 years over the AD 900-1300 period."

With respect to the cause of the great megadroughts of the past, Cook et al. say that of central importance to North American drought formation "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." What is more, they add that "La Niña-like conditions were apparently the norm [italics added] during much of the Medieval period when the West was in a protracted period of elevated aridity and solar irradiance was unusually high [italics added]."

Springer et al. (2008) derived a multi-decadal-scale record of Holocene drought in east-central North America based on Sr/Ca ratios and δ13C data obtained from stalagmite BCC-002 from Buckeye Creek Cave (BCC), West Virginia (USA) that grew from ~7000 years B.P. until its collection in 2002. This history revealed seven significant Mid- to Late-Holocene droughts, six of which, the researchers tell us, "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" by 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 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 state that "these latter values are very similar to the second (725-years) and third (480-years) harmonics of the 1450 ± 500-years IRD periodicity." Consequently, the five scientists 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."

Working with data obtained from hundreds of moisture-sensitive Scots pine tree-ring records originating in Finland, and using regional curve standardization (RCS) procedures, Helama et al. (2009) developed what they describe as "the first European dendroclimatic precipitation reconstruction," which "covers the classical climatic periods of the Little Ice Age (LIA), the Medieval Climate Anomaly (MCA), and the Dark Ages Cold Period (DACP)," running all the way from AD 670 to AD 1993. Based on their findings, they state that "the special feature of this period in climate history is the distinct and persistent drought, from the early ninth century AD to the early thirteenth century AD," which interval, in their words, "precisely overlaps the period commonly referred to as the MCA, due to its geographically widespread climatic anomalies both in temperature and moisture." In addition, they report that "the reconstruction also agrees well with the general picture of wetter conditions prevailing during the cool periods of the LIA (here, AD 1220-1650) and the DACP (here, AD 720-930)."

In further discussing their findings, the three Finnish scientists note that the global medieval drought they discovered "occurred in striking temporal synchrony with the multicentennial droughts previously described for North America (Stine, 1994; Cook et al., 2004, 2007), eastern South America (Stine, 1994; Rein et al., 2004), and equatorial East Africa (Verschuren et al., 2000; Russell and Johnson, 2005, 2007; Stager et al., 2005) between AD 900 and 1300." Noting that this widespread evidence "argues for a common force behind the hydrological component of the MCA," they indicate that "previous studies have associated coeval megadroughts during the MCA in various parts of the globe with either solar forcing (Verschuren et al., 2000; Stager et al., 2005) or the ENSO (Cook et al., 2004, 2007; Rein et al., 2004; Herweijer et al., 2006, 2007; Graham et al., 2007, Seager et al., 2007)," stating that "the evidence so far points to the medieval solar activity maximum [italics added] (AD 1100-1250), because it is observed in the Δ14C and 10Be series recovered from the chemistry of tree rings and ice cores, respectively (Solanki et al., 2004)."

And so the evidence continues to mount for a global and solar-induced Medieval Warm (and Dry!) Period, which likely eclipsed (in both categories) what the world has so far experienced during the Current Warm Period. And in light of the findings of the several other palaeoclimatic studies discussed herein, there would appear to be little question but what variations in solar activity have also been responsible for much of the higher-frequency drought variability of the Holocene that is known to have occurred throughout the world.

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Last updated 14 October 2009