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Extreme Weather Events: Are they Influenced by Rising Atmospheric CO2?

3.2.3. Other Factors Driving Observed Drought Trends

A number of researchers have identified various natural forcings that may help to explain the occurrence of historical droughts. These forcings have operated for hundreds and even thousands of years independently of the atmosphere's CO2 concentration; and they must be accounted for in drought attribution studies.

Among the most commonly-mentioned of these forcings are those related to various solar phenomena. In this regard, for example, Paulsen et al. (2003) employed high-resolution stalagmite records of δ13C and δ18O from Buddha Cave "to infer changes in climate in central China for the last 1270 years in terms of warmer, colder, wetter and drier conditions." Among the climatic episodes evident in their data were "those corresponding to the Medieval Warm Period, Little Ice Age and 20th-century warming, lending support to the global extent of these events." More specifically, their record begins in the depths of the Dark Ages Cold Period, which ends about AD 965 with the commencement of the Medieval Warm Period, which continues to approximately AD 1475, whereupon the Little Ice Age sets in and holds sway until about AD 1825, after which the warming responsible for the Modern Warm Period begins.

With respect to hydrologic balance, the last part of the Dark Ages Cold Period was characterized as wet. It, in turn, was followed by a dry, a wet, and another dry interval in the Medieval Warm Period, which was followed by a wet and a dry interval in the Little Ice Age, and finally a mostly-wet but highly moisture-variable Modern Warm Period. Paulsen et al.'s data also reveal a number of other cycles superimposed on the major millennial-scale cycle of temperature and the centennial-scale cycle of moisture, most of which higher-frequency cycles they attribute to solar phenomena, concluding that the summer monsoon over eastern China, which brings the region much of its precipitation, may "be related to solar irradiance."

The significance of this study with respect to the present discussion on drought resides in the fact that the authors' data clearly indicate that Earth's climate is determined by a conglomerate of cycles within cycles, all of which are essentially independent of the air's CO2 concentration; and it demonstrates that the multi-century warm and cold periods of the planet's millennial-scale oscillation of temperature may have both wetter and drier periods embedded within them. Consequently, it can be appreciated that warmth alone is not a sufficient condition for the concomitant occurrence of the dryness associated with drought.

In another study suggestive of a solar-drought link, Springer et al. (2008) constructed a multi-decadal-scale history of east-central North America's hydroclimate over the past 7,000 years, based on Sr/Ca ratios and δ13C data obtained from stalagmite BCC-002 of Buckeye Creek Cave (BCC) in West Virginia, USA. In doing so, the authors detected seven significant mid- to late-Holocene droughts that "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. (1997, 2001). In addition, they found that "the Sr/Ca and δ13C time series display periodicities of ~200 and ~500 years," and that "the ~200-year periodicity is consistent with the de Vries (Suess) solar irradiance cycle," and that the ~500-year periodicity is likely "a harmonic of the IRD oscillations." They also report that actual "cross-spectral analysis of the Sr/Ca and IRD time series yields statistically significant coherencies at periodicities of 455 and 715 years," which latter values "are very similar to the second (725-years) and third (480-years) harmonics of the 1450 ± 500-years IRD periodicity." Such findings, in the words of the five researchers, "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)," and that their high-resolution time series "provide much stronger evidence in favor of solar-forcing of North American drought by yielding unambiguous spectral analysis results."

In another study, Woodhouse and Overpeck (1998) reviewed what is known about the frequency and severity of drought in the central United States over the last two thousand years based upon empirical evidence of drought from various proxy indicators. Their study indicated the presence of numerous "multidecadal- to century-scale droughts," leading them to conclude that "twentieth-century droughts are not representative of the full range of drought variability that has occurred over the last 2000 years." In addition, they noted that the 20th century was characterized by droughts of "moderate severity and comparatively short duration, relative to the full range of past drought variability."

With respect to the causes of drought, Woodhouse and Overpeck suggest a number of different possibilities that either directly or indirectly induce changes in atmospheric circulation and moisture transport. However, they caution that "the causes of droughts with durations of years (i.e., the 1930s) to decades or centuries (i.e., paleodroughts) are not well understood." Hence, they conclude that "the full range of past natural drought variability, deduced from a comprehensive review of the paleoclimatic literature, suggests that droughts more severe than those of the 1930s and 1950s are likely to occur in the future," and, it might be added, irrespective of whatever the air's CO2 concentration or temperature might be doing in future years.

Gray et al. (2003) examined fifteen tree ring-width chronologies used in previous reconstructions of drought for evidence of low-frequency variations in five regional composite precipitation histories in the central and southern Rocky Mountains. In doing so, they found "strong multi-decadal phasing of moisture variation was present in all regions during the late 16th-century megadrought," and that "oscillatory modes in the 30-70 year domain persisted until the mid-19th century in two regions, and wet-dry cycles were apparently synchronous at some sites until the 1950s drought." And they thus speculate that "severe drought conditions across consecutive seasons and years in the central and southern Rockies may ensue from coupling of the cold phase Pacific Decadal Oscillation with the warm phase Atlantic Multidecadal Oscillation," which is something they envision as having happened in both the severe 1950s drought and the late 16th-century megadrought. Thus, there is reason to believe that episodes of extreme dryness in this part of the country may be driven in part by naturally-recurring climate "regime shifts" in the Pacific and Atlantic Oceans.

Also suggesting that ocean oscillations might bear a good deal of the blame for large-scale drought in the western U.S. was Seager (2007), who studied the global context of the drought that affected nearly the entire United States, northern Mexico and the Canadian Prairies-but most particularly the American West-between 1998 and 2004. Based on atmospheric reanalysis data and ensembles of climate model simulations forced by global or tropical Pacific sea surface temperatures over the period January 1856 to April 2005, Seager compared the climatic circumstances of the recent drought with those of the five prior great droughts of North America: (1) the Civil War drought of 1856-65, (2) the 1870s drought, (3) the 1890s drought, (4) the great Dust Bowl drought, and (5) the 1950s drought. In doing so, Seager reports the 1998-2002 period of the recent drought "was most likely caused by multiyear variability of the tropical Pacific Ocean," noting the recent drought "was the latest in a series of six persistent global hydroclimate regimes, involving a persistent La Niña-like state in the tropical Pacific and dry conditions across the mid-latitudes of each hemisphere."

Of additional note, there was no aspect of Seager's study that implicates global warming, either CO2-induced or otherwise, as a cause of-or contributor to-the great turn-of-the-20th-century drought that affected large portions of North America. Seager notes, for example, that "although the Indian Ocean has steadily warmed over the last half century, this is not implicated as a cause of the turn of the century North American drought because the five prior droughts were associated with cool Indian Ocean sea surface temperatures." In addition, the five earlier great droughts occurred during periods when the mean global temperature was also significantly cooler than what it was during the last great drought.

Working in eastern Canada, Girardin et al. (2004) developed a 380-year reconstruction of the Canadian Drought Code (CDC, a daily numerical rating of the average moisture content of deep soil organic layers in boreal conifer stands that is used to monitor forest fire danger) for the month of July, based on 16 well replicated tree-ring chronologies from the Abitibi Plains of eastern Canada just below James Bay. Cross-continuous wavelet transformation analyses of these data, in their words, "indicated coherency in the 8-16 and 17-32-year per cycle oscillation bands between the CDC reconstruction and the Pacific Decadal Oscillation prior to 1850," while "following 1850, the coherency shifted toward the North Atlantic Oscillation."

These results led them to suggest that "the end of [the] 'Little Ice Age' over the Abitibi Plains sector corresponded to a decrease in the North Pacific decadal forcing around the 1850s," and that "this event could have been followed by an inhibition of the Arctic air outflow and an incursion of more humid air masses from the subtropical Atlantic climate sector," which may have helped reduce fire frequency and drought severity. In this regard, they further note that several other paleo-climate and ecological studies have suggested that "climate in eastern Canada started to change with the end of the 'Little Ice Age' (~1850)," citing the works of Tardif and Bergeron (1997, 1999), Bergeron (1998, 2000) and Bergeron et al. (2001), while further noting that Bergeron and Archambault (1993) and Hofgaard et al. (1999) have "speculated that the poleward retreat of the Arctic air mass starting at the end of the 'Little Ice Age' contributed to the incursion of moister air masses in eastern Canada."

In another paper examining historic droughts in the U.S., Herweijer et al. (2006) find evidence for a "linkage between a colder eastern equatorial Pacific and persistent North American drought over the last 1,000 years," noting further that "Rosby wave propagation from the cooler equatorial Pacific amplifies dry conditions over the USA." In addition, they report that after using "published coral data for the last millennium to reconstruct a NINO 3.4 history," they applied "the modern-day relationship between NINO 3.4 and North American drought ... to recreate two of the severest Mediaeval 'drought epochs' in the western USA," again demonstrating the importance of understanding other forcings on drought independent of atmospheric CO2.

Finally, Morengo (2009) worked with hydrometeorological indices for the Amazon basin and its several sub-basins in an effort designed "to explore long-term variability of climate since the late 1920s and the presence of trends and/or cycles in rainfall and river indices in the basin," which analyses were based on northern and southern Amazonian rainfall data originally developed by Marengo (1992) and Marengo and Hastenrath (1993), and which were subsequently updated by Marengo (2004). According to the Brazilian researcher, the results of this effort indicate "no systematic unidirectional long-term trends towards drier or wetter conditions [were] identified." Instead, he found that "the rainfall and river series show variability at inter-annual scales." And of the patterns he uncovered, Morengo writes that they are "characteristic of decadal and multi-decadal modes," which he describes as "indicators of natural climate variability" that are linked to the El Niño Southern Oscillation, "rather than any unidirectional trend towards drier conditions (as one would expect, due to increased deforestation or to global warming)."

Clearly, numerous studies fail to support the climate-alarmist claim that CO2-induced global warming is increasing both the frequency and severity of drought conditions around the world; and the many findings presented above persuasively demonstrate that several other factors - unrelated to rising CO2 - dominate historical drought records. And, therefore, proper accounting for their influence must be conducted before assessing a potential role for anthropogenic CO2.

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