Carbon Dioxide and Earth's Future Pursuing the ... - Magazooms

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The 16th-century megadrought, as they describe it, persisted “from the 1540s to 1580s in
Mexico, from the 1550s to 1590s over the [U.S.] Southwest, and from the 1570s to 1600s over
Wyoming and Montana,” and it “extended across most of the continental United States during
the 1560s.” It also recurred with greater intensity over the Southeast during the 1580s to
1590s; and so horrendous was this climatic event, that the researchers unequivocally stated
that “the ‘megadrought’ of the 16th century far exceeded any drought of the 20th century.” In
fact, they said that “precipitation reconstruction for western New Mexico suggests that the
16th century drought was the most extreme prolonged drought in the past 2000 years.”
To put these various sets of droughts in perspective, we turn to the study of Stahle et al. (2007),
who used an expanded grid of tree-ring reconstructions of summer Palmer Drought Severity
Indices covering the United States, southern Canada, and most of Mexico to examine the
timing, intensity, and spatial distribution of decadal to multidecadal moisture regimes over
North America. This work revealed that during the Current Warm Period, “the Dust Bowl
drought of the 1930s and the Southwestern drought of the 1950s were the two most intense
and prolonged droughts to impact North America,” as did the studies of Worster (1979), Diaz
(1983) and Fye et al. (2003). During the Little Ice Age, on the other hand, they report the
occurrence of three megadroughts, which they define as “very large-scale drought[s] more
severe and sustained than any witnessed during the period of instrumental weather
observations (e.g., Stahle et al., 2000).” However, they report that still “stronger and more
persistent droughts have been reconstructed with tree rings and other proxies over North
America during the Medieval era (e.g., Stine, 1994; Laird et al., 2003; Cook et al., 2004).” In
fact, they say that these latter megadroughts were so phenomenal that they decided to refer to
them as “no-analog Medieval megadroughts.”
So with megadroughts occurring at cooler-than-present temperatures and with no-analog
megadroughts occurring at warmer-than-present temperatures, one must consider the
possibility that something other than temperature is the driving force behind their occurrence.
And there are a number of scientists who feel that that “something other” is solar variability,
such as Black et al. (1999), who stated that “small changes in solar output may influence
Atlantic variability on centennial time scales,” Yu and Ito (1999), who felt forced “to consider
solar variability as the major cause of century-scale drought frequency in the northern Great
Plains,” Dean and Schwalb (2000), who 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,” Verschuren et al. (2000), who
indicated that variations in solar activity “may have contributed to decade-scale rainfall
variability in equatorial east Africa,” Hodell et al. (2001), who wrote that “a significant
component of century-scale variability in Yucatan droughts is explained by solar forcing,”
Mensing et al. (2004), who concluded that “changes in solar irradiance may be a possible
mechanism influencing century-scale drought in the western Great Basin” of the United States,
Asmerom et al. (2007), who suggest that a 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), who emphasize that the
positive correlation of Lake Masoko hydrology with various solar activity proxies “implies a
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