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

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forcing of solar activity on the atmospheric circulation and thus on the regional climate of [a]
part of East Africa,” and Springer et al. (2008), who say their findings “corroborate works
indicating that millennial-scale solar-forcing is responsible for droughts and ecosystem changes
in central and eastern North America,”
In one final and exceptionally perceptive paper dealing with North American droughts, Cook et
al. (2009) wrote that “IPCC Assessment Report 4 model projections suggest that the subtropical
dry zones of the world will both dry and expand poleward in the future due to greenhouse
warming,” and that “the US southwest is particularly vulnerable in this regard and model
projections indicate a progressive drying there out to the end of the 21st century.” However,
they then wrote that “the USA has been in a state of drought over much of the West for about
10 years now,” and that “while severe, this turn of the century drought has not yet clearly
exceeded the severity of two exceptional droughts in the 20th century,” so that “while the
coincidence between the turn of the century drought and projected drying in the Southwest is
cause for concern, it is premature to claim that the model projections are correct.”
We begin to understand this fact when we compare the “turn of the century drought” with the
two “exceptional droughts” that preceded it by a few decades. Based on gridded instrumental
Palmer Drought Severity indices for tree ring reconstruction that extend back to 1900, Cook et
al. calculated that the turn-of-the-century drought had its greatest Drought Area Index value of
59% in the year 2002, while the Great Plains/Southwest drought covered 62% of the US in its
peak year of 1954, and the Dust Bowl drought covered 77% of the US in 1934. In terms of
drought duration, however, things are not quite as clear. Stahle et al. (2007) estimated that the
first two droughts lasted for 12 and 14 years, respectively; Seager et al. (2005) estimated them
to have lasted for 8 and 10 years; and Andreadis et al. (2005) estimated them to have lasted for
7 and 8 years, yielding means of 9 and 11 years for the two exceptional droughts, which
durations are to be compared to 10 or so years for the turn-of-the-century drought, which
again makes the latter drought not unprecedented compared to those that occurred earlier in
the 20th century.
Real clarity, however, comes when the turn-of-the-century drought is compared to droughts of
the prior millennium. Cook et al. write that “perhaps the most famous example is the ‘Great
Drouth’ (sic) of AD 1276-1299 described by A.E. Douglass (1929, 1935).” Yet this 24-year
drought was eclipsed by the 38-year drought that was found by Weakley (1965) to have
occurred in Nebraska from AD 1276 to 1313, which Cook et al. say “may have been a more
prolonged northerly extension of the ‘Great Drouth’.” But even these multi-decade droughts
truly pale in comparison to the “two extraordinary droughts discovered by Stine (1994) in
California that lasted more than two centuries before AD 1112 and more than 140 years before
AD 1350.” And each of these megadroughts, as Cook et al. describe them, occurred, in their
words, “in the so-called Medieval Warm Period.” And they add that “all of this happened prior
to the strong greenhouse gas warming that began with the Industrial Revolution [authors’
italics].”
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