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

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Pacific and dry conditions across the midlatitudes of each hemisphere.” In fact, there was no
aspect of this study that implicated 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 noted, 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.
Another far-ranging study was that of Cook et al. (2007), who discussed the nature of a number
of megadroughts that occurred over the past millennium and clearly exceeded in all aspects all
droughts of the instrumental period. Indeed, they state that “these past megadroughts dwarf
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.”
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.” And in further explaining the mechanics of this phenomenon, on which they
say 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 during
much of the Medieval period when the West was in a protracted period of elevated aridity and
solar irradiance was unusually high.”
In light of these several observations, it would appear that throughout the AD 900-1300 period
of what Cook et al. call “significantly elevated aridity” in North America, the tropical Pacific
Ocean likely experienced significantly elevated temperature, which may well have been far
greater than anything experienced over the course of the 20th century, because there was no
period of time over the last several hundred years when North America experienced anything
like the seemingly endless aridity of that 400-year megadrought that coincided with the great
central portion of the Medieval Warm Period. And in light of this observation, we conclude that
much of the Medieval Warm Period had to have been much warmer than even the warmest
portion of the 20th century, or any time since. In fact, there is reason to believe that the world
as a whole may well have been warmer during the bulk of the Medieval Warm Period than it is
currently, for Cook et al. write that “the persistent droughts over North America all arose as
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