Geoscience Reference
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when the PDO is in a “positive,” or warm, phase, sea surface temperatures are
warmer than average. Researchers who use paleoclimate data to reconstruct
past sea surface temperatures are able to see patterns of change in the phase
of the PDO and even in the strength of the PDO. Such studies suggest, for
example, that the PDO was generally in the negative (cool) phase during
the middle part of the Holocene, about 7,000 to 4,000 years ago, so sea
surface temperatures along the Pacii c coast were cooler. Conditions during
this period were wetter in the Pacii c Northwest and drier in the Southwest
and interior western North America (as described in chapter 7). h e wet-
ter conditions of the Neoglacial that followed (as described in chapter 8)
correspond to a period when the PDO was in a positive (warm) phase.
h e PDO returned to the negative (cool) phase again in the late Holocene
for the four centuries from AD 900 to 1300. As we described in chapter 9,
this was a period marked by extreme and prolonged droughts. h e connec-
tion between the PDO phase and this megadrought period has been inferred
from tree-ring records from Southern California and western Canada—
which are at the extreme ends of the North American region af ected by the
PDO. Tree rings from these regions reveal that the Southwest was dry and
the Northwest was wet at that time. A study using moisture-sensitive tree-
ring chronologies from Southern California and northern Baja California
has shown that the PDO was unusually weak during the Little Ice Age com-
pared to that of the centuries immediately before and at er it.
Although ocean conditions of the past seem to explain the broader pat-
terns of precipitation in the West, other hypotheses have been put forward
to explain some of the more frequently recurring climate cycles that we see
in the past. h e more intriguing of these hypotheses are ones that involve
extraterrestrial causes of climate change.
extraterrestrial causes of climate
change: sunspot cycles
Whereas the PDO and the ENSO are climate oscillations produced internally
within the earth's ocean-atmosphere system, our planet's climate is also inl u-
enced by extraterrestrial events, including changes in the output of energy
from the sun. h e study of solar variations has a long history, extending back
to the early seventeenth century when Galileo Galilei systematically studied
the sun at dawn and dusk with his rudimentary telescope. He observed dark
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