Geoscience Reference
In-Depth Information
Sediments on the coastal ocean l oor contain the remains of single-celled
organisms that form calcite shells, called foraminifera (or “forams”), which
contain information about oceanic conditions like sea surface temperature.
Of the coast of Santa Barbara, these forams have been used to assess sea
surface temperatures over the past ten thousand years using the oxygen
isotopic composition of a particular species of foram,
Globigerina bulloides.
h is species lives exclusively in surface waters, so its shell incorporates oxygen
directly from the surface waters and faithfully records changes in ambient sea
surface temperature. Analysis of these shells from cores broadly coincident
with the Medieval period has shown that the surface water temperatures
were 2-3°F colder from AD 700 to 1300 than they are today. Cooler than
average waters in the eastern Pacii c are ot en associated with La Niña condi-
tions, which bring drought to the American Southwest, but more evidence
from the tropical Pacii c Ocean would be needed to coni rm the connec-
tion during Medieval times. We will discuss this and other evidence for the
ocean-atmosphere changes that occurred during this time in chapter 11.
In light of the evidence for a warm, dry period in the West between about
1,800 and 550 years ago, with particularly deep droughts toward the end of
that period (from 1,050 to 550 years ago), current inhabitants of the western
United States face sobering questions: How common are such severe and pro-
longed droughts? Do they represent the most extreme events we can expect?
When will the next one strike? Will they be more frequent and severe as the
region warms in the coming decades? How would they af ect society today?
And what of the opposite climate extreme—the deluges and l oods that
ended the droughts? How ot en do these l oods, with magnitudes similar
to the 1861-62 event or even greater, recur? To further explore these ques-
tions, we turn next to the Little Ice Age. h is cool period, which followed
the Medieval Climate Anomaly and lasted until the nineteenth century,
brought several megal oods of a magnitude that no one alive today has yet
experienced.
