Page 275 - Earth's Climate Past and Future

CHAPTER 14

Millennial

Oscillations

of Climate

Large climatic oscillations have occurred over intervals considerably shorter
than the orbital cycles. Because they last for a few thousand years, they are
called millennial oscillations. Many of these fluctuations began and ended in
decades, fast enough to be of possible relevance to human concerns about
future climate. The cause of these fluctuations remains unknown, and this
chapter describes accumulating evidence about their distribution in time and
space and their possible origin.
The oscillations were largest when glacial ice sheets existed in the northern
hemisphere, and they have been much smaller during interglacial climates like the
current one. Very large oscillations have been found in Greenland ice cores and in
North Atlantic sediment cores, and significant fluctuations also appear in records
from many other regions, primarily in the northern hemisphere. Smaller fluctua-
tions occur in south-polar regions, and their timing is different (almost opposite)
compared to the fluctuations in the north. The oscillations appear to be largely
random, rather than cyclic. Scientists have narrowed the range of explanations to
three possibilities. They could be driven by internally generated fluctuations in the
margins of northern hemisphere ice sheets, by external changes in the strength of
the Sun or by interactions of the ice sheets, the atmosphere, and the ocean.