Biology Reference
In-Depth Information
Another forcing mechanism for climate change is the presence of the
ice itself. At the maximum of the last glaciation, the two centers of con-
tinental ice, the Laurentide ice sheet centered over Hudson Bay and the
Cordilleran ice sheet in the northern Rocky Mountains, fused to form a
dome of ice more than a mile thick. This divided the polar jet stream into
a northern and southern arm, with the latter displacing the high pressure
system formed by the descending arm of the Hadley circulation cell over
southwestern North America. There was also a southern shift of another
high pressure system, the Bermuda-Azores High, and the result of both
movements was a change from arid climates in the southwest to cooler and
moister pluvial regimes.
The geological and biological records show the expected eighteen to
twenty cycles of glaciation, on a periodicity of 100,000 years for about the
past 780,000 years. Prior to that time, glaciation followed the tilt cycle of
43,000 years, which actually results in greater differences in solar insola-
tion. The cause for the change in timing is not fully understood, but ampli-
fi cation of the eccentricity cycle is thought to be modulated by the dimin-
ishing trend in atmospheric CO
2
concentration until recent times. There
are three implications of these patterned changes in climate. One is that
for about 90,000 years of each eccentricity cycle, it was getting cold, it
was very cold, or it was still cold, while in only about 10,000 years of the
cycle was it warm like the present. This means that in areas of western
North America, the desert vegetation, so characteristic of today, is the ex-
ception, and that for about 90 percent of the last 100,000 years the typical
ecosystem was a more mesic shrubland/chaparral-woodland-savanna. The
second realization is that any change in climate or vegetation during the last
glacial/postglacial interval was repeated at least eighteen to twenty times
since about 2.6 Ma. For example, if it is determined that recent fl uctuations
in Quaternary climate affected the lowland neotropical rain forest, as it did,
then it must also be assumed that there were similar effects multiple times
during the Quaternary. In other words, ecosystems are revealed as far more
dynamic than if there were only four glaciations lasting 175,000 years and
affecting only the high latitudes. This is another important paradigm shift
in our view of recent climatic change and biotic history.
The third implication is that if ecosystems experienced such profound
and frequent changes in range and composition during glacial intervals, it is
unlikely they would regain the exact same range and composition after each
climatic reshuffl ing (Davis 1981, 1986). During each time segment within
a fl uctuating climate, the prevailing ecosystems in a region are composed of
ecologically compatible species that have had the opportunity to assemble
