And Then There Were None: Major Extinction Events in Earth’s History - Geology - page 324

During certain periods in earth's history, most of life was lived in shallow oceans. A shift
in sea level would have had dramatic effects on the environments supporting shallow
marine life. Lower sea levels would force shallow sea life into dry, waterless environ-
ments. Higher sea levels would leave them in deeper water with less access to sunlight
and oxygen found near the ocean's surface.
Such sea-level changes could have been the result of climate changes (the melting or
growing of large ice caps, which would change the amount of water in the oceans) or
tectonic plate movements (which I describe in Chapter 9).
Changing climate
Most scientists now consider climate change to be the most important factor in mass ex-
tinctions. The earth's climate changes in response to many different factors, including
impacts, tectonic plate movements, and volcanic eruptions.
In looking at evidence for mass extinctions in the geologic records, scientists conclude
that global-scale changes can most reliably be explained through changes in a global
system, such as the climate. Other geologic evidence indicates that periods of mass ex-
tinction commonly occur during global warming or glaciations, leading scientists to con-
clude that shifting climate conditions changed global environments so dramatically that
many species could not adapt, and perished.
End Times, at Least Five Times
Species go extinct all the time; extinction is part of the natural order of things. Normal
rates of extinction through time are part of what scientists call the background extinction
rate expected to occur on earth. The mass extinctions described in this section are peri-
ods when the rate of extinction, as indicated in the fossil record, is much more dramatic
and extreme than the normal (background) rate.
Figure 22-2 is a graph that illustrates extinction rates throughout earth's history, high-
lighting the five major extinction events that I describe in this section.
Figure 22-2: Extinc-
tion rates for five
major extinction
events.