Geography Reference
In-Depth Information
advance and retreat of ice sheets produces a close match to cycles
of solar energy predicted by orbital forcing theory. For example,
there have been eight large glacial build-ups over the past 800,000
years on an approximately 100,000-year cycle each coinciding with
minimum eccentricity. Smaller decreases or surges in ice volume
have come at intervals of approximately 23,000 years and 41,000
years in keeping with the frequency of the other two orbital
mechanisms.
While orbital forcing theory matches the timing of the cool and
warm periods it is not sufficient on its own to explain the magni-
tude of changes in temperature experienced. In fact there appears
to be a 4° to 6°C shortfall. Furthermore, the orbital cycles mathe-
matically predict a smooth rise and fall of temperature. However,
the evidence for the actual build-up and retreat of ice shows a saw-
tooth pattern. Over tens of thousands of years ice sheets built-up
several kilometres thick, souring and scarring the landscape as far
south as central Europe and Midwestern USA (Figure 2.2). But
each cycle ended abruptly. Within a few thousand years the ice
sheets melted back to present-day patterns. So orbital forcing seems
to be a driver of climate change through the Quaternary, but other
feedbacks must be operating to deal with the temperature shortfall
and the sawtooth pattern. Understanding these feedbacks might be
important for us so that we may understand how the climate may
evolve in the future.
One positive feedback comes from increased reflection (
albedo
)
of the Sun's energy from the ice causing a further drop in tempera-
tures, allowing ice sheets to expand further. This would be
enhanced by sea level fall following ice sheet growth allowing ice
sheets to expand further on land enhancing albedo. Nevertheless,
these feedbacks are still not sufficient to explain the magnitude of
temperature changes observed.
An additional feedback which has been the centre of research
for the last two decades is related to changes in the thermohaline
deep water circulation system in the oceans (described in Chapter
1) and the role of the oceans in changing atmospheric composition.
Ice cores from the middle of the oldest ice sheets in Greenland and
Antarctica contain bubbles of gas. These bubbles contain air from
the time when the snow fell that later formed the ice. Data from
bubbles contained within ice cores indicates that carbon dioxide
