Geoscience Reference
In-Depth Information
time had a period approximating 41 000 years. Although this cannot be attributed to
a single event (again rather a combination of circumstances) it is perhaps useful to
re-emphasise that the Isthmus of Panama formed before some 2.5 mya, physically
separating the Pacific and Atlantic Oceans. Even though the isthmus formed then,
as the two continents drew closer, the water flow between the two oceans would
have gradually declined over previous millennia and this factor might have become,
in association with others, more climatically significant at some point prior to the
continents' physical joining. The 41 000-year periodicity subsequent to the onset
of northern hemisphere glaciation reflects Milankovitch obliquity and it domin-
ated global climate variations 1.75-1 mya (see Chapter 1). However, the Earth sys-
tem went through a climate threshold called the Mid-Pleistocene Transition (MPT)
around a million years ago. After this the Milankovitch eccentricity cycle of roughly
100 000 years began to dominate global climate fluctuations. As to what caused this
switch, there is some evidence from modelling to suggest that as each successive
glacial got deeper a point was reached when the two components of the Laurentide
glacial ice sheet in North America joined together. Prior to 1 mya the ice sheet had
two distinct components. This joining would have conferred a regional albedo effect.
This meant that the overall North American ice sheet (with the now joined com-
ponents) could outride the obliquity component of Milankovitch solar variation and
so the eccentricity component dominated. This it has done through to the present.
(Even in the depth of the last glacial the overall shape of the North American ice
sheet exhibits the echo of its two separate components as they were in the depth of
pre-MPT glacials: see Fig. 4.3.)
In addition to the changes in surface ocean circulation, which helped cool the higher
latitudes, there were amplifying feedbacks. One of these came to play a significant
part once the Earth had undergone some initial cooling and so encouraged further
cooling.
Carbon resides in a number of reservoirs in the biogeosphere (see Chapter 1).
This includes reservoirs in the atmosphere (of climate significance) and both in the
surface and deep waters of the oceans. Both these latter reservoirs are larger than the
atmospheric reservoir and so flows to and from the oceans have a major impact on
the atmospheric reservoir; indeed, the size of the ocean deep-water carbon reservoir
is over 45 times that of the atmosphere's (see Figure 1.3). In fact, the flow of carbon
between these three reservoirs exceeds that of the flow to the atmosphere by human
fossil fuel burning by at least an order of magnitude and so these natural flows
are important determinants of the global climate. For carbon dioxide to return to
the atmosphere from the deep ocean there needs to be mixing with surface waters.
Sigman et al. (2004) suggested that before about 2.7 mya, at high latitudes the oceans
were sufficiently warm that, by contrast, the sea in winters would have had a cool
surface with a temperature not far from the deep-water temperature (which was
warmer than that today). This would allow surface water to sink and for deep water
to surface elsewhere and return carbon to the atmosphere. However, after 2.7 mya the
Earth's deep oceans became so cold that vertical mixing was reduced. This is because
warm water, when cooled by a degree, becomes proportionally more heavy than cool
water cooled a degree. In other words, the relationship between sea-water density
and temperature is not linear (see Figure 4.1). It is an idea that seems to fit in with
Search WWH ::




Custom Search