The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 131

that there was an inception of a strong carbon dioxide/greenhouse gas feedback and
amplification of orbital forcing at approximately 2.7 mya that has connected the fate
of northern hemisphere ice sheets with global ocean temperatures since that time.
This marginal sea-surface temperature leading global changes in ice volume and
deep-ocean temperature over the past 3.5 million years is exactly what would be
expected with Milankovitch pacemaking of glacial and interglacials. Indeed, as ice
cores indicate, surface temperatures lead carbon dioxide change 4 . Building on the
discussion in section 4.4, what is happening is this: By the Quaternary period, contin-
ental plate tectonic movement had placed most of the Earth's land area in the northern
hemisphere (see section 1.5). This and the rise of the Tibetan Plateau in the northern
hemisphere and its reflective waxing and waning ice (section 4.1) meant that the solar
energy received by the Earth at these latitudes in the summer (which determines how
much of the previous winter's ice melts) is indicative of Milankovitch pacemaking.
Depending on whether warming or cooling (more or less energy is received) is
taking place will determine how much carbon is released or sequestered from wet-
lands, soils and other terrestrial biosphere carbon pools and so alter atmospheric
carbon dioxide and methane, which in turn amplifies the said warming or cooling.
So Milankovitch pacing slightly leads carbon dioxide change. The subsequent amp-
lified warming or cooling will determine global ice volume. Finally, it takes time
for temperature change on the surface of the Earth to be carried by ocean currents
to deeper waters, and so deep-ocean temperatures also lag behind Milankovitch
pacing.
Whereas glacial-interglacial temperature changes were smaller in the tropics com-
pared to the poles, climate-related events in high latitudes near the north polar circle
did affect the tropics. Biological productivity in the tropics off Venezuela appears to
have increased and decreased, reflecting periods of iceberg discharge from the large
glacial Laurentide ice sheet over northern North America, as represented in the cli-
mate change recorded in the Greenland ice sheet. Chemical change in the sediments
off Venezuela bears an even closer correlation with the Greenland ice-core record
for the 18 O isotope, which more than adequately reflects regional climate change
during the glacial (Peterson et al., 2000). Having said this, the Venezuela sediment
record may relate more to deep-ocean ventilation than the tropical climate per se
because, as we shall shortly see, global ocean circulation helps transport heat hemi-
spherically and link the oceans globally and transport heat away from the tropics.
Changes in ocean circulation also manifested themselves elsewhere in the tropics.
For example, the Kalahari Desert in Africa saw significant arid events approximately
41 000-46 000, 20 000-26 000 and 9 000-16 000 years ago, which are thought to be
times of circulation change (Stokes et al., 1997).
The detail required to understand and climatically model the planet is far greater
than we have so far acquired, as we shall see in Chapters 5 and 6. So there is a

4

Some climate change sceptics use the argument that initial (Milankovitch-paced) glacial-interglacial
temperature change leads carbon dioxide change to say that changes in atmospheric carbon dioxide
concentration are not very important with regards to global climate change. This example in the public
climate debate shows how easy it is to mislead those with little understanding of the science with a
specious argument.