The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 128

global climate change over the past few million years, ice cores from Antarctica
have been useful in ascertaining regional climate change in the southern hemi-
sphere over the past few glacial-interglacial cycles. Greenland ice cores have
provided similar information, particularly relating to the last glacial-interglacial
cycle. (Ice cores can be analysed for variations in deuterium, dust and greenhouse
gases as discussed in section 2.3.1.) The longest ice core to date comes from Dome
C in Antarctica. It provides a continuous record of chemical climate proxy data
covering 740 000 years and several glacial-interglacial cycles. The core's physical
length is about 3 km. We will shortly return to ice cores, but for now note that the
Dome C record does strongly suggest that iron-dust deposition was higher in the
cool parts of glacials (when globally there was less rain, and hence it was drier on
land) and that subsequently the glacials reached their coldest phase. It is thought
that iron fertilisation of the oceans encouraged algal blooms that further drew down
carbon dioxide into the oceans, so causing the coldest periods of glacials (Wolff et al.,
2006).
One question to arise is the nature of glacial pacemaking. It is clear from so
much evidence that the Milankovitch orbital parameters drive the glacial-interglacial
cycles. However, do the Milankovitch orbital variations lead the driving, or forcing, of
the climate? Or, in other words, does global climatic change lag some time behind the
parameters, as opposed to the parameters having a more or less instantaneous effect?
Finally, do the waxing and waning of the ice sheets that amplify such forcing do so
in time with either global climatic change and/or Milankovitch forcing? As noted
in section 1.5, Milankovitch factors provide the pacemaker for glacial-interglacial
cycles, but are the two (orbital and glacial timing) and the changes in atmospheric
greenhouse gases perfectly synchronised?
Nicholas Shackleton of Cambridge University's Godwin Laboratory addressed
these questions in 2000. After all, Milankovitch forcing is weak. Although we are
considering changes of about 15% of 440 W m − 2 on the Earth's surface at 65 ◦ ,this
is countered by opposite changes in the southern hemisphere. The actual difference
taken globally is less than 1 W m − 2 , or less than 0.1% of the solar constant (the
energy received per square metre perpendicular to the Sun in the Earth's orbit).
This is due to the solar energy the entire planet receives and absorbs, which in
turn arises from the summation of a variety of factors: its albedo at various locales
about the planetary sphere, the time of year these locales receive the energy and
the distance from the Sun, again integrated across the planetary sphere. So how
does this weak signal magnify into a glacial-interglacial transition? Shackelton shed
some light on this when comparing the timing of changes in benthic 18 O foram
palaeorecords with the ice core and trapped gas Antarctica records in addition to
June insolation at 65 ◦ N (another often-used latitude for Milankovitch forcing curves
that crosses both the former Laurentide and Fennoscandian ice sheets). He found
that while the extremes in Milankovitch forcing are in time with changes both in
global climate and atmospheric carbon dioxide, ice volume lags behind by a few
thousand years. This might suggest that northern hemisphere summer sunlight (or
perhaps the northern hemisphere thermal growing season?) affects the carbon cycle
so as to drive atmospheric carbon dioxide concentrations and that this in turn has an
immediate affect on temperature. This, while initially plausible, is unlikely because