The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 135

Depth (m)

0 500 1000 1500 2000 2500 2750 3000 3200 3300

280

260
240
220
200

a

2
0

− 2

− 4

b

− 6

700
600
500
400

− 8

c

− 0.5

0.0
0.5
1.0

d

100
50
0

e

− 50

0 50 000 100 000 150 000 200 000

Age (years ago)

400 000

250 000 300 000 350 000

Carbondioxideandmethanerelatetotemperature.(a)Carbondioxide,(b)temperature(asderivedfrom
isotopiccompositionoftheice)and(c)methane.Thepatternsforallthreearebroadlysimilar.(d)Track
changesintheice's 18 Oisotope.(e)Milankovitchinsolation(orsolarenergyreceived)ascalculatedforJuneat
65 ◦ N.ThisshowshowpeaksandtroughsinMilankovitchclimateforcingactasapacemakerof
glacial-interglacialcycles.ReproducedwithpermissionfromPetitetal.(1999).

Fig. 4.5

southern hemisphere glacial-interglacial transition being out of phase (or rather,
not exactly in phase) is one such seeming contradiction. If the global Broecker
thermohaline slows then heat ceases to be transported to the North Atlantic from
the southern oceans. This in turn means that the southern oceans warm and the
northern oceans cool. At the end of the last glacial we have the apparent para-
dox of glaciers in Patagonia, South America, growing while those in Europe were
retreating. This trading of hot and cold between hemispheres is called the bipolar
see-saw.
In 2009 a British and North American team led by Stephen Barker and Paula
Diz of Cardiff University, and including Wallace Broecker of global thermohaline
circulation renown, looked at both Antarctica and Greenland ice core records together
with marine sediment cores (for 18 O analysis from forams) from off the US east coast,
off Spain, off Chile and in the South Atlantic off South Africa. The results showed that
northern hemisphere ice sheets began to dwindle some 21 000-19 000 years ago (in
agreement with Milankovitch theory), yet Antarctic temperatures and carbon dioxide
only started rising 18 000 years ago, and more abruptly (not quite as more gradually
as Milankovitch theory would suggest). So far so good with respect to the bipolar see-
saw. But was change in the strength of the Broecker thermohaline conveyor involved