An introduction to climate change - Climate Change: Biological and Human Aspects - page 21

2

0
-2
-4
-6
-8
-10

700

600

Methane

500

400

1990
level
of CO 2

300
280
260
240
220
200

Carbon dioxide

180

0 40 80

Age (thousand years before present)

120 160

Fig. 1.6 Atmosphericcarbondioxideandmethanepalaeorecordforthepastglacial-interglacialcycleplottedwith

regionaltemperaturechange,asindicatedbytheicehydrogenisotopeproxy.AdaptedfromBarnolaetal.
(1987)andChappelazetal.(1990),reproducedfromtheIPCC(1990)withpermission.

1.5 Pacemakeroftheglacial-interglacialcycles

There have been 20 or so of these glacial-interglacial cycles over the past 2 million
years and, at first sight, they have an apparent regularity. The question then arises as
to whether there is anything causal driving this periodicity.
In the 1920s the Serbian mathematician Milutin Milankovitch (or Milankovic) -
although owing a debt to the work of James Croll in 1864 - suggested that minute
variations in the Earth's orbit could affect the sunlight reaching its surface. This
happens largely because the Earth's geography is currently asymmetric: the north-
ern hemisphere is land-dominated while the southern is ocean-dominated, so the
hemispheres differ in the way they absorb the Sun's heat. Also at the present time
the positioning (through plate tectonics) of the North and South American and the