Ice with everything - Antarctica: Global Science from a Frozen Continent - page 89

sea-level rise. This estimate suggests that the Antarctic ice sheet probably
contributed to the observed sea-level rise of that period. The areas concerned may
have involved the marine portions of the West Antarctic ice sheet as well as the
Aurora/Wilkes Land sector of the East Antarctic ice sheet. However, there are still
huge uncertainties on the sequence of events involved in the bipolar warming and
ice sheet decay of the last interglacial period. Current deep drilling operations have
recently been conducted in West Antarctica and in the Ross Sea sector, with the goal
of improving the characterisation of the local climate and ice sheet evolution.
The Dome C climate record also shows interesting features of the relationships
between the onset of ice ages and the variations of the Earth

s orbit. The orbital
theory of palaeoclimate relates the ice age changes with a latitudinal and seasonal
redistribution of incoming solar radiation due to past changes in three features of
the Earth

'

s rotation around the Sun (excentricity, obliquity and precession of the
equinoxes).
In this theory, the onset of an ice age is directly related to low levels of
summer radiation received from the sun in the northern hemisphere. Indeed,
both palaeoclimatic records and climate models show that such reduced summer
insolation can prevent the melting of the snowfall accumulated during winter on the
northern hemisphere high latitude continents. Climate feedbacks then amplify this
to an initiation of ice sheet growth. Larger extents of snow increase the surface
albedo and therefore the high latitude cooling effect. Increased Arctic sea ice extent
and persistence, as well as changes in continental vegetation, amplify this high
latitude albedo feedback, followed by changes in ocean circulation and in greenhouse
gas concentrations, propagating the polar cooling towards the lower latitudes. A key
question is then to identify which threshold of northern hemisphere solar insolation
minimum is necessary to trigger the onset of an ice age. The Dome C record shows a
particularly long interglacial period, about 400 000 years ago. Over the 20 to
30 000 year duration of that exceptionally long interglacial period, one small
minimum of northern hemisphere summer insolation was not deep enough to
provoke the onset of an ice age, while the second minimum was strong enough to
trigger a glaciation. All other ice ages were triggered by stronger insolation minima.
What is the relevance of this

'

is orbit
can be calculated with high accuracy over millions of years. Over the next tens
of thousands of years, the northern hemisphere incoming solar radiation will be
exceptionally stable, due to the low excentricity of our orbit around the Sun, and
will not cross this

finding? Future changes in the Earth '

threshold. This result suggests that, even without
anthropogenic interference through greenhouse gas emissions, we are going to enjoy
an exceptionally long interglacial period. In other words, we cannot count on the
orbital forcing to compensate for the long-term warming induced by anthropogenic
greenhouse release.

' glaciation '