Ice with everything - Antarctica: Global Science from a Frozen Continent - page 86

Antarctic climatic conditions; climate changes in the sea ice and ocean moisture
and salt source areas; aridity in South America.
The climate of our planet responds to perturbations of the global radiative
budget. Natural factors, such as changes in the orbit of our planet, changes in the
activity of the Sun, or injection of aerosols in the stratosphere by tropical explosive
volcanoes can alter this radiative budget. Because such processes will force the
climate to react, they are commonly called

. While the orbital
forcing can be accurately calculated from astronomical equations, speci

' climate forcings '

c aerosols
archived in ice cores allow estimation of the variability of the solar and volcanic
activity during the last millennia, which are indispensable inputs for climate models
in order to simulate past natural climate variations. Both changes in solar activity
and changes in the Earth field can modulate the incoming cosmic rays

at the top of the atmosphere. These cosmic rays produce a new isotope of beryllium
(cosmogenic 10 Be), characterised by a million year long residence time.
Measurements of 10 Be in ice cores have different applications: they provide a proxy
for past variations in solar activity, which could play a signi

' s geomagnetic

cant role in decadal to
centennial climate variability. Excursions or reversions of the Earth ' s magnetic eld

can be detected in Antarctic ice core 10 Be flux, which therefore provides a useful

chronological marker for ice-core dating. Volcanic eruptions also act as an agent of
climate change. Explosive tropical volcanoes distribute sulfate aerosols in the global
stratosphere. These particles have both a direct effect on incoming solar radiation,
re ecting it back into space, and an indirect effect as they act as cloud condensation

nuclei. Elementary and chemical analyses of ice cores are used to characterise the
magnitude and frequency of volcanic eruptions. On the scale of ice core records,
volcanic activity appears randomly distributed in time. Climate reconstructions and
climate models suggest that the small variations in solar activity and the frequency
of volcanic eruptions are the main forcing agents for decadal to centennial climate
pre-industrial fluctuations of the past millennia. Ice cores are therefore extremely

precious as they provide information not only on the climate history, but also on
some of the natural forces that act on climate fluctuations.

Finally, ice cores are the only direct archives of the past composition of our
planet akes into ice crystals entraps air

bubbles. The analysis of the ice core air provides a direct archive of past changes in
atmospheric composition. Present in small concentrations, some trace gases have
huge impacts on the global climate system through their absorption of infra-red
radiation (the so-called greenhouse effect). Ice cores reveal the natural variability
of carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) concentrations
back in time, and highlight the human imprint on the global atmosphere.
The ice-core scienti

' s atmosphere. The compaction of snow

c community has made great efforts to obtain the oldest
possible ice core climate records. The formula is well known: you need to drill at