Ice with everything - Antarctica: Global Science from a Frozen Continent - page 91

The Antarctic records of atmospheric composition are also bringing important
insights into the coupling between the climate system and the global carbon
cycle. Warm and cold Antarctic periods are associated with increased and reduced
concentrations of carbon dioxide and methane in the atmosphere. Detailed
measurements conducted on high-resolution ice cores suggest that during both the
onset and the termination of ice ages, Antarctic temperature changes, driven by
changes in insolation, occur just before or in phase with changes in atmospheric CO 2
concentration, which precede Nothern Hemisphere warming and methane
concentration rise. At the glacial

interglacial scale, natural variations of carbon
dioxide appear to be mostly controlled by changes in the ocean carbon uptake, with
a key role for the Southern Ocean. Natural variations of methane, by contrast, are
driven by methane emissions mostly provided by wetlands located in the tropics and
boreal lands. Stable over the past 800 000 years, the apparent correlation between
climate and greenhouse gas variations demonstrates the so-called carbon cycle/
climate feedbacks. In response to an initial perturbation (here, orbital), climate
change induces changes in the vegetation, soils and wetlands on the continents, and
in the ocean temperature, gas solubility, circulation and sedimentation processes.
These land surface and ocean changes in turn modify the sources and sinks of
greenhouse gases. A climate warming induces an increased concentration of
greenhouse gases in the atmosphere, itself inducing a supplementary absorption of
infra-red radiation and a warming of the lower atmosphere and of the Earth

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'

s
surface. Such a feedback is expected to continue to operate for the future: current
anthropogenic emissions of greenhouse gases are inducing global warming, which is
itself modifying the natural carbon sinks. Other climate feedbacks are revealed by
the ice core data such as the amplifying role of changes in austral sea ice and
Patagonian dust over the course of glacial interglacial cycles.

Most climate models containing an explicit representation of the carbon cycle show
that the doubts on future climate change arise from three key uncertainties:

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rst,
human emissions of greenhouse gases; second, the response of climate to increased
greenhouse gas concentrations ( ); and third, the carbon cycle

feedbacks, with a saturation of the ocean and vegetation sinks leading to a larger
proportion of the anthropogenic emissions staying in the atmosphere. The ice core
data provide a benchmark to test the realism of these coupled climate and carbon
cycle models, and provide observational constraints on climate sensitivity.
Based on the magnitude of past Antarctic temperature variations, the available
ice core data clearly support the prediction that a doubling of pre-industrial
carbon dioxide concentrations in the atmosphere would induce a global
warming of ~3ºC.
Ice core data clearly show that the present-day levels of greenhouse gas
concentrations are unprecedented over the past 800 000 years, and that the

' climate sensitivity '