Ice with everything - Antarctica: Global Science from a Frozen Continent - page 92

current rate of increase is unprecedented over the past 20 000 years, a time
interval for which the resolution of ice core measurements is large enough to
make this comparison. In fact, we can now say that the global atmospheric
composition is at present controlled by the human emissions of greenhouse
gases, which are well mixed throughout the atmosphere. The ice core perspective
on the step change between the natural carbon cycle historical variability and the
present industrial period is impressive, showing that we humans have become a
'

at the planetary scale. With 30% more carbon dioxide and
200% more methane now than during any warm pre-industrial period, we have
enteredintoanewgeologicalperiodwhere humans are the dominant climate
forcing factor: the

geological force '

.
Emissions of greenhouse gases are already reaching 30 billion tonnes
of carbon dioxide per year, and are increasing year on year. Future climate
change scenarios have been explored by climate modelling groups, coordinated
within the International Panel on Climate Change. Future global temperature
changemayvarybetween1.5andmorethan4 C, depending on the emissions of
greenhouse gases. Climate models consistently show a polar ampli

' Anthropocene '

cation of
global temperature changes. In the central Antarctic plateau, the simulated
temperature rise is about 20% higher than the global average. How would an
Antarctic warming of a few degrees over the next century compare with the
natural variability of Antarctic temperatures? There is evidence for interglacial
periods when Antarctica (but not necessarily the whole planet) was up to 5 C
warmer than at present. Therefore, a warming of several degrees above present
day is not unprecedented. Past sea-level data show that the polar ice sheets
were vulnerable to such warmth and that they contributed to an increased global
sea-level rise by several metres. The fastest historical temperature rise we have
detailed data for comes from the Dome C ice core and took place during the last
deglaciation, rising at most 4 C within a thousand years. Worryingly, the future
temperature rise simulated by present climate models could reach the same
warming magnitude but about 10 times faster than at any time in the last
800 000 years.
Antarctic and Greenland ice cores do not only document the glacial -

interglacial cycles linked with slow changes in the Earth ' s orbit. They also provide

clear evidence for rapid climatic fluctuations operating over glacial periods.

Greenland ice core data reveal 25 abrupt climate changes punctuating the last glacial
period. These Greenland events appear as prolonged cold

periods, followed by an abrupt warming reaching up to 16 C in a few decades,
and then a progressive cooling back to another cold episode. These rapid instabilities
are not restricted to the high latitudes of the northern hemisphere, but they are
also recorded in marine and terrestrial palaeoclimatic records at temperate and

' Dansgaard - Oeschger '