ATMOSPHERIC AND OCEANIC IMPACTS OF
ANTARCTIC GLACIATION ACROSS THE EOCENE–
OLIGOCENE TRANSITION
A. T. Kennedy, A. Farnsworth, D. J. Lunt, C. H. Lear, P. J. Markwick
The Eocene-Oligocene Transition occurred approx. 34 million years ago and was one of the
biggest climatic shifts since the end of the Cretaceous (with the extinction of the dinosaurs).
The Earth dramatically cooled and the Antarctic ice sheet first formed, but the cause and
nature of the cooling remain uncertain. Using a climate model we looked at the effect of ice
sheet growth and palaeogeographical change (i.e. continental reconfiguration as Australia
separated from Antarctica) on the Earth’s steady-state climate. We utilised four simulations: a
late Eocene palaeogeography with and without an ice sheet and an early Oligocene
palaeogeography with and without an ice sheet.
The formation of the Antarctic ice sheet causes a similar atmospheric response for both
palaeogeographies: cooling of the air over Antarctica, intensification of the polar atmospheric
cell and increased winds over the Southern Ocean. The sea surface temperature response to
the growth of ice is very different, however, between the two palaeogeographies. For the
Eocene palaeogeography there is a 6°C warming in the South Pacific sector of the Southern
Ocean in response to ice growth, but very little change (or even a slight cooling) for the
Oligocene palaeogeography. Why, under the same forcing (the appearance of the ice sheet),
do these different palaeogeographies have such different sea surface temperature responses?
The stronger winds over the Southern Ocean force more-saline water from the southern
Indian Ocean into the less-saline southern Pacific Ocean. This is particularly important for the
Eocene simulations, where the narrow gap between Australia and Antarctica limits flow from
the Indian to the Pacific Ocean. As salinity in the southern Pacific Ocean increases the water
becomes denser and sinks, releasing heat. This accounts for the increase in sea surface
temperature in the Eocene simulations. In the Oligocene simulations, flow is already much
greater between the Indian and Pacific Oceans, and so there is no marked increase in density,
sinking or sea surface temperature following glaciation. There is only a mild cooling due to
the presence of the large, cold ice sheet.
Whether in reality the dominant ocean response to glaciation was warming or cooling may
have impacted the growth of the ice sheet at this major transition in the Earth’s history.
However, more importantly, this research highlights that sensitivity to subtle changes in
palaeogeography can potentially have very large effects on the modelled climatic response to
event such as Antarctic glaciation. This could be very important for understanding
palaeoclimate records and interpreting climate model results.
This research is featured in a special issue of the Philosophical Transactions of the Royal
Society A, on the theme of ‘Feedbacks on climate in the Earth System’. The full special issue
and paper can be accessed here. Citation: Kennedy A.T., Farnsworth A., Lunt D.J., Lear C.H.,
& Markwick P.J. (2015) Atmospheric and oceanic impacts of Antarctic glaciation across the
Eocene–Oligocene transition. Phil. Trans. R. Soc. A, 373, 20140419,
doi:10.1098/rsta.2014.0419.
Composite satellite image of what the Earth may have looked like Prior to Antarctic
glaciation during the late Eocene (image by Alan Kennedy).
Special issue cover (image from Royal Society).