The Earth hasn’t always been struggling with global warming. Around 34 million years ago at the Eocene-Oligocene Transition (EOT) the Earth was undergoing a period of global cooling. This significant shift in climate led to the formation of the first permanent ice sheets of the Cenozoic Era over Antarctica, as shown by the dramatic shift (in a geologic sense) in the oxygen isotope records . The cooling, likely a result of declining atmospheric carbon dioxide levels but potentially also coinciding with Southern Ocean gateway changes, turned Antarctica from a green forested continent to the land of ice we know today. This is illustrated with an image of how this world might have looked.
Guest post by Alan Kennedy, University of Bristol
[I think this image is a great tool to communicate complex paleoclimate evidence – Ed]
The mock satellite image below, featured in a recent Science Perspectives piece, shows many of the key features that are known today about the EOT. The base map was adapted from those provided by Getech and vegetated according to simulations carried out using the fully coupled climate model HadCM3L and the dynamic vegetation model TRIFFID [2,3]. At elevated atmospheric CO2 levels relative to today, both Antarctica and Australia would have been covered with mixed forest during the Eocene, shown both by models and the fossil record .
As the Earth cooled and its orbit favoured cool Southern Hemisphere summers, ice would have started to accumulate over mountainous regions of the continent. The incipient ice caps shown here are taken from the seminal work of DeConto and Pollard on the EOT . As the ice started to expand there would be a number of feedbacks with the atmosphere, including the height-mass balance feedback and an albedo feedback. These feedbacks would have stabilised the early ice sheet from melt and caused rapid growth once the ice caps reached a critical mass and altitude where they could start to expand across large plateaus of the continent.
The Tasman Seaway between Australia and Antarctica would have been narrow and shallow at the EOT . There is evidence that it may have been highly biologically productive [7,8], which is shown in the image by the phytoplankton blooms off the continental margins and in the seaway. The role of this ocean gateway at the EOT is still largely unknown. Its impact on productivity and what affect this productivity might have on the carbon cycle on a geologic time scale for example, or its constraints on ocean circulation and how this might affect the physical climate response at the EOT provide a complex challenge for modellers.
Palaeoclimate research might not seem immediately accessible to a lay-audience. I constructed this map by merging modern day satellite images to help visualise some of the concepts discussed here. The outlines of the continents are hopefully quite easy to recognise and it is often only once the viewer has taken a moment to reflect on the image that they notice the colours and position seem ‘wrong’. I hope this will then encourage the viewer to think why the world appears different and engage with ideas of global change: that the world has always changed and will always continue to change, on a range of spatial and temporal scales.
1: Zachos, J.C., Shackleton, N.J., Revenaugh, J.S., Pälike, H. & Flower, B.P. (2001). Science, 292, pp. 686-693, doi: 10.1126/science.1059412
2: Lunt, D.J., Farnsworth, A., Lopston, C., Foster, G.L., Markwick, P.J., O’Brien, C.L., Pancost, R.D., Robinson, S.A. & Wrobel, N. (2015). Clim. Past Discuss. 11, 5683, doi: 10.5194/cpd-11-5683-2015
3: Kennedy, A. T., Farnsworth, A., Lunt, D. J., Lear, C. H. & Markwick, P. J. (2015). Philos. Trans. R. Soc. Lond. A 373, 20140419, doi: 10.1098/rsta.2014.0419
4: Thorn, V. C. & R. DeConto (2006). Palaeogeogr. Palaeoclimatol. Palaeoecol., 231, 134–157, doi: 10.1016/j.palaeo.2005.07.032
5: DeConto, R. M. & Pollard, D. (2003). Nature, 421, 245-249, doi: 10.1038/nature01290
6: Stickley, C.E., Brinkhuis, H., Schellenberg, S.A., Sluijs, A., Rohl, U., Fuller, M., Grauert, M., Huber, M., Warnaar, J. & Williams, G.L. (2004). Paleoceanography, 19, PA4027, doi: 10.1029/2004001022
7: Zachos, J.C., Quinn, R.M., & Salamy, K. (1996). Paleoceanography, 11, pp. 251-266, doi: 10.1029/96PA00571.
8: Diester-Haass, L. & Zahn, R. (2001). Palaeogeogr. Palaeoclimatol. Palaeoecol. 172, 153, doi: 10.1016/S0031-0182(01)00280-2