The Last Interglacial

The Last Interglacial was a period of the Earth’s geological history (between 130 000 and 115 000 years BP) characterized by a climate warmer than today, with a higher global sea level and smaller ice-sheets. It is regarded as an important period for investigating the ice-sheet sensitivity to climate change in a global warming scenario.

The Last Interglacial (LIG) was a period of the Earth’s geological history (between 130 000 and 115 000 years BP) characterized by a climate warmer than today, with a higher global sea level and smaller ice-sheets. During the LIG, polar temperatures were ~3-5 °C higher than today [1], the global sea level was at least 6.6 m above present [2] and the global surface temperature was ~ 1 °C warmer compared to the pre-industrial era (~ 1850-1900) [3].

A few interesting analogies exist between the Last Interglacial climate and the Earth’s future climate. For example, the IPCC projections [4] of future climate change show that, by the end of the 21st century, the global surface temperature is likely to increase by 1.5 °C (or more) relative to the pre-industrial era. Furthermore, this global warming will be accompanied by a 3-6 °C Artic warming and by a global mean sea level rise of 0.55m to 0.98m (depending on the emissions scenario).
Although the warmer last interglacial climate was caused by changes in the Earth’s orbital configuration, and the predicted global warming is a response to greenhouse gas emissions, the Last Interglacial is regarded as an important period for investigating the ice-sheet sensitivity to climate change in a global warming scenario.

In this project, we will investigate the substantial retreat (of 50-60%) of Antarctic sea ice between 130 000 and 116 000 years Before Present (BP) [5]. This major retreat compares with IPCC model-based predictions of a future sea ice retreat. In particular, according to the most recent IPCC report, current climate models predict a reduction of the Antarctic sea ice of about 50–60% by the next two centuries.

We will use the UK Earth System Model (UKESM1) to simulate the SH sea ice and test the following hypotheses on the causes of the LIG sea ice retreat:

• During the LIG the eccentricity of the Earth’s orbit was much higher than (in fact more than twice) the present value. The different orbital configurations during the LIG result in different seasonal and latitudinal distribution of the top-of -atmosphere insolation (i.e. incoming solar energy). We will investigate the role of the radiative forcing in causing the minimum in SH sea ice extent. See CMIP6 for further details.
• Evidence from sediment cores from beneath the West Antarctic Ice Sheet (WAIS), and far field sea level records, indicate that the WAIS disintegrated within the last 1.3 million years [6] and that the loss likely occurred during the LIG.
  The removal of the WAIS increases East Antarctic temperature to about + 1.5 °C above its pre-industrial value [7, 8]. This will also cause changes in the atmospheric circulation because of the lowered topography that triggers anomalous cyclonic circulations [8]. At the same time, the decrease in the height of the Antarctic ice sheets reduces the strength of katabatic winds [9].
  We will investigate the impact of this complex changes in temperature and atmospheric circulation on sea ice extent.
• The Atlantic Meridional Overturning Circulation (AMOC) transports vast quantities of heat from the Southern Hemisphere (SH) to the Northern Hemisphere (NH). If the AMOC is weakened through changes in NH surface buoyancy (because of the melt of NH ice sheets), Northern Atlantic cooling and Southern Atlantic warming occurs [10].
  We will investigate the melt of the Greenland ice sheet (GIS), which continued into the LIG providing a mechanism to warm the Southern Ocean, as a possible cause of the SH sea ice retreat.

References

[1] Jansen, E. et al. in Climate Change 2007: the physical science basis. Cambridge Univ. Press. 2007.
[2] Kopp, R. E. et al. Nature 462, 863–867 (2009).
[3] Otto-Bliesner, B. L. et al. Philosophical Transactions A 371, 20130097 (2013).
[4] Meehl GA et al., Global climate projections. In Climate Change 2007: The Physical Science Basis. Cambridge Univ. Press. 2007.
[5] Holloway, M. D., Sime, L. C., et al. Nature Communications 7 (2016): 12293.
[6] Scherer, R. P. et al. Science 281, 82–85 (1998).
[7] Holden, P. B. et al. Climate of the Past 6, 431–443 (2010).
[8] Steig, E. J. et al. Geophysical Research Letters 2015GL063861 (2015).
[9] Sime, L. C. et al. Quaternary Science Reviews 67, 59–80 (2013).
[10] Stouffer, R. J. et al. Journal of Climate 19, 1365–1387 (2006).