Antarctic ice-sheet response to atmospheric CO<sub>2</sub> and insolation in the Middle Miocene

Abstract

Abstract. Foraminiferal oxygen isotopes from deep-sea sediment cores suggest that a rapid expansion of the Antarctic ice sheet took place in the Middle Miocene around 13.9 million years ago. The origin for this transition is still not understood satisfactorily. One possible cause is a drop in the partial pressure of atmospheric carbon dioxide (pCO 2 ) in combination with orbital forcing. A complication is the large uncertainty in the magnitude and timing of the reconstructed pCO 2 variability and additionally the low temporal resolution of the available pCO 2 records in the Middle Miocene. We used an ice sheet-climate model of reduced complexity to assess variations in Antarctic ice sheet volume induced by pCO 2 and insolation forcing in the Middle Miocene. The ice-sheet sensitivity to atmospheric CO 2 was tested for several scenarios with constant pCO 2 forcing or a regular decrease in pCO 2 . This showed that small, ephemeral ice sheets existed under relatively high atmospheric CO 2 conditions (between 640-900 ppm), whereas more stable, large ice sheets occurred when pCO 2 was less than ∼600 ppm. The main result of this study is that the pCO 2 -level must have declined just before or during the period of oxygen-isotope increase, thereby crossing a pCO 2 glaciation threshold of around 615 ppm. After the decline, the exact timing of the Antarctic ice-sheet expansion depends also on the relative minimum in summer insolation at approximately 13.89 million years ago. Although the mechanisms described appear to be robust, the exact values of the pCO 2 thresholds are likely to be model-dependent.

Boundary conditions for the Middle Miocene Climate Transition (MMCT v1.0)

“…This value is within the range of published estimates and in reasonable agreement with the values by Langebroek et al (2009), Oerlemans (2004 as well as Gasson et al (2016) …”

“…Most studies present ice volume estimates in units of sea water δ 18 O. Backstripping methods provide sea-level rather than ice volume estimates. More direct Antarctic ice volume estimates can be derived from modelling studies Langebroek et al, 2009;Oerlemans 2004). Gasson et al (2016) performed a series of simulations with an ice-sheet model asynchronously coupled to a regional climate model and an isotope-enabled GCM using Middle Miocene palaeogeography and a range of atmospheric CO 2 concentrations and extreme astronomical configurations.…”

The multimillennial sea-level commitment of global warming

“…Similar to Greenland, the much larger East Antarctic Ice Sheet, which is predominantly grounded above sea level, has the potential of threshold behavior in response to increased surface melting (38). Current models (4,5) suggest that the required warming is much larger than anticipated in the near future (equivalent to ∼4 times preindustrial level of CO 2 or higher).…”

Modeling the Miocene Climatic Optimum. Part I: Land and Atmosphere*

“…Various numerical models have been used to explore Miocene climate, including sensitivity to CO 2 (Micheels et al 2009a;Steppuhn et al 2007;Tong et al 2009;You et al 2009), SSTs (Herold et al 2010;Lunt et al 2008a;Steppuhn et al 2006), vegetation (Dutton and Barron 1997;Micheels et al 2009b;Micheels et al 2007), bathymetry (Barron and Peterson 1991;Bice et al 2000;Butzin et al 2011;von der Heydt and Dijkstra 2006), or some combination of these (Fluteau et al 1999;Henrot et al 2010;Herold et al 2011a;Kutzbach and Behling 2004;Langebroek et al 2009;Micheels et al 2011;Ruddiman et al 1997). However, due to the relatively recent advent of coupled atmosphere-ocean models for deep time paleoclimate analysis, the majority of the above studies have relied on prescribing SSTs or ocean heat fluxes.…”