Ice AtmosphereOcean Modelling

Modelling the effect of atmospheric CO2 on the sensitivity of the Antarctic Ice Sheet to orbital forcing by DeConto and Pollard (2003), and how this may have initiated Antarctic Ice Sheet formation, was a turning point in understanding past ice-sheet behaviour. For three decades, it was believed that the main cooling influence came from thermal isolation with the formation of oceanic ''gateways'' (Kennett, 1977). The modelling showed, albeit simply and with significant assumptions, that ~ 80% of the influence could have been due to the cooling of atmospheric temperature from declining atmospheric CO2 levels. This has gained support from evidence of a 1 km drop in the carbonate compensation depth in the latest Eocene (Coxall et al., 2005) and a proxy record based on 813C from alkenones indicating an irregular decline in atmospheric CO2 levels from more than 1,000 ppm to less than 400ppm between 40 and 24 Ma (Pagani et al., 2005). Changes in ocean circulation through opening gateways have also been accommodated (Huber and Nof, 2006). As a result of this group of studies, the significance of CO2 as a greenhouse gas influencing Antarctic climate, ice sheets and ocean chemistry has been further enhanced.

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