Atmospheric Carbon Dioxide

While there has been no specific simulation of the influence of atmospheric CO2 on ice-sheet growth at the Oligocene-Miocene boundary, DeConto and Pollard (2003a) demonstrated the potential link through simulations across the Eocene-Oligocene boundary. Their modelling demonstrated that icesheet inception occurred below a threshold of 3 x pre-industrial atmospheric CO2 levels. Model results also demonstrated a strong response of ice volume to orbital forcing as atmospheric CO2 approaches the glaciation threshold, and decreasing orbital variability of an established ice sheet as CO2 approaches pre-industrial levels. Despite a predicted rewarming to pre-Oligocene-Miocene boundary levels in the early Miocene and a major Antarctic glaciation in the middle Miocene (Zachos et al., 2001a), this was not matched by a parallel changes in levels of atmospheric CO2 as determined by geochemical proxies (Pagani et al., 1999, 2005; Pearson and Palmer, 2000; Fig. 9.8). The apparent decoupling between Miocene temperatures and atmospheric CO2 levels led Pagani et al. (1999, 2005) to conclude that, despite a significant decrease in atmospheric CO2 from ~500ppmv in the late Oligocene to new near-modern values at the Oligocene-Miocene boundary, changing atmospheric CO2 levels may have been secondary in driving Miocene Antarctic climatic and ice-sheet evolution.

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