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要旨

Significant controversy exists surrounding the nature of the Antarctic ice sheet during the period of middle Pliocene warmth. This paper outlines the output from three sensitivity experiments for the middle Pliocene, using the UKMO GCM and the USGS PRISM2 data set. Different East Antarctic ice sheet configurations were prescribed in each modelling experiment conforming to a dynamic (sea level 35 m higher than at present), intermediate (Pliocene ‘control’, sea level 25 m higher than at present) and stable (sea level 12–15 m higher than at present) scenario for the nature of the Antarctic ice sheet during the middle Pliocene. All three scenarios would have provided middle Pliocene sea levels across the spectrum of estimates provided by known geological data. Model outputs indicate that surface temperature increases are greatest in the Pliocene ‘dynamic’ experiment. Climatic conditions for the Pliocene ‘control’ and ‘stable’ experiments remain harsh over Antarctica with surface temperatures rising above freezing (mean +4°C) for a maximum of 2 months per year. In contrast, mean surface temperatures from the Pliocene ‘dynamic’ experiment rise above freezing (mean +10°C) for 4 months of the year. Climatic outputs from all of the Pliocene experiments were used in a biome model (BIOME 4). For the Pliocene ‘control’ and ‘stable’ experiments, BIOME 4 predicted the occurrence of exclusively tundra type vegetation on Antarctica. For the ‘dynamic’ experiment, climatic conditions ameliorated sufficiently to allow BIOME 4 to predict deciduous taiga montane forests on Antarctica. Thus the results from the ‘dynamic’ experiment are consistent with reported Pliocene Nothofagus occurrences on Antarctica. In the future, more sophisticated palaeoclimate modelling studies need to employ atmospheric general circulation models (GCMs) coupled to dynamic ocean and ice sheet models. This will allow the GCM to predict rather than prescribe an Antarctic ice sheet configuration in equilibrium with reconstructed middle Pliocene sea surface temperatures. Such an approach may also shed light on how the Antarctic ice sheet responded to climate variability during the middle Pliocene.