Orbital forcing and its importance in understanding the warm Pliocene

The Pliocene is traditionally viewed as an epoch with a warm and stable climate. Data-Model comparisons for the mid-Pliocene (~3.3 – 3 Ma) have identified regions where models do not agree with geological proxies. Palaeoenvironmental syntheses used in these comparisons are time-averaged. It has been hypothesised that orbital cyclicity within the mid-Pliocene, not accounted for in previous model simulations or data syntheses, could contribute to data-model discord. Study of the Pleistocene (~11.7ka – 2.6 Ma) has established the importance of understanding climate variability and distinguishing the specific character of separate interglacial or glacial events. This thesis confirms such variability should be expected in the Pliocene. Using a climate model, two interglacials (MIS KM5c and K1) in the Pliocene are compared, and results demonstrate changes in the surface air temperatures (SATs) due to changes in orbital forcing can be substantial and differ between interglacials. A further two interglacials (G17 and KM3) are investigated, and changes in regional vegetation patterns and the summer Indian monsoon in response to orbital forcing over the four interglacial events are analysed. A notable vegetation response is seen in the continental interiors of North America and Eurasia, where forests are replaced by grassland and shrubland, and is most widespread for interglacials with the strongest orbital forcing (most different from present day). The Indian monsoon is slightly stronger than pre-industrial in KM5c (an interglacial with near-modern orbit), driven by higher CO2, and is significantly more intense in G17, K1 and KM3 than KM5c, due to orbital-driven increased seasonal SATs. Orbital forcing throughout Pliocene interglacials is found to have a significant effect on the simulation of regional climate, vegetation and the Indian monsoon system within the modelling framework used here. Time-averaged palaeoenvironmental synthesis therefore, cannot be expected to concur with climate model outputs using time specific orbital forcing.