This links directly to the emerging paradigm of Earth system sensitivity . Furthermore, the mPWP has been proposed as an important interval to assess the sensitivity of climate to current or near future concentrations of carbon dioxide (CO 2) in the longer term (hundreds to thousands of years) . Given the abundance of proxy data, the mid-Pliocene has become a focus for data–model comparisons that attempt to analyse the ability of climate models to reproduce a warm climate state in the Earth's history . The East Asian summer monsoon as well as other monsoon systems may have been enhanced . Meridional and zonal temperature gradients were reduced, which had a significant impact on the Hadley and Walker circulations . Based on model predictions, the global annual mean temperature may have increased by more than 3☌ . On land, the global extent of arid deserts decreased and forests replaced tundra in the Northern Hemisphere . Sea-ice cover also declined substantially . Between 2.7 and 3.2 Ma BP the peak sea level is estimated to have been 22☑0 m higher than modern , and it appears that SSTs were warmer , particularly in the higher latitudes and upwelling zones . During warm phases of the mid-Pliocene, highlighted by negative excursions in δ 18O from benthic foraminifera, Antarctic and/or Greenland ice volume may have been reduced . Therefore, the use of the PRISM dataset is evolving from specifying boundary conditions in models towards a model evaluation approach .īoth geological data and model outputs have shed considerable light on the nature of mid-Pliocene climate and environments. In later years, single-site SSTs and land-cover data are increasingly being used to evaluate model outputs, as climate models have developed and can now predict SSTs and vegetation (coupled atmosphere–ocean–vegetation–climate models-AOGCMs and AOVGCMs). These models required global information on SST, sea-ice cover as well as land cover, as they are not predicted variables in such models. Until 2004, atmospheric general circulation models (AGCMs) were the only type of climate model applied in a mid-Pliocene context . These reconstructions were developed with a dual purpose: to provide greater understanding of climate and environments in a warmer world, and to provide geographically continuous boundary conditions to facilitate Pliocene climate model experiments . Estimates of sea level as well as topographic differences between the mid-Pliocene and present day have been produced . The PRISM project has documented patterns of sea-surface temperature (SST) and land cover using multiple proxy techniques, as well as reconstructing deep-ocean temperatures . The interval is synonymous with the Pliocene Research Interpretation and Synoptic Mapping (PRISM) time slab for which a global dataset of palaeoenvironmental conditions has been developed by the US Geological Survey and international collaborators . According to the geological time scale of Gradstein et al. , it sits within the Piacenzian Stage of the late Pliocene. (a) The importance of the mid-Pliocene warm periodĬompared with the Pleistocene, the mPWP represents an interval of relatively warm and stable climate between 3.264 and 3.025 Ma BP . Climate model simulations indicate that proxy temperature estimates are unlikely to be significantly affected by orbital forcing for at least a precession cycle centred on the time slice, with the North Atlantic potentially being an important exception. It occurred during a period of orbital forcing that was very similar to present day. It is a warm interval characterized by a negative benthic oxygen isotope excursion (0.21–0.23‰) centred on marine isotope stage KM5c (KM5.3). A time slice centred on 3.205 Ma BP (3.204–3.207 Ma BP) has been identified as a priority for investigation. Here, we describe the rationale and approach for identifying a time slice(s) for Pliocene environmental reconstruction. This uncertainty comes, in part, from the reconstruction of a time slab rather than a time slice, where forcings required by climate models can be more adequately constrained. Uncertainties in prescribed forcings and in proxy data limit the utility of the interval to understand the dynamics of a warmer than present climate or evaluate models. While there is agreement between models and data, details of regional climate differ. The characteristics of the mid-Pliocene warm period (mPWP: 3.264–3.025 Ma BP) have been examined using geological proxies and climate models.
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