The influence of palaeogeographic uncertainty on the simulation of Pliocene climate

Although the Pliocene is a well-studied epoch, key sources of uncertainty remain that add significant complexity to our understanding of climate at the time. From a climate/earth system modelling perspective palaeogeography is a key source of uncertainty, especially in terms of the veracity of specified model boundary conditions. Small variations in prescribed conditions can potentially lead to different local, regional and even global simulated climate states.

Here we explore this uncertainty within the experimental framework of the 3^rd phase of the Pliocene Model Intercomparison Project (PlioMIP3). Building on the foundations of PlioMIP1 and 2, PlioMIP3 outlines three experiments within the Core and Core-Extension experimental design that are capable of directly addressing some of the key sources of palaeogeographic uncertainty in models, and which are potentially relevant to our understanding of both the Early and Late Pliocene. We use the Hadley Centre Coupled Climate Model Version 3 to perform the PlioMIP3 Core and Core-Extension experiments integrated for 4000 years. This includes the control simulation for the Late Pliocene, an alternative Late Pliocene simulation incorporating minimum land/sea mask changes from present-day, as well as an additional experiment that opens the Central American Seaway, and which is used as a single possible realisation of the Early Pliocene.

We compare and contrast climate states within the Late Pliocene and between the Eary and Late Pliocene.  Palaeogeographic uncertainty within the Late Pliocene is shown to have a significant enough impact on climate conditions to influence outcomes of localised data/model comparison, but it does very little to influence model results in terms of large-scale features of climate, or the simulated global annual mean temperature. In contrast, the Central American Seaway being open in the Early Pliocene simulation leads to significant variations in the simulated climate state, and even in the global annual mean temperature with identical greenhouse gas forcing as the Late Pliocene.

This underlines the importance of continued research to better understand palaeogeographic evolution through the Pliocene, which adds new detail to the complex tapestry of what we understand to be ‘Pliocene’ climate.