Modelling Pliocene warmth: contribution of atmosphere, oceans and cryosphere

Abstract

The relative role of the atmosphere, oceans and cryosphere in contributing towards middle Pliocene warmth (ca 3 Ma BP) is investigated using the HadCM3 coupled ocean–atmosphere general circulation model. The model was initialised with boundary conditions from the USGS PRISM2 data set and a Pliocene atmospheric CO₂ level of 400 ppmv and run for 300 simulated years. The simulation resulted in a global surface temperature warming of 3°C compared to present-day. In contrast to earlier modelling experiments for the Pliocene, surface temperatures warmed in most areas including the tropics (1–5°C). Compared with present-day, the model predicts a general pattern of ocean warming (1–5°C) in both hemispheres to a depth of 2000 m, below which no significant differences are noted. Sea ice coverage is massively reduced (up to 90%). The flow of the Gulf Stream/North Atlantic Drift is up to 100 mm s⁻¹ greater in the Pliocene case. Analysis of the model-predicted meridional streamfunction suggests a global pattern of reduced outflow of Antarctic bottom water (AABW; up to 5 Sv), a shallower depth for North Atlantic deep water formation and weaker thermohaline circulation (3 Sv). The decrease in AABW occurs mainly in the Pacific rather than Atlantic Ocean. Model diagnostics for heat transports indicate that neither the oceans nor the atmosphere are transporting significantly more heat in the Pliocene scenario. Rather, these results indicate that the major contributing mechanism to global Pliocene warmth was the reduced extent of high-latitude terrestrial ice sheets (50% reduction on Greenland, 33% reduction on Antarctica) and sea ice cover resulting in a strong ice-albedo feedback. These results highlight the need for further studies designed to improve our knowledge regarding Pliocene terrestrial ice configurations.

Introduction

The middle Pliocene (ca 3 Ma BP) represents a recent period in geological history when the climate of the Earth was significantly warmer than present-day. The period has been studied extensively by palaeoenvironmental/climatologists. Geological data supporting the assertion of a warmer than present climate includes sea surface temperatures (SSTs) reconstructed from planktonic foraminifera [1], [2], [3], [4], [5], ostracods [6], [7], siliceous microfossil records [8], diatom records [9], terrestrial vegetation records [10], [11], [12] and numerous records of higher than present sea levels [13], [14], [15].

Numerous modelling studies have been conducted for the period. The first study used the PRISM0 8°×10° digital data set and the GISS (Goddard Institute for Space Studies) atmospheric general circulation model (GCM; 8°×10° model resolution) which focussed on the middle Pliocene climate of the Northern Hemisphere [16]. The second model investigation used a 2°×2° version of the (PRISM1) digital data set to prescribe the boundary conditions for the NCAR (National Centre for Atmospheric Research) GENESIS atmospheric GCM (4.5°×7.5° model resolution) which examined the nature of the middle Pliocene climate on a global scale [17]. A global scale palaeoclimate reconstruction for the middle Pliocene was conducted using the updated PRISM2 digital data set and the UKMO (United Kingdom Meteorological Office) atmospheric GCM, running with an enhanced spatial resolution of 2.5°×3.75° [18]. The same model has been used to carry out detailed data/model comparisons on a regional scale [19], [20] and to conduct ice sheet sensitivity/biome experiments for the period [21], [22], [23].

Despite these modelling studies, the cause of middle Pliocene warmth continuous to be the subject of much debate [16], [17], [24], [25], [26], [27]. A combination of modelling studies and evidence from Pliocene proxy data indicate that CO₂ concentrations at 3 Ma BP were greater (absolute value 400 ppmv) compared to mid-19th century levels (∼280 ppmv) [28], [29], [25], [30], [31]. However, an increase in CO₂ alone would be expected to raise surface temperatures at all latitudes, and, hence, cannot explain the reconstructed pattern of little or no SST change at the equator [2], [3], [4].

Variations in oceanic circulation and increased heat advection away from the equator to high latitudes have been proposed as a significant causal factor in producing the climate of the middle Pliocene [25], [26], [3]. Such an increase in heat advection could be a response to enhanced thermohaline circulation in the oceans or enhanced surface gyre flow resulting from increased regional atmospheric winds and wind stresses [19]. Thus far, all palaeoclimate modelling studies of the middle Pliocene have been restricted to using atmospheric GCMs utilising prescribed SSTs and/or a simple slab-ocean model [32], [22]. Although slab-ocean models are capable of simulating part of the feedbacks of the oceans on climate, they are incapable of simulating changes to horizontal ocean heat transports, and related changes to ocean currents and thermohaline circulation. These have been recognised as potentially pivotal mechanisms in forcing climate change for the Pliocene and many other time periods [33], [34], [35].

Thus it has not been possible to robustly examine the oceans role in generating and or maintaining mid-Pliocene warmth. This study aims to remedy this situation through the use of a fully coupled atmosphere–ocean model.