Modelling Pliocene warmth: contribution of atmosphere, oceans and cryosphere
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 3degreesC compared to present-day. In contrast to earlier modelling experiments for the Pliocene, surface temperatures warmed in most areas including the tropics (1-5degreesC). Compared with present-day, the model predicts a general pattern of ocean warming (1-5degreesC) 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.