PP23E-01 INVITED
Lessons from the Pliocene Warm Period and the Onset of Northern Hemisphere Glaciation
The Pliocene warm period (~5-3 Ma), the most recent time when Earth's global temperature was warmer than present for a sustained period of time, was followed by an ice age climate with large Northern Hemisphere glaciations (NHG). The processes that determined Pliocene warmth and the subsequent transition to NHG would be expected to have had different regional expressions, and as such, their study requires comparative regional analyses. This is possible because the Ocean Drilling Program has recovered Pliocene sediments in some critical regions in the world's oceans. During the Pliocene warm period, high latitude warmth was accompanied by warmer than modern sea surface temperatures (by 2-9°) in tropical and subtropical eastern boundary upwelling regions. In contrast, in the west, the Indo-Pacific warm pool and North Pacific subtropical gyre were not much warmer, resulting in reduced zonal temperature gradients compared to today. This early Pliocene mean climate state is referred to as permanent El Niño- like conditions, or El Padre (to distinguish its cause from processes that determine short-lived El Niño events). Paleoceanographic data indicate that the tropical subsurface thermocline was warmer or deeper; thus, the underlying cause of El Padre is related to the relatively warm temperature of upwelling subsurface water. Both data-based and modeling studies show that warm upwelling anomalies drive far-field effects, such as warmer North American continental temperatures. At the same time, low latitude upwelling regions are influenced by conditions at higher latitudes where thermocline waters originate. The Pliocene warm period provides compelling evidence that processes that determine thermocline properties and that link high and low latitude climate and ocean conditions need to be well-represented in coupled climate models in order to accurately predict long term climate change. Global cooling and NHG occurred at the end of the warm Pliocene period, but the timing of this transition in high and low latitudes differed indicating that regional climate thresholds and/or sensitivities play a role in the regional expression of a global climate transition. Atmospheric carbon dioxide concentrations in the Pliocene were about 30% higher than in preindustrial times, similar to today's concentration. If decreasing pCO2 was responsible for the transition to cool climate and NHG, the implied climate sensitivity (change in temperature relative to change in pCO2) was similar to the highest values predicted by coupled climate models used to predict future global warming.