Modeling the mid-Piacenzian warm climate using the water isotope-enabled Community Earth System Model (iCESM1.2)

Abstract

The mid-Piacenzian warm period (MPWP, ~3.264–3.025 Ma, previously referred to as the mid-Pliocene warm period), is the most recent geological period with atmospheric CO2 concentrations (400ppmv) close to today, but global surface temperatures were higher than today and in equilibrium with the CO2 concentrations. Therefore, the mid-Piacenzian equilibrated climate is often compared to the modern transient climate. In this study, we conduct a water isotope-enabled Community Earth System Model (iCESM1.2) simulation to study the large-scale features of the MPWP following the protocols of Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). This MPWP simulation exhibits considerable warming in the high latitudes comparable to high-latitude MPWP surface warming evidenced in proxy records (i.e., polar amplification) that has been often underestimated in previous simulations. The improved performance of iCESM1.2over the PlioMIP2 models is due to a larger contribution of iCESM1.2-simulated downward clear-sky surface long wave radiation fluxes affecting polar amplification. Compared to the PI period, the total precipitation simulated by iCESM1.2 is generally wetter than the PlioMIP2 multi-model ensemble mean (MME) except for the opposite performance between iCESM1.2 and PlioMIP2 MME over the regions [~30S°–10N]. A heavier δ18Op during the MPWP mainly occurred in the tropical Indian ocean and surrounding Asian-African-Australian monsoon regions. There are contrasting changes in the tropical meridional and zonal atmospheric circulations (Hadley and Walker cells) during the MPWP. A weakened and expanded Hadley circulation (toward the poles) led to a reduction in tropical precipitation [~30S°–10N] and a poleward shift of the edge of the northern subtropical arid zone. In contrast, the tropical zonal atmospheric overturning circulation (Walker cell) and the global ocean meridional overturning circulation (MOC) are generally enhanced during the MPWP compared to the PI period.

Reproducing the Mid-Piacenzian Warm Period Climate in the 2020s.

“…Salzmann et al (2011) note the MPWP was not only warmer than present-day conditions, but wetter, and that temperate and boreal vegetation zones were shifted northwards from their present positions. Sun et al (2023) concur, finding considerable warming in the higher latitudes, a wetter climate, but less warming in the tropics.…”

Reproducing the Mid-Piacenzian Warm Period Climate in the 2020s.

“…Salzmann et al (2011) note the MPWP was not only warmer than present-day conditions, but wetter, and that temperate and boreal vegetation zones were shifted northwards from their present positions. Sun et al (2023) concur, finding considerable warming in the higher latitudes, a wetter climate, but less warming in the tropics.…”

“…It is argued that the MPWP was an equilibrium climate, and so is not, or not strictly, comparable to today's climate, which is a transient one (Sun et al, 2023). The parallel is between equilibrium climate sensitivity (ECS), which is the change in surface atmospheric temperature (SAT) in response 3 to a doubling of CO 2 , and transient climate response (TCR).…”