Abstract
Abstract. We compare results obtained from modeling the mid-Pliocene warm period using the Community Earth System Models (COSMOS, version: COSMOS-landveg r2413, 2009) with the two different modeling methodologies and sets of boundary conditions prescribed for the two phases of the Pliocene Model Intercomparison Project (PlioMIP), tagged PlioMIP1 and PlioMIP2. Here, we bridge the gap between our contributions to PlioMIP1 (Stepanek and Lohmann, 2012) and PlioMIP2 (Stepanek et al., 2020). We highlight some of the effects that differences in the chosen mid-Pliocene model setup (PlioMIP2 vs. PlioMIP1) have on the climate state as derived with COSMOS, as this information will be valuable in the framework of the model–model and model–data comparison within PlioMIP2. We evaluate the model sensitivity to improved mid-Pliocene boundary conditions using PlioMIP's core mid-Pliocene experiments for PlioMIP1 and PlioMIP2 and present further simulations in which we test model sensitivity to variations in paleogeography, orbit, and the concentration of CO2. Firstly, we highlight major changes in boundary conditions from PlioMIP1 to PlioMIP2 and also the challenges recorded from the initial effort. The results derived from our simulations show that COSMOS simulates a mid-Pliocene climate state that is 0.29 ∘C colder in PlioMIP2 if compared to PlioMIP1 (17.82 ∘C in PlioMIP1, 17.53 ∘C in PlioMIP2; values based on simulated surface skin temperature). On the one hand, high-latitude warming, which is supported by proxy evidence of the mid-Pliocene, is underestimated in simulations of both PlioMIP1 and PlioMIP2. On the other hand, spatial variations in surface air temperature (SAT), sea surface temperature (SST), and the distribution of sea ice suggest improvement of simulated SAT and SST in PlioMIP2 if employing the updated paleogeography. Our PlioMIP2 mid-Pliocene simulation produces warmer SSTs in the Arctic and North Atlantic Ocean than those derived from the respective PlioMIP1 climate state. The difference in prescribed CO2 accounts for 0.5 ∘C of temperature difference in the Arctic, leading to an ice-free summer in the PlioMIP1 simulation, and a quasi ice-free summer in PlioMIP2. Beyond the official set of PlioMIP2 simulations, we present further simulations and analyses that sample the phase space of potential alternative orbital forcings that have acted during the Pliocene and may have impacted geological records. Employing orbital forcing, which differs from that proposed for PlioMIP2 (i.e., corresponding to pre-industrial conditions) but falls into the mid-Pliocene time period targeted in PlioMIP, leads to pronounced annual and seasonal temperature variations. Our result identifies the changes in mid-Pliocene paleogeography from PRISM3 to PRISM4 as the major driver of the mid-Pliocene warmth within PlioMIP and not the minor differences in forcings.
Highlights
In 2050, the global population is expected to have increased by 2.7 billion relative to its 2005 value (Bongaarts, 2009)
We have tested to what degree changes between the Community Earth System Models (COSMOS) model setups of PlioMIP1 and PlioMIP2 are able to create different realizations of simulated mid-Pliocene warm period (mPWP) climate and how different these alternative mPWP climate states are from PlioMIP2 core simulations Eoi400 and PlioM1, both produced with the same climate model as employed in this study
We find that the global patterns of mPWP climate, as expressed in sea surface temperature (SST) and surface air temperature (SAT), are similar in PlioMIP2 and PlioMIP1
Summary
In 2050, the global population is expected to have increased by 2.7 billion relative to its 2005 value (Bongaarts, 2009). The mid-Pliocene warm period (mPWP) (3.264– 3.025 million years (Ma) before present (BP); Dowsett et al, 2016) has been suggested as a time slice which could provide possible insight into future climate in terms of temperature (Jansen et al, 2007). Evidence from the geologic record suggests that in the past the Arctic was more vegetated than today, e.g., during the mid-Pliocene warm period (Rybczynski et al, 2013) These records of the past offer us a glimpse into a climate of increased temperatures in polar regions that may return in the decades (Overland et al, 2014) as an effect of the human influence on climate. The mid-Pliocene can be considered a useful, but not direct, analog for future warmth (Jansen et al, 2007)
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