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Past climate states hold valuable insights into future climate change. Among those states, mid-Pliocene (3.3 - 3.0 Ma) is often studied as an important analog to near future climate change following an intermediate warming pathway. This time interval featured topography and geography like present-day, yet with retreated polar ice sheets and expanded boreal forests, potentially reflecting equilibrium earth system responses to CO2 forcing at a centennial to millennial time scale. Despite the prolific research on Pliocene climate, little is known about the amount of radiative forcing, especially from changing boundary conditions, that drives the Pliocene climate. Existing constraints mainly focused on well-mixed greenhouse gases and aerosols. Here, we applied the methodology commonly used to quantify radiative forcing of future climate and its sources to constrain radiative forcing of the mid-Pliocene climate using three generations of Community Earth System Models (CCSM4, CESM1.2, and CESM2). To calculate ERF, the difference in net top of the atmosphere radiative fluxes is computed between a pre-industrial control and a mid-Pliocene simulation. Both are carried out with prescribed pre-industrial sea surface temperature. The three mid-Pliocene simulations separately feature a 400 ppm CO2 (the level of mid-Pliocene), mid-Pliocene geography and topography, and mid-Pliocene ice and vegetation. Changing atmospheric temperature, water vapor, surface albedo, and clear vs total sky radiative fluxes are further extracted from these simulations to calculate radiative adjustments with published radiative kernels for CESM. In our preliminary results with CESM1.2, we found that ERF is 1.754 W m-2 for CO2 forcings, 1.143 W m-2 for vegetation and ice sheet forcing, and -0.339 W m-2 for geographic and topographic forcing. Further, ERF from boundary condition changes mostly arises from changing surface albedo with 1.626 W m-2 for vegetation and ice sheet changes and –0.54 W m-2 for geographic and topographic changes respectively. Radiative adjustments from water vapor responses tend to amplify the instantaneous forcing with the most profound effect induced by vegetation and ice sheet changes. These results underscore the importance of constraining radiative forcing from changes in boundary conditions, which is potentially key to understanding drivers of past climate warmth and inter-model spread in simulated past climate states. 

Equilibrium climate sensitivity (ECS) quantifies the amount of warming resulting from a doubling of the atmospheric CO2 forcing. Despite recent advancements in climate simulation capabilities and global observations, there remains large uncertainty on the degree of future warming. To help alleviate this uncertainty, past climates provide a valuable insight into how the Earth will respond to elevated atmospheric CO2. However, there is evidence to suggest that ECS is dependent on background climate warmth, which may interfere with the direct utilization of paleo-ECS to understand present-day ECS. Thus, it is important that a range of different climate states are considered to better understand the factors modulating the relationship between CO2 and temperature. In this study, we focus on three time intervals: the mid-Pliocene Warm Period (3.3 – 3.0 Ma), the mid-Miocene (16.75 – 14.5 Ma), and the early Eocene (~50 Ma), in order to sample ECS from Cenozoic coolhouse to hothouse climates. Here, we combine the Bayesian framework of constraining the ECS and its uncertainty with several published methods to estimate the global mean surface temperature (GMST) from sparse proxy records. This framework utilizes an emergent constraint between the simulated GMST changes and climate sensitivities across the model ensemble. For each time interval, we employ a combination of parametric and non-parametric functions, coupled with a probabilistic approach to derive a refined estimate. Preliminary results for the Pliocene indicate a GMST reconstruction of approximately 19.3°C, which is higher than previous estimates that were derived using only marine records. Using this estimate, we calculate an ECS that is also higher than previously published values, especially due to the inclusion of high-latitude terrestrial temperature records into our estimates. Intriguingly, using the consistent methodology, our calculated ECS for the early Eocene is lower than that of the mid-Pliocene. This result does not support an amplified ECS in hothouse climate, and points to a potentially important role of ice albedo feedback in amplifying the ECS in coolhouse climate. Ongoing work will apply the same methodology to the mid-Miocene and further investigate the source for the estimated ECS state dependency between these climate intervals. 

Abstract. The mid-Pliocene Warm Period (mPWP, 3.3–3.0 Ma) was characterised by an atmospheric CO2 concentration exceeding 400 ppmv with minor changes in continental and orbital configurations. Simulations of this past climate state have improved with newer models but still show some substantial differences from proxy reconstructions. There is little information about atmospheric aerosol concentrations during the Pliocene, but previous work suggests that it could have been quite different from the modern period. Here we apply idealised aerosol scenario experiments to examine the importance of aerosol forcing on mPWP tropical precipitation and the possibility of aerosol uncertainty explaining the mismatch between reconstructions and simulations. The absence of industrial pollutants leads to further warming, especially in the Northern Hemisphere. The Intertropical Convergence Zone (ITCZ) becomes narrower and stronger and shifts northward after removal of anthropogenic aerosols. Though not affecting the location of monsoon domain boundary, removal of anthropogenic aerosol alters the amount of rainfall within the domain, increasing summer rain rate over eastern and southern Asia and western Africa. This work demonstrates that uncertainty in aerosol forcing could be the dominant driver in tropical precipitation changes during the mid-Pliocene: causing larger impacts than the changes in topography and greenhouse gases.

 

Abstract. During the mid-Pliocene warm period (mPWP; 3.264–3.025 Ma), atmospheric CO2 concentrations were approximately 400 ppm, and the Antarctic Ice Sheet was substantially reduced compared to today. Antarctica is surrounded by the Southern Ocean, which plays a crucial role in the global oceanic circulation and climate regulation. Using results from the Pliocene Model Intercomparison Project (PlioMIP2), we investigate Southern Ocean conditions during the mPWP with respect to the pre-industrial period. We find that the mean sea surface temperature (SST) warming in the Southern Ocean is 2.8 °C, while global mean SST warming is 2.4 °C. The enhanced warming is strongly tied to a dramatic decrease in sea ice cover over the mPWP Southern Ocean. We also see a freshening of the ocean (sub)surface, driven by an increase in precipitation over the Southern Ocean and Antarctica. The warmer and fresher surface leads to a highly stratified Southern Ocean that can be related to weakening of the deep abyssal overturning circulation. Sensitivity simulations show that the decrease in sea ice cover and enhanced warming is largely a consequence of the reduction in the Antarctic Ice Sheet. In addition, the mPWP geographic boundary conditions are responsible for approximately half of the increase in mPWP SST warming, sea ice loss, precipitation, and stratification increase over the Southern Ocean. From these results, we conclude that a strongly reduced Antarctic Ice Sheet during the mPWP has a substantial influence on the state of the Southern Ocean and exacerbates the changes that are induced by a higher CO2 concentration alone. This is relevant for the long-term future of the Southern Ocean, as we expect melting of the western Antarctic Ice Sheet in the future, an effect that is not currently taken into account in future projections by Coupled Model Intercomparison Project (CMIP) ensembles.