Earth System Models (ESMs) are advanced numerical tools that enable researchers to evaluate responses in the Earth’s system to deliberate changes in climate forcings and boundary conditions, like greenhouse gas concentration, orbit, and continental configuration. Through simulations of past warm climates, we can measure shifts in temperature, the hydrological cycle, or the carbon cycle that may offer insights into modern warming trajectories and model biases. This dissertation examines changes in atmospheric and marine processes in response to warming across novel simulations of past climates with environmental proxy data to further understand the Earth system under elevated CO2 levels.

In Chapter 2, we investigate the orbital sensitivity of Earth’s hydrological cycle under different CO2 background states within an Early Eocene setting by conducting water isotope-enabled Community Earth System Model (CESM) simulations at 3x and 6x pre-industrial (PI) CO2 levels. This chapter suggests that seasonal hydrological responses to orbital changes are greater than CO2-driven changes in several regions – with these orbital differences more pronounced in lower CO2 climates – and, therefore, the orbit in place during proxy archive formation can provide critical context for interpreting oxygen isotopic signals.

In Chapter 3, we explore equilibrium climate sensitivity (ECS) across CESM slab ocean model (SOM) simulations of four distinct past climate intervals: the late Cretaceous (~90 Ma), the early Eocene (~55 Ma), the late Oligocene (~25 Ma), and PI. We analyze the contributions of model boundary conditions, like CO2 state, geography, and ocean heat transport (OHT), to ECS differences and decompose the total climate feedback parameter in order to provide new constraints on ECS sensitivity and variability through Earth’s history.

In Chapter 4, we simulate the Miocene Climatic Optimum (MCO) at pre-MCO 280 ppm CO2 and MCO 560 ppm CO2 levels using CESM coupled with the Marine Biogeochemistry Library (MARBL) to study phytoplankton community structure and marine primary productivity shifts during this recent warm period. We find that an elevated CO2 level leads to surface warming, sea ice melt, weakened overturning, inhibited upwelling, and changes in nutrient distribution. These consequences result in poleward migration and increased productivity by small phytoplankton and decreased productivity by larger diatoms.

Together, the results of this dissertation demonstrate the value of advanced ESM simulations, particularly when analyzed alongside empirical data, in determining potential hydrological, radiative, or biogeochemical alterations in response to elevated atmospheric CO2 concentrations throughout Earth’s history.