| 24 Feb 2026
Simulation of climate during the Miocene Climatic Optimum under different CO2 forcings
Abstract. The Miocene Climatic Optimum (MCO; 17 – 14 Ma), characterized by global mean surface temperatures ~7 °C higher than preindustrial (PI), offers a target for validating models for warmer-than-present climate states. Here, we use a water isotope tracer enabled version of the Community Earth System Model to simulate the MCO under 1x (MCO1x), 2x (MCO2x), and 4x (MCO4x) PI CO2. Our simulations show significant warming due to MCO boundary conditions as well as a small increase in equilibrium climate sensitivity with higher CO2. All simulations exhibit a decreased mean equator-to-pole temperature gradient relative to PI. However, the spatial patterns of warming are distinct between simulations with relatively greater high latitude warming between MCO1x and MCO2x and relatively greater low latitude warming between MCO2x and MCO4x. Warming is associated with enhanced precipitation and enriched precipitation δ¹⁸O (δ¹⁸Op) globally.
We compare the MCO model outputs with proxy of surface temperature, precipitation, and δ¹⁸Op. Like many other MCO modeling studies, our simulations underestimate the reduced latitudinal temperature gradient reconstructed with proxies. We find better model-proxy agreement for terrestrial and marine temperature records in the MCO1x and MCO4x experiments, respectively. Precipitation and δ¹⁸Op records show the best agreement with the MCO2x and MCO1x experiments, respectively, but there are large uncertainties due to limited proxy data and large reconstruction uncertainties. The MCO2x simulation is warmer than the projected 2080–2100 climate under RCP8.5, highlighting the importance of both boundary conditions and equilibrium versus transient climate system response to increased CO2.
Competing interests: I declare that neither I nor my co-authors have any competing interests.
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