Potential role of methane and other non-CO2 trace gases in past warm climates

Past warm climates allow the evaluation model predictions of the response of the Earth System to elevated greenhouse gas levels. However, Earth System model simulations routinely underestimate high-latitude warmth for past states, meaning that the forcings provided to models, the models themselves or the climate reconstructions are in error. We focus on the first of these and review the potential role of varying levels of atmospheric trace gases besides carbon dioxide (non-CO₂ trace gases), which has been investigated in relatively few studies to date. Using a combination of terrestrial biogeochemistry models and simplified atmospheric chemistry scheme we make first-order estimates of the radiative forcing by non-CO₂ trace gases for the mid-Pliocene and compare these with new results for the Miocene. We will discuss the main uncertainties involved and review potential avenues for future research.