New Insolation Forcing for Paleoclimate Studies

Abstract Insolation forcing at the top of the atmosphere is widely used in paleoceanographic and paleoclimatological studies. In paleoclimate simulations, it must reflect Earth’s orbital history. General Circulation Models (GCMs) typically use a modern orbital configuration to allow direct comparison to modern and near‐future scenarios, often with time slice orbital experiments (DeepMIP, PlioMIP, etc.). However, especially in transient simulations (e.g., of the past 3 Myr), orbital parameters from outdated astronomical solutions (e.g., those labeled Ber78 and BL91 ) are still used, potentially due to their integration in widely used models such as the Community Earth System Model (CESM). These routines are particularly inappropriate for deep‐time simulations, for example, those targeting the Eocene, when the eccentricity is incompatible and the obliquity amplitude is too large compared to modern solutions. Furthermore, prior to Myr, the frequently used La04 solution should not be used. Our orbital solutions, ZB18a and ZB20a , best match geologic data to 58 and 71 Ma, respectively. In this study, we show that outdated astronomical solutions yield incorrect top‐of‐atmosphere insolation. The difference between insolation from Ber78 and our solution ZB18a (1,1) increases periodically with increasing age, reaching significant offsets (up to 24 at kyr and at 2.1 Myr)—of the same order of magnitude as the orbital forcing itself. To facilitate the use of updated astronomical solutions (AS) in paleoclimate simulations, we provide ZB18a (1,1) and ZB20a (1,1) and Fortran subroutines that interpolate the astronomical parameters and compute insolation. We provide drop‐in replacements for existing CESM routines, hoping that proper AS inputs be used in future studies.