New Insolation Forcing for Paleoclimate Studies

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 timeslice 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 However, especially in transient simulations (e.g., of the past 3 Myr), orbital parameters from outdated astronomical solutions (e.g., those labeled Ber78 and BL91) are still used, potentially due to their integration in widely-used models such as the Community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, e.g., those targeting the Eocene, when the eccentricity is incompatible and the obliquity amplitude is too large compared to modern solutions. Furthermore, before −40 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 In this study, we show that outdated astronomical solutions yield incorrect top-of-atmosphere insolation.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 Wm −2 at −934 kyr and −51 Wm −2 at 2.1 Myr)—of the same order of magnitude as the orbital forcing itself. To facilitate the use of updated astronomical solutions in paleoclimate simulations, we provide ZB18a(1,1) and ZB20a(1,1) along with Fortran subroutines that interpolate the astronomical parameters and compute insolation. We also provide drop-in replacements for CESM routines, hoping that proper insolation routines be used in future studies.