New Insolation Forcing for Paleoclimate Models

In paleoclimate simulations, the insolation forcing at the top of the atmosphere needs to be altered to reflect Earth's orbital history during the time interval of interest. General Circulation Models (GCMs) often rely on either a modern orbital configuration to allow for direct comparison to modern and near-future climate simulations—sometimes with snapshot sensitivity experiments (DeepMIP, PlioMIP, etc.)—or they use the Berger (1978) (Ber78) routines to compute the orbital parameters. For snapshot simulations targeting older time periods such as the Eocene, using the modern orbital configuration is inappropriate, because the obliquity amplitude, for example, was much smaller than in the recent past. Our astronomical solutions ZB18a and ZB20a have been shown to produce the best match with geologic data to 58 Ma and 71 Ma, respectively (Zeebe & Lourens 2019, Kocken & Zeebe 2024). The eccentricity of the Ber78 solution diverges from these astronomical solutions already at ~33 ka, shows a different amplitude throughout, and drifts out of phase at ~1.6 Ma. It has been noted in the literature as well as code that the Ber78 routines are not appropriate for an analysis of time periods older than ~1 Myr. However, even recent transient simulations of the past 3 Myr sometimes fall back to using these outdated routines for the full time period. This is likely because of their ease of use; for example, the Ber78 routines are well-integrated into the Community Earth System Model (CESM).

In this study, we analyze the effects of using recent astronomical solutions on the insolation at the top of the atmosphere. Here we show that the absolute difference between insolation from Ber78 and our solution ZB18a increases periodically with increasing age, reaching values up to 88 Wm⁻² at 2.68 Ma. This difference is of the same order of magnitude as the difference between a precession minimum and maximum. To facilitate using recent astronomical solutions in GCMs such as the CESM, we make the ZB18a and ZB20a orbital solutions readily available: We provide Fortran subroutines that calculate insolation and interpolate the astronomical parameters to a certain calendar date, and provide drop-in replacements to existing Fortran subroutines from the CESM. In this presentation we will show several examples of previous studies that could have benefited from these new routines.

Berger, A. (1978). Long-term variations of caloric insolation resulting from the earth’s orbital elements, Quaternary Research, 9, 139–167. https://doi.org/10.1016/0033-5894(78)90064-9

Zeebe, R. E., & Lourens, L. J. (2019). Solar System chaos and the Paleocene–Eocene boundary age constrained by geology and astronomy. Science, 365(6456), 926–929. https://doi.org/10.1126/science.aax0612

Kocken, I.J., & Zeebe, R. E. (2024). Testing Astronomical Solutions With Geological Data for the Latest Cretaceous: An Astronomically Tuned Time Scale. Paleoceanography and Paleoclimatology, 39(11). https://doi.org/10.1029/2024PA004954