The Largely Linear Response of Earth’s Ice Volume to Orbital Forcing

 

We investigate the effect of Earth’s orbitally governed incoming solar radiation on global ice volume over the past 800,000 years. It is well established that the orbital parameters play some role in the pacing of the glacial-interglacial cycles. However, due to limited data and enigmatic dynamics, the mechanics that could facilitate this relationship remain unresolved. We therefore consider a simple linear model of ice volume that imposes minimal assumptions about its dynamics. This model adequately reproduces the ice volume data for most of the 800,000 year period, with the exception of Marine Isotope Stage 11. This suggests that, aside from a few extrema, the ice volume dynamics primarily result from an approximately linear response to orbital forcing. We substantiate this finding by addressing some of the key criticisms of the orbitally forced hypothesis. In particular, we show that eccentricity can significantly vary the ocean temperature without the need for amplification on Earth. We also present a feasible mechanism to explain the absence of eccentricity’s 400,000 year period in the ice volume data. This requires part of the forcing from eccentricity to be lagged via a slow-reacting mechanism, resulting in a signal that closer approximates the change in eccentricity. A physical interpretation of our model is proposed, using bulk ocean and surface temperatures as intermediate mechanisms through which the orbital parameters affect ice volume. These show reasonable alignment with their relevant proxy data, though we acknowledge that these variables likely represent a combination of mechanisms.