Mending Milankovitch's theory: obliquity amplification by surface feedbacks

C R Tabor,D Pollard,C J Poulsen

doi:10.5194/cp-10-41-2014

C R Tabor, D Pollard + Show 1 more

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https://doi.org/10.5194/cp-10-41-2014

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Abstract

Abstract. Milankovitch's theory states that orbitally induced changes in high-latitude summer insolation dictate the waxing and waning of ice sheets. Accordingly, precession should dominate the ice-volume response because it most strongly modulates summer insolation. However, early Pleistocene (2.588–0.781 Ma) ice-volume proxy records vary almost exclusively at the frequency of the obliquity cycle. To explore this paradox, we use an Earth system model coupled with a dynamic ice sheet to separate the climate responses to idealized transient orbits of obliquity and precession that maximize insolation changes. Our results show that positive surface albedo feedbacks between high-latitude annual-mean insolation, ocean heat flux and sea-ice coverage, and boreal forest/tundra exchange enhance the ice-volume response to obliquity forcing relative to precession forcing. These surface feedbacks, in combination with modulation of the precession cycle power by eccentricity, help explain the dominantly 41 kyr cycles in global ice volume of the early Pleistocene.

Highlights

Paleoclimate proxy records often display variations on timescales of 104–106 yr
Milankovitch cycles are thought to be responsible for the growth and retreat of the large Northern Hemisphere (NH) ice sheets that characterize the Pleistocene through the influence of Earth’s three orbital/rotational parameters on highlatitude summer insolation
Our results demonstrate that internal climate feedbacks not considered in Milankovitch’s theory help explain the relatively strong obliquity signal observed in the early Pleistocene δ18O sediment records

Summary

Introduction

Paleoclimate proxy records often display variations on timescales of 104–106 yr These climate variations, known as Milankovitch cycles, are quasi-cyclic. They are attributed to the direct and combined effects of changes in Earth’s degree of axial tilt (obliquity), direction of axial tilt (precession), and circularity of orbit (eccentricity) (Hays et al, 1976). According to Milankovitch’s theory, times of high (low) summer insolation produce high (low) rates of summer melting, leading to NH ice-sheet retreat (growth). This theory is the most widely accepted explanation for the strong correlation between ice-volume proxy records and orbital variations (Hays et al, 1976)

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