Resolving Milankovitch: Consideration of signal and noise

Abstract

Milankovitch-climate theory provides a fundamental framework for the study of ancient climates. Although the identification and quantification of orbital rhythms are commonplace in paleoclimate research, criticisms have been advanced that dispute the importance of an astronomical climate driver. If these criticisms are valid, major revisions in our understanding of the climate system and past climates are required. Resolution of this issue is hindered by numerous factors that challenge accurate quantification of orbital cyclicity in paleoclimate archives. In this study, we delineate sources of noise that distort the primary orbital signal in proxy climate records, and utilize this template in tandem with advanced spectral methods to quantify Milankovitch-forced/paced climate variability in a temperature proxy record from the Vostok ice core (Vimeux and others, 2002). Our analysis indicates that Vostok temperature variance is almost equally apportioned between three components: the precession and obliquity periods (28%), a periodic "100,000" year cycle (41%), and the background continuum (31%). A range of analyses accounting for various frequency bands of interest, and potential bias introduced by the "saw-tooth" shape of the glacial/interglacial cycle, establish that precession and obliquity periods account for between 25 percent to 41 percent of the variance in the 1/10 kyr - 1/100 kyr band, and between 39 percent to 66 percent of the variance in the 1/10 kyr - 1/64 kyr band. These results are approximately two to four times greater than those published by Wunsch (2004) for the same Vostok time series. In all cases, most of the remaining variance is accounted for by the "100,000" year cycle, which is distinct from a background continuum that resembles autoregressive "red noise." Our analysis high-lights the importance of a comprehensive assessment of the climate signals in geologic records, and underscores the significance of orbital forcing/pacing as a primary agent of Pleistocene climate change.