Orbital Influences on Conditions Favorable for Glacial Inception

Glacial cycles are driven by cyclical changes in Earth's orbital parameters: obliquity, precession, and eccentricity. A common interpretation of Milankovitch's orbital theory suggests high-latitude June insolation is the dominant forcing in driving glacial cycles, which places emphasis on the importance of precession. However, there is abundant evidence for the importance of obliquity. Here, we use linear reconstructions based on single-forcing climate model simulations to produce time series of climate variables important in the growth and decay of ice sheets to examine the relative influences of obliquity and precession in locations important to glacial inception. We show that obliquity can cause changes in positive degree-days (sum of mean daily temperatures above 0°C) of larger magnitude than those of precession for Scandinavia and Baffin Island. Our positive degree-day time series were best represented by Milankovitch's caloric summer insolation metric. Snowfall is dominated by obliquity in Scandinavia and by precession in Baffin Island.