Paleo Slide Set: The Ice Ages
Earth's axial tilt, adapted from Pisias and Imbrie [1986/1987]

Croll was aware that the obliquity of earth's axis varied through time, but Leverrier's 
astronomical calculations only allowed Croll to include eccentricity and precession in his theory. 
Milankovitch, on the other hand, benefited from innovations in astronomy that made it possible 
to incorporate changes in tilt into his calculations. Earth's axial tilt varies from 24.5 degrees 
to 22.1 degrees over the course of a 41,000-year cycle. Changes in axial tilt affect the distribution 
of solar radiation received at the earth's surface. When the angle of tilt is low, polar regions 
receive less insolation. When the tilt is greater, the polar regions receive more insolation during 
the course of a year. Like precession and eccentricity, changes in tilt thus influence the relative 
strength of the seasons, but the effects of the tilt cycle are particularly pronounced in the high 
latitudes where the great ice ages began.

With Pilgrim's new calculations as his guide, Milankovitch embarked on an exhaustive series of 
calculations. Without a computer or even a calculator, the task was arduous indeed. While the 
calculations were complex, the reasoning behind them was quite simple. Croll had argued that winter 
insolation was the key factor in understanding the ice ages, but Milankovitch thought that summer 
insolation was more important. During periods of lower summer temperatures, he reasoned, less of 
the previous winter's snow would melt. Glaciation would soon begin after the snows of several 
winters piled up. Milankovitch set out to determine how variations in precession, eccentricity, 
and obliquity affected the amount of solar radiation received during the summer at particular latitudes.

Photo Credits:
Thomas G. Andrews
NOAA Paleoclimatology Program