For half a century since Milankovich proposed the idea, climatologists have been intrigued by the possibility that cyclical convergences of variations in the earth's orbit and rotation have brought on ice ages. These variations include the planet's precession, the precession of the equinoxes, and the configuration of earth's orbit about the sun.
But the most sophisticated mathematical models incorporating these changes have not yielded the conditions for ice ages, according to National Center for Atmospheric Research (NCAR) investigator Tzvi Gal-Chen. This, even though the models include a feedback mechanism that would reproduce cumulative effects. Milankovich suggested that cooler summers in the northern hemisphere would produce less melting of snow and ice in the summer. Because snow and ice reflect sunshine well, their accumulation would lead to more cooling, creating a snowball effect and setting off an ice age.