<P> The Milankovitch cycles are a set of cyclic variations in characteristics of the Earth's orbit around the Sun . Each cycle has a different length, so at some times their effects reinforce each other and at other times they (partially) cancel each other . </P> <P> There is strong evidence that the Milankovitch cycles affect the occurrence of glacial and interglacial periods within an ice age . The present ice age is the most studied and best understood, particularly the last 400,000 years, since this is the period covered by ice cores that record atmospheric composition and proxies for temperature and ice volume . Within this period, the match of glacial / interglacial frequencies to the Milanković orbital forcing periods is so close that orbital forcing is generally accepted . The combined effects of the changing distance to the Sun, the precession of the Earth's axis, and the changing tilt of the Earth's axis redistribute the sunlight received by the Earth . Of particular importance are changes in the tilt of the Earth's axis, which affect the intensity of seasons . For example, the amount of solar influx in July at 65 degrees north latitude varies by as much as 22% (from 450 W / m2 to 550 W / m2). It is widely believed that ice sheets advance when summers become too cool to melt all of the accumulated snowfall from the previous winter . Some believe that the strength of the orbital forcing is too small to trigger glaciations, but feedback mechanisms like CO may explain this mismatch . </P> <P> While Milankovitch forcing predicts that cyclic changes in the Earth's orbital elements can be expressed in the glaciation record, additional explanations are necessary to explain which cycles are observed to be most important in the timing of glacial--interglacial periods . In particular, during the last 800,000 years, the dominant period of glacial--interglacial oscillation has been 100,000 years, which corresponds to changes in Earth's orbital eccentricity and orbital inclination . Yet this is by far the weakest of the three frequencies predicted by Milankovitch . During the period 3.0--0.8 million years ago, the dominant pattern of glaciation corresponded to the 41,000 - year period of changes in Earth's obliquity (tilt of the axis). The reasons for dominance of one frequency versus another are poorly understood and an active area of current research, but the answer probably relates to some form of resonance in the Earth's climate system . Recent work suggests that the 100K year cycle dominates due to increased southern - pole sea - ice increasing total solar reflectivity . </P> <P> The "traditional" Milankovitch explanation struggles to explain the dominance of the 100,000 - year cycle over the last 8 cycles . Richard A. Muller, Gordon J.F. MacDonald, and others have pointed out that those calculations are for a two - dimensional orbit of Earth but the three - dimensional orbit also has a 100,000 - year cycle of orbital inclination . They proposed that these variations in orbital inclination lead to variations in insolation, as the Earth moves in and out of known dust bands in the solar system . Although this is a different mechanism to the traditional view, the "predicted" periods over the last 400,000 years are nearly the same . The Muller and MacDonald theory, in turn, has been challenged by Jose Antonio Rial . </P>

When was the last glacial maximum (commonly known as the ice age )