Snowball Earth, if it existed, ended many millions of years ago. No one suggests that the world could freeze over again to that extent, but most scientists agree that there will be more ice ages.
When Louis Agassiz proposed it, the Great Ice Age was believed to have been a single event that ended once and for all; it was confined to the Pleistocene epoch, and we now live under quite different conditions, in the Holocene. Then, as geologists unraveled more of the Earth's recent history, it became evident that there have been many ice ages. They have not come to an end, and the Holocene is no more than an interglacial.
Our present interglacial has already lasted for about 10,000 years. This is the average duration for an interglacial, so we should expect the next ice age to commence sometime soon—"soon" being within the next few thousand years.
The present eccentricity of Earth's orbit favors an interglacial, however, and the onset of ice ages seems to be linked to the 100,000-year orbital cycle (see the section "Milutin Milankovitch and his astronomical cycles" on pages 52-60). On the other hand, the obliquity of the Earth's axial tilt is decreasing, and the Tropics—the latitude in each hemisphere below which the Sun is directly overhead at noon on one day in the year— are retreating toward the equator at about 0.07 inch (1.7 mm) an hour. This change favors an ice age, as does the present timing of the equinoxes, because Earth presently reaches aphelion in July. A July aphelion depresses summer temperatures over the vast continents of the Northern Hemisphere, restricting the summer melting of snow.
Despite present concerns about a warming of the global climate, the fairly certain long-term prediction is of a return of the ice sheets. No one knows when they will start advancing, but there is likely to be a new ice age within the next few thousand years—and it could begin much sooner than that.
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