The big questions are, of course, what caused those glaciers to spread, and will it happen again? Actually, no one is yet completely sure. But an intriguing idea, due to work in the 1930s by the Serbian astronomer Milutin Milankovich, may link them to the precession which Hipparchus discovered.
As already noted, the Earth's orbit is not perfectly round, but is slightly elongated. The Earth therefore comes closest to the Sun in the first week of January (the exact day varies a little). It means that just when the northern hemisphere experiences winter and receives the least amount of sunlight, the Earth as a whole receives the most (the swing is about 3%, peak to peak). This makes northern winters milder, and northern summers are milder too, since they occur when the Earth is most distant from the Sun.
The opposite is true south of the equator: the beginning of January occurs there in summer, and therefore one expects southern summers to be hotter, and southern winters colder, than those north of the equator. This effect is however greatly weakened, because by far most of the southern hemisphere is covered by ocean, and the water tempers and moderates the climate.
Right now, northern winter occurs in the part of the Earth's orbit where the north end of the axis points away from the Sun. However, since the axis moves around a cone, 13,000 years from now, in this part of the orbit, it will point towards the Sun, putting it in mid-summer just when the Earth is closest to the Sun.
At that time one expects northern climate to be more extreme, and the oceans then have a much smaller effect, since the proportion of land in the northern hemisphere is much larger. Milankovich argued that because winters were colder, more snow fell, feeding the giant glaciers. Furthermore, he said, since snow was white, it reflected sunlight, and with more severe winters, the snow-covered land warmed up less effectively once winter had ended. Climate is maintained by a delicate balance between opposing factors, and Milankovich argued that this effect alone was enough to upset that balance and cause ice ages.
Milankovich was aware that this was just one of several factors, since it turns out that ice ages do not recur every 26,000 year, nor do they seem common in other geological epochs. The eccentricity of the Earth's orbit, which determines the closest approach to the Sun, also changes periodically, as does the inclination of the Earth's axis to the ecliptic. But overall the notion that ice ages may be linked to the motion of the Earth through space may be currently our best guess concerning the causes of ice ages.
Postscript, 28 July 1999. The magnitude of the “Milankovich effect" depends on the difference between largest and smallest distances from the Sun. That, in its turn, depends on the eccentricity of the Earth's orbit, which varies with a 100,000-year cycle, on which a 413,000-year cycle is superposed. J. Rial (Univ. of North Carolina) found signatures of those cycles in the oxygen isotope content of deep-sea sediments, in full agreement with the Milankovich theory. His work is in Science, vol. 285, p. 564, 23 July 1999; a non-technical explanation “Why the Ice Ages Don't Keep Time " is on pages 503-504 of the same issue.
The sea-bottom results have now been compared to hydrogen isotope ratios in deep boreholes in the ice sheets of Antarctica, which took nearly a million years to accumulate (Science, 11 June 2004, p. 1609). Deep-sea sediments show that in the last million years, but not before, the variation is dominated by a periodicity around 100,000 years. Its origin, the article states, “is one of the unanswered, yet fundamental questions." Ice cores could help explain it.