http://www.eduplace.com/kids/hmsc/6/d/cricket/ckt_6d.shtmlWhat Really Causes the Ice Ages?
by Jeanne Miller
The last million years on Earth have been one long ice age, interrupted regularly by interglacials, or brief periods of warmth. The warm spells have usually lasted between 10,000 and 20,000 years. We're in one now that began about 12,000 years ago. So
any millennium now the temperature will drop, glaciers will grow, and ice sheets thousands of kilometers thick will advance on the continents, devouring a large fraction of the land on the planet.For most of the last million years these cycles of glaciation have occurred worldwide every 100,000 years—80,000 to 90,000 years of ice followed by 10,000 to 20,000 years of warmth. Why are they so regular?
In the mid-1800s, when the existence of ice ages was discovered, there was no way to know the cause or frequency of these climate changes. James Croll, a self-taught Scottish scientist, suggested that variations in the Earth's orbit around the Sun produced the effect. In the 1920s, Milutin Milankovitch, a Serbian engineer, developed Croll's hypothesis further.