Younger Dryas:'Milankovitch Variations'
Milankovitch variations are variations in sunlight. They are cyclical changes in Earth's orbit that change the seasonal distribution of sunlight falling on the Northern Hemisphere.
These cycles seemed to explain the advance and retreat of the northern ice sheets for the past 700,000 years or so with fits and starts lasting tens to hundreds of thousands of years. This theory could not however explain more rapid climate fluctuations, as those seen in the last ice age. The shortest fluctuation it has been able to explain is ~20,000 years in length.(Broecker, 1988)