The earth experiences an ice age when the overall climate becomes much cooler. during an ice age, there are several periods of glaciation, which are periods when glaciers and ice sheets cover more of the earth's surface. ice reflects more sunlight back into outer space than does bare or vegetated ground. because ice sheets and glaciers are more reflective, they absorb less sunlight and therefore release heat. this causes the earth's climate to become during periods of glaciation.

The given blank can be filled with less and even cooler.

A long duration of decline in the temperature of the surface of the Earth and atmosphere, leading to the expansion or existence of the polar and continental ice sheets and alpine glaciers is known as an ice age.  

During an ice age, there are many periods of glaciation that are periods when ice sheets and glaciers cover the majority of the Earth's surface.  

The ice reflects more sunlight back into outer space than does the vegetated or bare ground. As the glaciers and ice sheets are more reflective, they captivate less sunlight and thus discharge less heat. This makes the climate of the Earth to become even cooler during the times of glaciation.