Simplify the following text for a Grade 7 native English-speaking person.
Original: An `` ice age '' or , more precisely , `` glacial age '' is a generic geological period of long-term reduction in the temperature of the Earth 's surface and atmosphere , resulting in the presence or expansion of continental ice sheets , polar ice sheets and alpine glaciers .
Simplified: An ice age is a period when for a long time the temperature of Earth 's climate is very low . This leads to an expansion of the continental ice sheets , polar ice sheets and mountain glaciers .