ice age

noun

Etymology: Perhaps a calque of German Eiszeit.

1. A period of long-term reduction in the temperature of Earth's surface and atmosphere, resulting in the presence of major polar ice sheets that reach the ocean and calve icebergs.

   “The earth is currently in the Late Cenozoic Ice Age, an ice age that began 33.9 million years ago with the Antarctic Ice Cap.”

2. Any of several glacial periods (within a scientific ice age) marked by episodes of extensive glaciation alternating with episodes of relative warmth (interglacial).

   “The cause of ice ages, Köppen decided, is to be found in cool summers, not brutal winters.”