A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate between glacial periods. The last glacial period ended about 15,000 years ago.<ref name="Severinghaus1999"> The Holocene epoch is the current interglacial. A time when there are no glaciers on Earth is considered a greenhouse climate state.