An
ice age is a period of long-term reduction in the temperature of
Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar
ice sheets and alpine
glaciers. Within a long-term ice age, individual pulses of cold climate are termed "
glacial periods" (or alternatively "glacials" or "glaciations" or colloquially as "ice age"), and intermittent warm periods are called "
interglacials".
Glaciologically,
ice age implies the presence of extensive
ice sheets in the northern and southern hemispheres. By this definition, we are in an interglacial period—the
Holocene—of the ice age that began 2.6 million years ago at the start of the
Pleistocene epoch, because the
Greenland,
Arctic, and
Antarctic ice sheets still exist.