Complex sentence: 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.
Split into simpler sentences: By this definition, we are in an interglacial period--the Holocene--of the ice age. The ice age began 2.6 million years ago at the start of the Pleistocene epoch, because the Greenland, Arctic, and Antarctic ice sheets still exist.