We've been in an ice age for 3 million years were ice has covered much of the earth. There are glacial periods where ice extends furthur to the equator (what most people think is an ice age) and there are interglacial periods. We are currently in an interglacial period. During this ice age, the earth hos more often than not ben in glacial periods. The last interglacial period lasted only 12 thousand years. We are currently getting relatively close to that time frame.
Basically "what temperature the Earth is supposed to be" is many degrees colder than this over the past 3 million years.