Given that the innards of these ice sheets are complicated and inaccessible, researchers rely heavily on what we can discover about their pasts. One key point in the past has been the warm interglacial period around 400,000 years ago. The cycles in Earth’s orbit that govern the timing of the ice ages conspired to produce an exceptionally long respite from the cold at this time—twice as long or more than the most recent interglacial period about 120,000 years ago. It may have been warmer, as well, and some estimates put sea level in the vicinity of six to 13 meters higher than it is today.
Probably caused by all the human-caused carbon burning extant 400,000 years ago.