Originally posted by @sonhouse
... CO2 was high back then and temperatures were 3 or 4 degrees C higher, enough to melt all the ice.
As the link explains, it takes a while for the ocean and atmosphere to respond to a rapid rise in carbon dioxide thus "the Arctic today is not in equilibrium with the greenhouse gas settings, meaning that we have to expect significant increases in [Arctic] temperature and precipitation even if the level of greenhouse gases keeps constant,"
The more up-to-date scientific analysis indicates CO2 levels were (for the early and middle Pliocene but not for the late Pliocene when the climate was colder than what it was now) probably higher back then than now.
This isn't explained in this above link but previous estimates of CO2 levels back them where initially based on relatively pure quality disturbed samples considered as unreliable and open to some interpretation but these much more recent good quality core samples have increased that estimate. (there was a link to that but don't remember its address. Does anyone have it to give it here? )
Also, more CO2 is not the only explanation why it was warmer back then as there were multiple natural causes, not just CO2.
Another explanation for most of the Pliocene period being warmer than the present day ( except the late Pliocene ) is that the albedo of the Earth would have been lower than what it is today due to less glaciers at the poles. Less ice means less albedo which means more solar radiation absorbed at the Earth's surface which means higher average global temperatures maintained. The late Pliocene had more glaciers as well as less CO2 which at least partly explains why it was cooler.
"...decreasing carbon dioxide levels during late Pliocene may have contributed substantially to global cooling and the onset of northern hemisphere glaciation...."
In other words, there is evidence that CO2 decreased below 400ppm in late Pliocene and the temperature also decreased And by a significant amount as well. The evidence for the causal link between CO2 concentration and climate it just overwhelming.