ice
it was hotter during the previous interglacial period than it is in this one. the polar ice caps didn't melt then. some folks think it's quite likely the polar ice caps will melt during this one. one wonders, why? it was hotter then than now. what's different? perhaps it was a celestial thing. like the sun shone on the equatorial regions more than the polar. or perhaps this time the heat is somehow being more uniformly distributed across the planet. which would much more likely be a compositional thing than a solar forcing thing. ie stuff people do.