Originally Posted by BobtheEnforcer

The interior of the earth doesn't directly impact the heat of the earth's surface. It does indirectly, by greenhouse gases being emitted volcanically. The surface of the earth is primarily controlled by solar radiation and climate/heat transfer. The important point here is that in the past the sun was less strong, and its getting stronger as time progresses. As helium builds in the core, the amount of solar radiation increases. So, early in earths history, the sun was about 6% less luminance.