I was in a hurry when I posted that.
The Earth as a whole, meaning the physical planet, along with the atmosphere, has been cooling since it's formation 4.6 billion years ago. Volcanism has been decreasing. There has been residual heat escaping form the entirety of the actual planet. This has been slowed slightly by the decay of radioactive minerals which provide some heat, and slow contraction of the planet by gravity. But eventually, the Earth will be just like most of the other planets in our solar system, cold and devoid of any self-created geological activity. The Moon still has quakes, but they are from gravitational forces.
18,000 years from now is hardly a flash in the pan of the geologic time scale. I do not know of any predicted hot periods in the future, until the eventual demise of the planet when the sun ultimately burns out. That will consume all of the inner planets. Unless more heat is created by contraction, radioactive decay (which is steady), or outside forces (ie, collision), there logically can be no increase in the earth's subsurface temperatures. What the temperature is today under our feet is a result of the forces that created the planet a long time ago.
As far as "core heat" that is already reaching us. There are no "core explosions" or some such nonsense. There
were flood basalts in the past which created things like the Deccan Traps in India, and the Columbia River Basalts in Washington. They were massive outpourings of basalt, and still considered by some geologists to have been the cause or contributing factors to past Mass Extinctions, since their timing coencides closely with ME events. There were also kimberlites in the past, which were explosions of subsurface materials. They are responsible for the diamond deposits in Africa and Canada, but were not ME events by any means, though would pretty effectively destroy your home town.
A massive cooling event in the future could come from a number of sources, and is probably far more likely than extreme high temperatures. I only really know geology, so my example:
A caldera eruption would decrease global temps signifcantly (think Yellowstone caldera, also Long Valley caldera in CA).
http://en.wikipedia.org/wiki/Long_Valley_Caldera Long Valley or Yellowstone would spell the end of the US as we know it. But if you look at the eruption of Mount Pinatubo in 1991, it ejected
10 cubic KM of ash. In comparison, the last eruption at Long Valley produced
600 cubic KM of ash. So what? Mount Pinatubo lowered global temps by about
1F. So any sort of eruption on that scale would be extremely bad, and would pose a significant threat to our survival. I think an event like this is more likely that we like to believe, though we have no data on when another one might occur, if ever. But there is always activity in these calderas, upwellings, and gas being vented...