How much of the Earth's heat is caused by the hot centre?
Given the insulating effect of 6370 km of rock (the average radius of the Earth) &amp; its low thermal conductivity, if the temperature at the center of the earth was 18 °C it contribution to warming the atmosphere would be negligible.

The temperature at the center of the Earth is about 7000 °C, while the average temperature of the surface is about 10 °C. So if the temperature at the Earth's center was 18 °C its contribution to the surface temperature would be exceedingly small.