What is the percentage of the Earth's warmth that the Earth's hot centre is responsible for? What would be the temperature of the atmosphere if the Earth's centre was 18° C (average temperature on the surface)?
Given the insulating effect of 6370 km of rock (the average radius of the Earth) & 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.