(Part of the How to Talk to a Climate Skeptic guide)
Objection: We all live on a thin crust that floats on a huge ball of molten iron, and at its core, the Earth’s temperature is over 5000 degrees C! It’s pretty far fetched to think a few parts per million of CO2 can have a bigger effect that all that heat!
Although there is nothing wrong with the statement that the Earth is truly very hot at its center (actually as hot as the surface of the sun) the notion that it is a significant source of heat at the surface is easily dismissed with a little critical thinking. If the inner heat were really the dominant factor, then surely the day-night cycle would not be what it is, nor would you expect such variation in climates over seasons and latitudes. How can the south pole be covered with thousands of meters of ice with all this heat supposedly bubbling up from the surface? Why would a little lower angle of sunlight cause the average temperature to drop from +20°C in the summer to -20°C in the winter?
The fact of the matter is, solid rock is an extremely good insulator and the heat from the mantle propagates up very slowly and diminishes very quickly (at about 20°C/km) to almost nothing by the time it is at the surface. At the surface, the earth is releasing less than one-tenth of one Watt/m2. If you could somehow capture all of the energy coming up from the earth’s core into the foundation of an average-sized home, you might have enough to power one 15W light bulb! Not a lot of of juice when you compare it to the sun, which provides on average some 342W/m2 of energy to the earth’s surface.
And let’s not forget that what we are talking about is climate change, not just climate. So we need some kind of change in this heat flux if we wish to explain a change in the global temperature. Scientists have calculated that increased greenhouse gases have resulted in a radiative forcing of 2.43 Wm-2 which means we need that many Watts/m2 of change to account for the current warming. Back to geothermal, this means the energy flow from the earth would have had to jump by over 200 times to be the cause of the approximately 0.8°C temperature rise.
It is pretty hard to imagine not noticing that!
This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Skeptic.