|MadSci Network: Earth Sciences|
You are correct that geothermal heat flux has decreased in the billions of years since the Earth was created. However, except for the first few hundred million years of Earth's history, this heat flux has had an utterly negligible impact on Earth's climate.
Today, the average geothermal heat flux is about .05 watts per square meter of Earth's surface. This is 4000 times less than the amount of heat provided by the sun. The main energy balance of the climate system is between the incoming solar energy and the outgoing infrared radiation.
I did a very simple thermal balance calculation which suggests that if the geothermal heat flux were to double, atmospheric temperatures might increase by .005 degrees C. That's tiny compared to the predicted 1-3 degree temperature rise from human-produced greenhouse gases, and even 1-3 degrees is difficult to distinguish from natural random temperature changes.
If you'd like details of the calculation, e-mail me.
Try the links in the MadSci Library for more information on Earth Sciences.