At my blog, I'm continually faced with many new and different arguments against anthropogenic global warming. This one, however, I found to be particularly interesting. A regular contributor, named the Grit, postulated that the heat that is generated from beneath the Earth's surface actually contributes to global warming:

What is important is the IPCC claim as to what the CO2 increase in the atmosphere contributes, which is down around, using their biased figures, 3 Wm2. This makes your unreferenced claim of 0.075 Wm2 a much more important consideration. - Reasic

This is not the first time such a theory has been proposed. I've also seen it at Tim Lambert's blog:

And why are we not being cooked to a frazzle on the earth’s surface by this enormous mass of matter at a temperature greater than 1000 Degrees Celsius underneath us? Since the temperature gradient between the earth and space is somewhat steep, one wonders about the scientific basis of climate science and the hypothetical construct of anthropogenic CO2 induced global warming, given the overwhelming contribution that the earth’s interior makes to the surface temperature of the earth and to space’s ability to absorb all this thermal energy. Given the mass of the solid earth is somewhat greater than that of the atmosphere, of which 0.033 percent is CO2, a simple physics 101 calculation of the heat balance might suggest that the contribution by CO2 to the earth’s surface temperature is, for practical purposes, irrelevant. - Deltoid

...to which Tim Lambert responded:

Does the earth’s interior make an overwhelming contribution to the surface temperature? This claim seems to be contradicted by the fact that it is warmer in daytime. And in summer. And closer to the equator. It takes a rare kind of talent to present an argument on climate change that is inconsistent with the existence of seasons.

However, I would like to take a different approach, which should more directly address the Grit's proposal. What I've come to find, through some brief research, is that the reason it's not mentioned in IPCC reports is that it has virtually no effect on climate change. First of all, the Grit was being very generous with the figure he gave for the contribution of CO2 to global warming. According to the latest IPCC report, this figure is actually more like 1.66 watts per square meter (W/m^2). So, assuming the second figure is correct, one might then conclude that the planet's internal heat is actually a significant contributor to climate change. However, the key word here is "change". 1.66 W/m^2 is the amount that CO2 contributes to a change in climate, but 0.075 W/m^2 is the total energy released from beneath the Earth's surface. Therefore, we should calculate the change in the second number over time, so that we can compare apples to apples, as they say. It was difficult to find a straight forward answer to this question, but the best I found was this tidbit about the history of Earth's energy balance:

Some measure of the time involved for a planet the size of the earth to cool by convection is that the earth has been geologically active for over 4 billion years, and will probably take another 4-5 billion years to loose all its heat -and become a dead planet. - The Heat History of the Earth

What I gather from this is that the Earth started out internally warmer than it is now, and will gradually cool for a very long time. I then deduce that the amount of energy reaching the Earth's surface will also decline over that time. However, the cooling takes place over such a long period of time that, over the time periods we are currently concerned with, the heat generated from within the Earth could be considered constant. In that case, the change in temperature from inside the planet is basically zero, which explains why the IPCC does not mention it.

The fact of the matter remains that the scientific community is in agreement that a) the Earth is warming, and b) human activity is the primary cause for it. The IPCC reports are a culmination of all of the relevant scientific research on the subject of climate change over the last 6 years from many different fields of study, not an attempt to refute misguided skeptical arguments. If there were any reasonable debate on the issue, it would be occurring within peer-reviewed scientific literature. This is simply not happening, although there is a fervent opposition from many blogs and conservative news outlets.

The main reason I continue to entertain such arguments is that I am aware that they can be very misleading. So, I try to provide an outlet for the general public to find answers to many of the confusing and dishonest arguments that are presented by global warming skeptics. I hope this post has been helpful in that capacity, and if you have any additional questions, or have encountered any puzzling arguments that you'd like to see countered, I'd love to hear from you in the comments.