Thermal Equilibrium

When any object is heated, by sunlight or any other method, its temperature goes up (obviously). A hot object attempts to get rid of this heat and return to equilibrium with its environment. This occurs by three methods, conduction (direct heat transfer to another object in direct contact), convection (removal of the heat by circulation of a liquid or gas) and radiation. Radiation is in the form of infrared radiation for objects at room temperature. The hotter an object gets, the more infrared it emits, and at higher frequencies. Get it hot enough and it will glow so you can see it, like molten steel or lava. This eventually cools it off until it reaches thermal equilibium with its environment (energy in = energy out).

The first two can't occur with the earth, because it is surrounded by the vacuum of space, so it must radiate the excess heat away, back into the cold of space. Space is very cold, only three degrees above absolute zero. It sucks radiation out of warm objects. This is why the desert is very hot in daylight (from solar radiation), but cools quickly at night (the dry air allows infrared radiation from the air and rocks to go quickly into space. It is also why the tops of mountains are so cold, there is less air and moisture to obstruct the radiation from escaping.

The greenhouse effect is a misnomer, greenhouses work mostly by preventing convection (warm air heated by solar radiation can't get back out), and the glass, although transparent to solar radiation in the visual range, is opaque to the invisible infrared radiation emitted by hot objects.

When the earth is heated during the day, it cools off again at night by emitting infrared, but greenhouse gases make it slightly harder for the infrared from escaping, i.e., they are partially opaque to infrared, so the earth must get hotter for the infrared to carry away the excess energy. Greenhouse gases work primarily by making the atmosphere more opaque to infrared radiation, so the earth has to get hotter and radiate more to radiate the heat back into space. Of course, eventually the temperature (and emission) gets high enough so more radiation is produced and equilibrium is achieved, but this will now be occurring at a higher temperature than before. .

To summarize, if the earth warms up, it radiates the excess heat into space. Greenhouse gases just slow down the process slightly.

Why does the earth get hot?

Besides the input from external sunlight, the earth also generates its own internal heat. We know this because the deeper down you dig, the hotter it gets. Thousands of miles below our feet, the heat is thousands of degrees, and the rock is molten. This heat comes from two sources, the first is gravitational, the weight of rock crushing the rock below, it is left over gravitational energy from when the earth was born, by the gravitational accumulation of debris during its formation.

The second source is radioactivity. But not the radioactivity of a thin film on the surface, which you seem so concerned about. It is from the billions of tons of radioactive atoms naturally occurring in the earths crust, mantle and core. The heat generated cannot easily escape because it is insulated from the cold of space by miles of rock. Sometimes a volcano spurts some out as lava, but in the overall heat budget of the planet, it is insignificant. When this combined internal heat finally works its way to the surface, it is slowly radiated as excess infrared into space, just like the solar heat. This has been going on for billions of years, and it far exceeds the tiny amount of radioactivity we have here on the surface, or produce with our reactors.

Fission and radioactivity.

You seem to have a misunderstanding of the difference between fission and radioactivity. A radioactive atom has an unstable nucleus, and occasionally it emits a particle or quantum of radiation which allows that atom to eventually become stable (non-radioactive). Sometimes there are intermediate stages, but eventually, if you wait long enough, all the radioactive nuclei will emit energy and the pile of atoms will become inert. This is why there is no naturally occuring Plutonium on the planet. It emitted radiation and became Lead, it is all gone now, until we make some more in a reactor. The remaining radioactive elements are constantly diminishing this way too. This is the natural radiation background, and we mine and collect some of these leftover radionuclides for our reactors and bombs and science experiments. This activity does generate heat, (a waste reactor fuel element must be kept in a pool of water or it will get hot enough to melt). But this heat due to radioactivity is not anywhere near enough to be used as a power source in our nuclear reactors.

The real energy is created by fission, a totally diffferent process which is often confused with radioactivity, probably because all fissile materials are already radioactive. But not all of the hundreds of radioactive nuclei are fissionable. Only a handful are. We mostly use Uranium and Thorium, and we sometimes make artificial Plutonium (because all the natural Plutonium disappeared billions of years ago). Fission occurs when a fissionable nucleus splits into several smaller nuclei, called fission products, releasing some radiation and an enormous amount of heat.
It is the radiation from radioactive fission products which makes it dangerous, not the heat. These pieces of the original nucleus are often radioactive but they are no longer fissionable. The heat produced in fission is millions of times more than the heat released by the radioactive decay of the fission products. Since the fission heat isn't anywhere near enough to heat up the planet, there is no way the radioactivity of the fission products is going to do so. So you have nothing to worry about.

Now you may ask yourself, how come the earth's natural radioactivty has been able to melt the interior of the planet, but the stuff we create in our fission reactors is not a problem? Because man has only been able to collect and refine a tiny portion of the tiny amount of fissionable materials that are lying about on the surface of the crust. The earth is a huge volume with hundreds of billions of times more radioactives inside than have ever made it to the surface. Our mining and power generation activities are truly insignificant. Another reason is that much, if not most, of earth's original radioactives have already decayed into harmless inert matter, but the heat they generated is still trapped deep beneath our feet by thousands of miles of rock, trying to get out.

Trust me, J. The radioactive waste we generate with fission doesn't release enough heat to matter. Even the fission heat we generate (which is millions of times greater) isn't enough to matter. (I proved in my earlier post it is totally swamped by the heat we get from the sun). Even if we burned ever ounce of carbon on the earth's surface it still would not aprreciably heat up the planet all that much. What may heat up the planet is if we start screwing around with the earth's ability to radiate it's heat to the cold of space. If we add greenhouse gases to the atmosphere, it will become slightly harder for this heat to be radiated away. The earth will heat up just enough so the increased heat loss will once again just equal the solar input. It may be only a few degrees, we probably won't even notice, but it will affect the climate. And we will notice that.