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This is probably a really stupid question, please forgive me. Is it possible that the use of nuclear fission on earth contributes to the increased energy in the Earth's system as according with the Laws of Thermodynamics regarding the Conservation of Energy?

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Catherine, could you make sure my edit to the title represents what you want to ask, and fix it if not? – David Z Sep 14 '12 at 19:43
I really don't wish to publicly embarrass myself any further yet I have this nagging common-sensy non-scienty idea that as nuclear fission occurs only very rarely in natural systems, it is all this extra energy that we are busily unlocking from atoms that is contributing to the extra energy in our climate systems. Please can you give me your opinion on this, Best wishes, – Catherine Hoy Sep 25 '12 at 21:44
You're not embarrassing yourself at all as long as you follow the evidence where it takes you. In other words, if you honestly don't believe some step in the argument, and you have a logical reason why that step doesn't make sense, by all means, ask about it! (You can edit your question to do so, or post a new question.) But if all you have is a "common-sensy non-scienty idea" that contradicts the reasoning you've been given, you're in the wrong place to ask about it. – David Z Sep 25 '12 at 23:55

No, it is not possible that nuclear fission is responsible for climate change. So although it's theoretically possible that we could do enough nuclear fission to raise the global heat content of the earth significantly, in reality, we've done very little nuclear fission industrially, and it's a tiny tiny contribution.

First, remember that burning fossil fuels also adds to the global heat content. And that way way outstrips nuclear fission: each year, nuclear contributes to about 6% of global energy demand. That's less than we get from renewables, and much much less than we get from natural gas, coal and oil (those last three total around 80% of the global energy supply). So the contribution to the global heat content from nuclear is smaller than that from fossil fuels. All in all, the combined output from nuclear, oil, gas and coal is the equivalent of about 12TW of extra heat into the Earth (in mean power terms).

Remember, as far as global heat content is concerned, releasing locked-up chemical energy is no different to releasing locked-up nuclear energy. They're both just the introduction of extra energy into global heat content; energy that would otherwise have been locked away in non-heat form: either as mass, or as chemical energy.

And the 12TW of extra heat is tiny compared to the impact on global heat content from anthropogenic greenhouse gases.

The Earth's surface is around $5\times 10^{14} m^2$, so 12TW is about $0.025W / m^2$. The radiative forcing from $CO_2$ is around $1.8W / m^2$

Increasing the atmospheric concentrations of greenhouse gases directly (in particular, $CO_2$, but also $CH_4$ and several others), and indirectly, through secondary feedbacks and forcings, causes an increase in global heat content that is orders of magnitude larger than the heat added to the Earth from all human activities.

Thanks to AlanSE for noting that in general, renewables don't contribute to global heat content: broadly speaking, the energy in hydro, wind, wave and solar, would all have ended up as low-grade heat anyway. Geothermal was obviously part of the global heat content to start with. One might argue that tidal power does add to global heat content, but that's absolutely miniscule amounts. Biomass might in some cases have added to global heat content; in other cases, it might add something to embodied energy locked up in soil chemistry. There's also the effect of changes in global albedo from changes in land-use for energy: but again, these are miniscule in comparison with the forcing effect of greenhouse gases.

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I suppose it would be an interesting distinction to note that renewables in principle don't contribute to the Earth's heat content. Actually, this would be as good of a definition for "renewable" as any. – Alan Rominger Sep 14 '12 at 12:44
@CatherineHoy I think that's already covered by the answer. There are no significant implications, because the effect is tiny: $0.025W / m^2$ from all non-renewable energy sources. I'll update the answer to clarify that releasing locked-up chemical energy is no different to releasing locked-up nuclear energy, as far as global heat content is concerned. – EnergyNumbers Sep 14 '12 at 13:23
@CatherineHoy "However, this law does not apply to nuclear energy because it is produced when atoms of matter are split or fused." The law of energy conservation perfectly holds for nuclear reactions. Why do you think the opposite? – Yrogirg Sep 14 '12 at 13:32
It's very important to understand that direct contributions to global heat content, while potentially interesting, are utterly meaningless in relation to climate change. Climate change is an alteration of the global equilibrium temperature, not the heat content at any particular instant. Above the equilibrium temp, any heat we add will simply radiate away; below equilibrium, the earth will absorb solar radiation. – Colin K Sep 14 '12 at 16:33
I had a similar thought to Ron, it's really misleading to start your answer off saying "yes" when the rest of the answer appears to be devoted to proving "no." – David Z Sep 14 '12 at 19:42

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