I've been reflecting on whether we want nuclear at all in the long term (compared to renewables like wind, solar, and hydro). There's a certain amount of heat (energy) entering our planet and leaving it. Greenhouse gases reduce the amount leaving, causing the planet to warm up. Nuclear power increases the input because it's energy that would not be released here without us. But the question to ask is what's the significance of the energy input from nuclear power. Say for example that future society becomes fully powered on fusion reactors, the energy input from these reactors would be roughly $10^{22}$ joules/year (approx 20 times 2013 world energy consumption). We can compare to the total energy input from the sun, which is $10^{25}$ joules/year. From these numbers, the input from nuclear would be about 0.1% the total solar input. Is that enough to cause disturbance to the energy balance of our planet and to worsen any global warming symptoms?

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    $\begingroup$ Nuclear power is, in fact, an efficient solution to global warming. Compare this to other methods, say burning of fossil fuels. What make you think so? $\endgroup$
    – QuIcKmAtHs
    Jan 7, 2018 at 14:37
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    $\begingroup$ Nuclear power is no more unnatural than the burning of fossil fuels. Both speed up the release of energy already stored by our planet. One enhances the fission rate of $U^{235}$ while the other speeeds up the chemical decomposition of long chain carbon compounds. Neither significantly depletes the energy stored by the planet, but burning fossil fuels does significantly increase the atmospheric levels of $CO^2$, $\endgroup$ Jan 7, 2018 at 14:55
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    $\begingroup$ I'm not comparing nuclear to fossil fuels, I'm comparing nuclear to renewables (sun, wind, hydro, etc., whose energy source is the sun near the time of energy collection). $\endgroup$ Jan 7, 2018 at 15:38
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    $\begingroup$ You've bumped our current energy consumption by a factor of 20, you've made it all fusion, and you've found that even then, it's a thousandth of the energy input from the Sun. If this is a problem, it is a problem for our children's children's children to solve. "Stop wishing for bad luck and knocking on wood." $\endgroup$ Jan 7, 2018 at 17:23
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    $\begingroup$ @CedricEveleigh If you'd like some extra probably unnecessary worry: what would be the impact of taking a large amount of energy out of the wind and water currents? This doesn't add "new" energy to the system, but one could conceive that at a large enough scale this shifts the climate with significant ecological impact. Certainly, damming rivers does this in the small scale. Collecting solar energy potentially increases the energy in the system by taking in visible light that would easily be radiated away and converting it to infrared (eventually) that is more easily trapped. $\endgroup$ Jan 7, 2018 at 19:02

6 Answers 6


There is a balance between heat input and radiation lost to space. To maintain this balance, if you increase the input by 0.1%, you must do the same to the output. Approximating the earth as a blackbody, the energy it radiates is proportional to the fourth power of the temperature. So the temperature would need to increase by 0.025%. That's less than a tenth of a degree, which doesn't seem very significant.

  • $\begingroup$ I just calculated a required temperature increase of 0.3C, which isn't far from your result and isn't much indeed. I'd be curious to see your calculations/assumptions. $\endgroup$ Jan 7, 2018 at 23:04
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    $\begingroup$ I took your value of 0.1% increase in heating. Using the fact that the total energy radiated by black bodies is proportional to the fourth power of their temperature, and the fact that, for small $x$, $(1+x)^4 = 1 + 4x$, I arrived at one quarter of 0.1%, or 0.025% for the increase in temperature. For an average surface temperature of 300K, that would be about 0.07 K. $\endgroup$
    – Ben51
    Jan 7, 2018 at 23:10
  • $\begingroup$ I made a mistake in my earlier calculation. The temperature increase is indeed 0.07 K. Thanks. $\endgroup$ Jan 7, 2018 at 23:59
  • $\begingroup$ This answer is elegant and concise, but my concern is that it seems a little overly idealized for such a complicated system. For instance, it doesn't take into account changes in emmissivity due to albedo of the Earth or greenhouse gasses, and assumes the earth is a uniform, constant temperature. These omissions means it would ignore effects like greenhouse gasses and obliquity of Earth's axis that have been shown to have a significant effect on global temperature, so I'm not confident about how accurate its prediction for the OP's question would be. $\endgroup$ Jan 8, 2018 at 14:54

The answer is "No", and here is the qualitative explanation. The Greenhouse Effect is not about extra energy input at the Earth's surface, nor is it about reducing the thermal output. It is about upwelling.

In the static, ocean-free black body model: consider and sensor in deep space looking back at Earth black-body radiating in the infrared: it does not see the surface, it sees 1-optical depth depth down into the atmosphere, and it is that chunk of atmosphere that is at the correct blackbody temperature to balance the energy input. As you penetrate more optical depths into the atmosphere, it gets hotter, down to the surface, where it's hottest.

So the surface is hottest, radiating isotropically, with energy transfer to cooler regions above, which do the same, and so on. The processes are refer to as upwelling and radiative transfer.

The addition of greenhouse gases increase the optical of the atmosphere, thereby raising the surface temperature without changing the energy input or output.

  • $\begingroup$ That's a good answer cheers i like that. $\endgroup$ Jan 8, 2018 at 10:34
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    $\begingroup$ This would explain the greenhouse effect, but the mechanism of warming being proposed here is something quite different: it is direct drive of the climate system by an input of energy into the atmosphere that is generated at the surface. So simply explaining the mechanism of greenhouse effect does not answer the question of what a direct drive - change in the input - will do to the system. As @Ben said, input must balance output, so the output must go up. This means the layer you mention has to either get larger, or hotter, or both, and that will mean the layers below will be hotter. $\endgroup$ Jan 8, 2018 at 10:51
  • $\begingroup$ This does not answer the question (except for the "No" part, but then the explanation is very unrelated to this). The questioner did not ask for one (of many possible) explanations of the Greenhouse Effect. $\endgroup$ Jan 8, 2018 at 14:36
  • $\begingroup$ @The_Sympathizer I was addressing the OP's statement: "Greenhouse gases reduce the amount [of energy] leaving, causing the planet to warm up."--which would be a state of permanent non-equilibrium. A good case is Venus at 0.72 A.U. it gets 2x solar energy so the upper optical depth should be about 20% warmer than the Earth--yet the surface is 860F. $\endgroup$
    – JEB
    Jan 8, 2018 at 16:10

According to this source, the total solar irradiance of Earth varies-- rather conveniently-- by about 0.1% over decade long solar cycles. Since the climate is very difficult to predict accurately, unfortunately I can't find any widespread consensus on how much this affects climate (although I'm no climate scientist so there may very well be one that a quick search isn't producing). However, it seems likely that these solar cycles have been around much longer than man-made climate change has been an issue. So, at a first glance, I would say that a 0.1% increase of heat input to Earth wouldn't be too big an issue.

EDIT: To address the OP's concern that the cycles involved in the solar cycle are too short lived to make a valid prediction, I went down a rabbit hole of climate science and learned about Milankovitch cycles. Essentially, the obliquity of earth's axis, it's precession, and the eccentricity of Earth's orbit around the sun all vary in long (tens of kYr) cycles. However, only eccentricity actually changes the total yearly insolation of Earth-- according to this source, by about 0.167%.

So, now we have a very long-lived cycle of insolation that we can compare with and get (hopefully) more accurate results. That same pdf helpfully compares a short-time Fourier transform of average global temperature to STFT's of other data-- namely obliquity, eccentricity, precession, and insolation at 65 N in July.

In the pdf, they show that the STFT of eccentricity does in fact coincide with that of temperature for certain frequencies. So, one might be tempted to conclude that the 0.167% change in total insolation caused by eccentricity was responsible for the $12^\circ$C fluctuations in average temperature over 800 kYr, which is certainly a worrying amount! However, there are many other factors at play, as obliquity also shows a strong correlation to temperature even though it doesn't affect global insolation, as does insolation at 65N in July. Of course, this all stems from the fact that the Earth's climate is very chaotic and interconnected. For instance, lower insolation at 65N is more likely to produce ice than at lower latitudes, which leads to higher albedo and a whole bunch of other cascading effects.

The takeaway from this is that it's pretty difficult to know conclusively what would happen if humans dumped an additional 0.1% of total heat into the environment without actually doing it. However, there are natural processes that change heat dumped to the environment by comparable amounts, so it's not completely unprecedented (although the time scales involved are obviously very different). I think it's also important to point out that 20 times the energy usage in 2013 is a very extreme estimate, so the real life effects would probably be significantly milder than this post makes them appear.

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    $\begingroup$ It sounds like a good comparison with the same percentage, but I doubt the validity of comparing the effect of a fluctuating change in heat input (with a short 10 year cycle) to a constant change in heat input on earth's climate. $\endgroup$ Jan 7, 2018 at 15:37
  • $\begingroup$ True, but given the complexity of the climate, I'd say it's difficult to get much more than an imperfect comparison. Also, I'd argue that 20 times 2013 world energy consumption is a very extreme estimate; that'd require a population of 10 billion to each consume nearly 4 times the current per capita consumption of the US, even though that number has actually been declining. $\endgroup$ Jan 7, 2018 at 16:35
  • $\begingroup$ Long term we are sliding into an ice age. Any tools that can delay that should be welcome in the world arsenal. en.wikipedia.org/wiki/Ice_age#Glacials_and_interglacials detail of present en.wikipedia.org/wiki/File:Holocene_Temperature_Variations.png . $\endgroup$
    – anna v
    Jan 7, 2018 at 16:44
  • $\begingroup$ @elduderino Those are good points. 20x is a rough estimate indeed. $\endgroup$ Jan 7, 2018 at 16:51
  • $\begingroup$ greenhouse effect is 1/2000 times more warming for the planet than nuclear. $\endgroup$ Jan 7, 2018 at 22:24

Nuclear amounts to =0,005 W/m2, most of it is not thermal, it's light, fridges and mechanical drives.

A change of CO2 from 280ppmv to 410ppmv traps about 2 W/m2.

Greenhouse effect is 1000 - 2000 times more warming for the planet than nuclear. The thermal efficiency of a conventional nuclear power station is around 33%, gas turbine ones are at 60%. Decay efficiency of the waste is nearly zero.

In environmental sciences, they measure Global Economic Systems of energy and chemistry:

enter image description here

All the nuclear before Fukushima did 2500,000 GW/h. it's 11 percent of human energy needs.

the earth has 510.1 million KM2, mult by 1mn to have the number in meters.

2500,000,000,000Wh / 510,000,000,000,000 m2 = 2500 / 510,000 =0,005 W/m2

the sun gives us 10,000 times more than all the energy we make, and our energy is used mostly for used for machine drives, TV, communications and lighting.

The others gave the most relevant factor of the equation, which is greenhouse gases.

enter image description here

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    $\begingroup$ "greenhouse effect is 1/2000 times more warming for the planet than nuclear." Are you saying nuclear is 2000 times more than greenhouse? The fraction is a bit confusing. $\endgroup$
    – axsvl77
    Jan 8, 2018 at 3:22
  • $\begingroup$ 1000 or 2000... times $\endgroup$ Jan 8, 2018 at 7:28

Why would the nuclear energy of unstable elements not turn into heat without us chain reacting it and using it for electricity first?

The radioactive isotopes also give off heat spontaneously as they decay to more stable elements naturally.

It would be interesting to see an estimate of how much energy is generated by spontaneous radioactive decay by all elements in the whole earth as a system. I would be very surprised if our nuclear output would be anywhere near the total amount of energy of all decaying atoms in earths crust and cores, (but I don't have any source for that.)

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    $\begingroup$ On your first point, the natural decay process of ${\,}^{235}U$ to ${\,}^{207}Pb$ releases much less energy than the fission of ${\,}^{235}U$ in a nuclear reactor or nuclear bomb into much smaller elements $\endgroup$
    – Henry
    Jan 7, 2018 at 19:49
  • $\begingroup$ @Henry : Yes, that sounds reasonable. But the question is about the total effect on the Earth as a system. I wonder how large mass of $^{235}U$ would need to fission to give off even the same order of magnitude as the natural energy given off by the mass of radioactive elements spontaneously decaying which we can't do anything about. $\endgroup$ Jan 7, 2018 at 19:59

All known other energy sources including solar and wind energy produce extra waste heat into the planet that would otherwise be lost. All human energy needs are adding extra tiny amounts of energy- nuclear is not special.

  • $\begingroup$ Nuclear power is "special" in the way that it is not nearly as efficient as solar. Nuclear produces huge amounts of waste heat during energy "production", similar as fossils. Solar cells almost don't produce any waste heat. $\endgroup$ Jan 8, 2018 at 14:44

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