I have wondered about this scenario for a while, and would like to know what is your opinion about it. Its assumptions are quite specific and probably won't be true, but they do appear realistic enough for me.

(1): Assume that nuclear fusion becomes an available energy source within a couple centuries, it will provide a cheap, plentiful, emission-free, and long lasting source of energy for human activities.

(If this assumption is wrong, we are probably in trouble)

(2): Assume that continued economical/technological development requires increasing energy consumption indefinitely.

(This is probably wrong if we utilise completely new physics in the future, but I don't think this assumption is unlikely)

(3): Assume that the generation of waste heat during energy generation/consumption cannot be dramatically lowered in the short-term future.

(This is also probably wrong. But it will hold true if we still have to use machines/engines/generators based on the same design principles as we do today, and I don't see that happening too soon)

The logical conclusion from the above three assumption:

At some point after the implementation of nuclear fusion, humanity's energy consumption might reach a level so high that the waste heat we release into the atmosphere will be altering the Earth's climate system not unlike what our carbon emissions are doing today.

(Since the source of any future fusion plant is likely hydrogen in seawater, for Earth it probably acts as an extra heat source independent of the sun)

The Earth is functionally a giant spacecraft, and spacecrafts usually have very sophisticated heat management systems to prevent them from overheating, so perhaps we have to work with that as well.

I haven't done too much number crunching yet, I might have gotten the figures wildly wrong.

We know today the amount of solar energy the Earth receives per year is about ~5000 times the amount of energy humanity consumes.

If humanity's energy consumption increases 100 times, and 50% of the energy is released into the atmosphere as waste heat, then we are releasing ~1% of solar energy into the atmosphere as heat.

That might have some serious climate implications if lasting for a long time, but I'm not certain about that yet.

Possible solutions:

(1): Geoengineering, that seems to be obvious. We try to reduce the solar energy input on Earth when the heat we release is too much. But that probably will negatively impact the biosphere a lot due to photosynthesis issues.

(2): Set "energy consumption targets" for countries/firms/etc like current climate policy.

Problem: while countries can continue to develop their economy and technology without increasing carbon emission (by adopting clean energy, etc), a limit on energy consumption seems be a hard cap on a country's development that cannot be worked with. So, probably no one would be compliant with such an agreement...

(3): Colonising other planets/solar systems

Each colony would also have to face that problem.

The Earth (and any other planet/moon we colonise) seems to be functionally the same as a giant space station. And space stations need sophisticated maintenance systems, including management of waste heat.


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Back-of-the-envelope equilibrium estimate: if we increase the energy added to the atmosphere by 1%, then the Stefan-Boltzmann law says that a blackbody would need to be warmer, or 0.4%, to radiate that much more. At the Earth's temperature of ~288 K, this would be ~0.7 K warmer.

This suggests to me that it will have a smaller impact than global warming. Whatever we use to solve global warming will probably work on this problem as well. It's still something to keep in mind, though.

Ah thanks, so the equilibrium is more robust than I initially assumed, didn't expect that to happen.

So the issue won't be as pressing as climate change could be, although some kind of ceiling still exists for energy consumption on Earth nevertheless...

Note: since most global warming statistics are presented to the American layman in degrees Fahrenheit, it is probably useful to convert 0.7 K to 1.26 F.

I would assume Kelvin users to outnumber Fahrenheit users on LW.
I'd assume the opposite, since I don't think physicists (and other thermodynamic scientists like some chemists) make up a majority of LW readers, but it's irrelevant. I can (and did) put both forms side-by-side to allow both physicists and non-physicists to better understand the magnitude of the temperature difference. (And since laymen are more likely to skim over the number and ignore the letter, it's disproportionately more important to include Fahrenheit.) Edit: wait, delta-K is equivalent to delta-C. In that case, since physicists ⋃ metric-users might make up the majority of LW readers, you're probably right about the number of users.
About half of LessWrong users (or at least visitors) are from places other than the U.S., which means there are a lot more metric users.

See this more rigorous analysis by Robert A. Freitas. Yes, it will cause some problems.

This is an interesting study, it seems that his numbers are not too far off what I plugged in as a placeholder (that our current energy consumption is within a couple magnitudes from becoming climate altering)

Though I'm not making sense of the nanobots yet haha

There's a post about this at the blog DoTheMath, which calculates we boil ourselves with waste heat in ~400 years, assuming GDP doubles every 100 years and per capita energy consumption increases at the same rate it has been for the previous ~400 years.

The usual economic retort is that the economy could look very different from the one we are used to, and decouple from energy consumption. But the assumption about waste heat is what is doing the work here, and we have recently developed thermal transistors. These transistors have been designed out of quantum objects. And it turns out we might be able to beat the Planck limit in the far field. Which is to say, we can build heat computers, and then waste heat could be converted into computation.

That doesn't solve the problem of too much energy use being bad, but if waste heat is computation then we can hit peak (safe) output, stay there, and still add value.

It possible to create a sphere out of mirrors around our earth that can be turned to reduce the amount of energy earth receives from the sun. It's very expensive to build such a system but I would expect it to be build if we get heat problems a few centuries down the line.

Geoengineering includes getting better at radiating heat as well as reducing heat received. Superconductor to dark-side cold farms might do the trick. Also, include bioengineering in your list of possible mitigations: if we can live in a very hot environment (or upload/emulate on a more durable substrate), it's less of a problem.

Longer-term, #3 is the only way. Intelligent life needs to elsewhere when this planet's used up.

I don't expect this to be a problem because by the time humans would be using this much energy we should be easily capable of constructing simple megastructures. One would only need to decrease the amount of IR light that hits the earth with massive but relatively cheap (at least once you have serious space industry) IR filters in order to decrease the earth's temperature without impacting anything dependant on the sun's light.

reversible computer gates exist and are Turing complete, which in human words means it's possible to do computers where the elections are not sent to ground willynilly. When you "not" a live wire, that in a very real sense is sending elections to ground.

Now I think Turing completeness is a terrible standard, we really need a more complex hierarchy like complexity thoery has, as my understanding of the proof these things are Turing complete is that they are generating waste outputs, so if you are stopping using not gate to prevent sending elections to ground and you generate waste outputs that in thoery your sending to ground there is not nesserily an improvement here, but maybe something comes out of it anyway.

Futher more if we have fusion don't we have economical carbon sinks? And space sillyness?

I'd like to question both assumptions (2) and (3).

For (2), it's not clear to me that "more energy" is the thing people want. Some things we do require a lot of energy (transportation, manufacturing) but some things that are really important use surprisingly little power (the internet).

For (3), it seems like we've been moving in the direction of more-efficiency for a long time (better engines and turbines to convert more of the fuel into useful energy, fewer losses to friction and transmission, etc.).

Overall, I think we're seeing an upward trend in power usage because more people are getting the full benefit of modern technology, not because modern technology is using more power per person. I wish I could find a long-term graph, but per-capital power consumption in the United States has been going down for a few years, and total power consumption in the United States has been flat since around 1995: https://en.wikipedia.org/wiki/Energy_in_the_United_States#Consumption Another way of putting this is that it's not that cars are using more gas, it's that a lot more people have cars. This means that in the short-term we can expect energy usage to continue going up for a while, but it could plausibly peak if/when the global population peaks and we finish the project of ending worldwide poverty.

For (1) I could nitpick since fission could also power our civilization for a long time, although I don't think it really effects the question you're asking.