The movie Back to Future revealed that Mr. Fusion (MF) produced by Fusion Industries (FI) was in widespread use in 2015. What the movie did not reveal was the FI was privately funded by Bill Gates. FI used 10,000,000,000 dollars and the entire first year of production at a cost of 2500 dollars for each unit and selling them at 5000 each. So on the first day to take orders (tomorrow - Sep 11 2015) there are 4,000,000 units available and they sell out completely very quickly once people realize that MF is legitimate. MF is thus widely deployed before the end of 2015.

A single MF unit can output 1 MW continuously as long as it receives a steady supply of fusion materials notably banana peels, Miller beer and of course the can itself. A a beer can is almost entirely aluminum or tin-coated steel, it is essentially useless for fusion as MF can only fuse hydrogen. Doc Brown tossed in the beer can as comedic element. The only real fuel was the hydrogen in the water, hydrocarbons, etc. Fortunately hydrogen is very common in banana peels and beer.

Note: to get the wattage required for time travel requires storing the energy over time and releasing it in a burst of energy -- this technology comes from a more distant future and is thus not available.

An MF unit lasts 20 years without maintenance when used continuously at full power, at the lowest power usage (10 kW) an MF unit can last up to 50 years without maintenance. FI has an absolute monopoly on the product by including the best tech ever designed to prevent reverse engineering as well as an iron-clad license agreement. For example, China remains ineligible for importing MF units until they totally revamp their IP laws. The end result is that you cannot maintain a MF unit, it has to be returned to FI for a 1000 dollar trade in credit on a new unit. What this really means is that electricity now costs about 0.001 cents per kWh or about 10,000 times cheaper then before MF. Not surprisingly, power usage grows dramatically at first, but eventually levels off to a 5% increase annually.

Due to widespread use of MF, global carbon dioxide emissions plummet over the following years. But earth faces a new danger. Waste heat from the operation of MF units. MF can only convert 20% of the fusion energy into electric power. Part of the loss is in the generation of magnetic fields, etc. the rest is lost in converting the nuclear energy into electricity itself. At current energy usage levels, the heat contribution to the Earth's energy balance is negligible, but at the new 5% rate of increase annually, we are going to overheat the earth soon. Of course, once the electricity is used, it ultimately converts to waste heat too unless it leaves the Earth in the form of light, etc.

You have been appointed to chair the presidential commission to fix the heat problem that is expected to be serious within 40 years if no corrective action is taken. How do you fix this for 40 years from now? How you fix the heat problem on an ongoing basis assuming that at least 10% of the population wants to continue living on the Earth.

Yes, I read Midas World quite a few years ago. The solution to the problem in that book depended upon moving the population into space, which is part of the reason why I mentioned the 10% Earth based population requirement.

I would also like to clarify what I mean by a significant heating problem in 40 years. At that point, the global warming due to fusion will reach 2 degrees Celsius above baseline. Currently Earth receives 173,000 TW of solar radiation. A 2 degree rise means that mankind is generating about 4,660 TW of fusion heat which means mankind has 932TW of electric energy coming from about 932,000,000 MF units and Bill Gates is now the first trillionaire. This is a huge increase from current power levels of about 18TW globally from all sources. With a 5% growth, the temperature rise will far exceed that before long as the doubling period is only 14.2 years.

  • $\begingroup$ Have you read Midas World by Frederick Pohl? That is a collection of novellas with a darkly comical take on the problems caused by cheap fusion, including the problem of waste heat. Might be good research; it's not a long book to read. $\endgroup$ Sep 10, 2015 at 10:15
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    $\begingroup$ If the US is any indication, 40 years is too far in the future for businesses to care so will lobby to keep things going status quo $\endgroup$
    – bowlturner
    Sep 10, 2015 at 14:29
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    $\begingroup$ This is a great question, but the thermodynamics of the situation are a bit off. If we solved the CO2 problem, then the retention effects of global warming go back down. Radiation from the earth to space remains proportional to temperature. Global energy consumption in 2012 was estimated to be equivalent to a continuous 17.7 terawatts, and the sun dumps 10000 times that on us continuously. And as an engineer, I LOVE waste heat. I can make it spin turbines and get even MORE power from the same process, or heat things that I need hot. $\endgroup$
    – user8827
    Sep 10, 2015 at 14:43
  • $\begingroup$ Also, it is worth pointing out that this essentially wiped out all energy consumption from any process used to manufacture other power stations, and in most cases the power stations themselves. Heat engines have hard limits set by Carnot efficiency, and after that typically about 6% is lost on the grid itself. Overall, institution of the MF systems will result in an initial decline in energy usage - really. I promise. $\endgroup$
    – user8827
    Sep 10, 2015 at 14:54
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    $\begingroup$ I did not mention using the waste heat for any number of purposes simply because it is not relevant as is does not affect the heat balance. Thanks for pointing it out, but basically it does not matter. Adding 5,000 TW of heat will make the earth noticeably hotter. $\endgroup$ Sep 10, 2015 at 16:08

7 Answers 7


My answer is to apply monetary economics. I shall assume that technological or bio-technological solutions to increase the ability of the planet to dispose of significantly more waste heat than it does today are not feasible or desirable.

Secondly, with all that MF power, lets get people making use of it to develop a spacefaring civilisation!

So my recommendation to the commission is roughly as follows:

  1. The (world) government shall establish a currency for which one unit can be redeemed against the rights to operate a MF that produces one unit of waste heat. This is similar to todays money which is redeemable against future consumption - now we explicitly tie money to entropy production. This currency replaces the current fiat system. Money based on energy usage has been quite widely discussed in the context of peak oil. This situation is essentially the same.
  2. The quantity of currency is fixed according to the capacity of the earth to dispose safely of the waste heat, so its different to todays money in which the economy is not energy constrained is is more constrained by supply and demand of finished goods.
  3. The government mandates that FI with their fantasticly secure technology that cannot be reverse engineered add a 'waste meter' to each MF that is loaded with the credits/money and will refuse to produce power when the credits run out.
  4. These constraints do not apply for MFs used off-world, and this stimulates movement of power hungry manufacturing processes, and in time, residential establishments to move off world. Asteroids and comets are fetched to power the off world MFs. Since the off-world economy is not energy constrained, it uses its own forms of private currency similar to modern bank created money/credit. These currencies float against the terrestrial currency.
  5. Because the rights to production of waste heat on earth cannot increase, those trying to save money in terrestrial heat-currency may be subject to a negative interest rate or inflation in the terrestrial currency if he actual quantity of waste heat that can be safely disposed of by the planet declines due to government decree. When the global heat production limit is increased the terrestrial currency appreciates versus the off world ones, and vice versa when the terrestrial limit decreases.
  6. The 10% remaining on earth must spend their terrestrial currency on power production to maintain their standard of living, but also on importing finished goods from off world manufacturies (the data centres may also be off world). The off-worlders selling goods to earth get in return terrestrial currency that they can only redeem by either coming to earth an living for a while (many off world workers probably don't live off world all the time), and by importing stuff which cannot easily be manufactured in space - mainly stuff like quality food and drink, works of art, things like that. Ultimately to work, there has to be an equilibrium in which neither the off world nor terrestrial economies run a persistent trade surplus.
  • $\begingroup$ Please add your observation in a comment re: the inherent problem in compound growth to your answer - embellishing if desired. Once you have done so, I will accept your answer and remove this comment. Thanks. $\endgroup$ Nov 8, 2015 at 1:30

Short Term Solution.

We have a big problem, and need to combine a number of changes to really make enough diference. Raising the price of MF by a large amount will crash the economy and save lots of energy, but the pain of such an approach makes it political suicide as well as being very likely to result in major wars. Fortunately widespread use of MF has greatly accelerated the economy and the rich Earth can afford needed mitigation steps. An energy tax should be part of these incentives.

Encourage energy conservation. In the 1950's oil was cheap and people did wasteful things like building houses where the walls were not insulated, drove cars that got 10 miles per gallon, etc. The incredibly cheap energy of MF has encouraged similar behaviors and eliminating the waste will save considerable energy. Economic and regulatory incentives will be needed to make this happen quickly. Processes that are particulaly energy intensive and wasteful may need special treatment.

Work with the FI company to increase the efficiency of MF itself. Since this technology is tightly controlled, it may be necessary to take measures against FI that will be politically damaging since most people are fond of the cheap energy. Assuming significant gains are archieved, economic incentives may be needed to push out the most efficient units more quickly. If FI data shows that most MF units are swapped out within 30 years, no incentives will be needed.

Implement some of the climate engineering solutions to reduce global temperatures that are being considered today. E.g., Change rooftops, roads, parking lots, etc. by using white paint or other diffuse reflectors. Note that non-compliance is easily detected making it easy to penalize if needed.

Note that geoenginering projects that focus on CO2 reduction will not be effective since CO2 production has been largely eliminated by use of MF.

The tight control of MF turns out to be useful as the government can embargo MF shipments to countries that do not take similar measures.

Long Term Solution

Develop space technogies that will help move industry and people to space. The space fountain and similar technologies must be included as it could be used as a basis in the future for increasing heat transfer away from earth and moving the earth as well as solar shades. Note that getting rid of waste heat in space is not as easy as radiation is the only way to shed excess heat however you really have no choice but to move some heat off planet as usage increases.

Move industrial, agricultural, commercial and residential processes to space. O'Neill cylinders may become very common in Earth and lunar orbits, solar orbits, etc. Some satellites use reflective foil on our surfaces to reflect nearly all of the sunlight away - a similar strategy would be used on O-Neill cylinders. Automated facilities could run hotter than Earth. Require near Earth facilities to direct their heat radiation away from Earth.

Move the earth's orbit outward. Done slowly, the moon orbit will be stable. Solar radiation is proportional to the square of the orbital radius so moving the earth outward by 1 percent will reduce radiation by about 2 percent. A space fountain that uses the sun's gravity to return the stream will change the orbit. We need to get very good at space fountains.

Use space fountains to increase Earth's heat loss. It is easy for the mass stream to spend a few hours in space for each round trip, plenty of time to radiate nearly all of the heat to space. The mass stream will also need to spend enough time on earth to pick up significant waste heat so a design modification to a space fountain where the stream loops repeatedly before returning to space allows the mass stream to absorb more waste heat. High temperature superconductors will make space fountains very efficient.

Final Solution

Stop increasing energy use without bound. Slowing the rate of growth helps only in the sense that it delays the negative outcomes. Bitcoin mining is an example of basically unproductive use of energy, i.e., no physical goods or services are created, but if you can make more money than you spend on energy and computers it will occur. With the incredibly cheap energy of MF there will be a number of low-value activities that suddenly become economically viable. An energy tax would reduce or eliminate low-value activity. Ultimately, though there is no further energy consumption possible. Note that some of the short term solutions are mostly intended to reduce the growth rate.

At 5% growth it is only 107 years from a 2 degree temperature rise until the MF energy use matches the energy coming from the sun and 441 years after than the Earth mankind is proucing more heat than the entire output of the sun.

One gram of hydrogen converted to helium releases 3.39E11 joules. Alternatively, 1 MF at full load consumes 46.5 grams of hydrogen per year. It really does not make any sense for MF to have to deal with banana peals, beer cans, etc. as feeding with pure hydrogen it entirely practical given the small quantity of fuel needed. Pure water would be an excellent feed stock.

So how much water? Water is 11.2% hydrogen by weight so a single MF will consume 0.415 kg/year of water, giving off 0.369 kg of oxygen and 0.043 kg of helium and 3 grams are converted into energy (E=MC^2). At the 2 degree global warming level, we are consuming 387 metric tonnes of water per year. A cubic km of water is a gigaton so that is 2580 years worth and the earth has 1.335E09 cubic km of water. Unfortunately the 5% growth rate means that 592 years after that, we consume the entire contents of the oceans in a single year (150 years after we exceed the energy output of the sun)

Einstein allegedly said that compound interest/growth was the powerful force in the universe. Although he likely never said this, he would have been right. Having a economy that must continuously grow in order to prevent collapse (as is true in most if not all modern economies) is indeed foolish.

Why did I answer my own answer? To be honest, none of the other answers dealt with the harsh reality that 5% growth in perpetuity is completely impossible. I did not notice that @rumguff referred to this problem in a comment when I wrote this answer. Since he also has a decent answer re: changing economic incentives I will be accepting his answer.

I would also like to observe that population growth once mankind expands to space would continue and even hosting 10% of the total population on Earth might be a problem eventually. Though growth rates have been observed to decline in modern economies I strongly suspect that the changes of incredibly cheap energy and space expansion would cause a renewed population boom.

@rumguff has apparently abandoned WorldBuilding, so I am accepting my own answer.


Using the thermal energy pumped into the Earth's atmosphere to power a series of lasers. Pointing the lasers offworld (potentially at colonies or ships) can be used to power human expansion across the globe.

OTEC is a reasonable way of stealing heat and converting it back into power at a global level, coupled with large offshore windfarms to pull back energy as warm air from developed landmasses causes climate change. You can build the laser arrays on the roof of the OTEC buildings (as they don't require any cooling towers or aboveground exhausts) and as long as the arrays are built to fire window wavelength lasers you can ship quite a lot of heat offworld.

Sadly: This approach is exorbitantly expensive, both in development costs, construction costs and maintenance costs. The maximal thermal-electrical conversion efficiency for a tropic-based OTEC plant is 7%, so you're going to have to produce an awful lot of OTEC plants to balance this out.

Another (slightly barmy) plan is to scoop up large amounts of water, heat it, load it into rockets (or your very own hovertrain), and fly in a straight line away from the earth, spraying water as you go. The water would cool due to blackbody radiative effects, and thanks to gravity would fall back to earth with less energy than it left with. If your launch mechanism is efficient enough you could theoretically create a lot of these 'heat spikes', increasing the effective radiative surface of the earth and thus cooling it down.

Of course, if you have enough tech and control ability to do that efficiently, you might as well just put up a giant, overengineered parasol at the L1 Lagrangian point and turn down the sun by 5%.

Or you could limit the number of MF modules available on-world such that the heat equation eventually balances out at a temperature you're comfortable with. The lower C02 should compensate for the warming effect by allowing enough of the excess heat to radiate away, at which point you're laughing.

  • $\begingroup$ "Human expansion across the globe" $\endgroup$
    – BlueWizard
    Sep 11, 2015 at 5:31
  • $\begingroup$ Sorry, on this planet we honour the second law of thermodynamics. From the OTEC page (emphasis by me): "Ocean thermal energy conversion (OTEC) uses the temperature difference between cooler deep and warmer shallow or surface seawaters to run a heat engine and produce useful work". Where would be the cold reservoir for converting earth's heat into laser radiation? $\endgroup$
    – celtschk
    Sep 13, 2015 at 9:04
  • $\begingroup$ The deep ocean is the cold reservoir. Below 100 feet the water is roughly 4 degrees. The 'cold' reservoir is that huge volume of cool water. Gravity gives us a hand here by acting as an aquatic Maxwell's Demon. :D $\endgroup$
    – Joe Bloggs
    Sep 14, 2015 at 8:39
  • $\begingroup$ No explanation of how to get that energy from the atmosphere into the ocean. $\endgroup$
    – RonJohn
    Sep 22, 2017 at 18:49
  • $\begingroup$ @RonJohn: You don't (at least, that's not the point). You use the temperature difference between the cold water deep in the ocean and the warm water near the surface to generate electricity, which you then use to power lasers pointing out of the atmosphere. $\endgroup$
    – Joe Bloggs
    Sep 22, 2017 at 21:01

Let's take this a step further than we already have. Let's take this where it was going - everyone has cheap power for their homes and industries, and no one anywhere on the planet has had any need for the grid in any form. In the worst case scenario, absolutely everyone on the planet has totally decommissioned their distribution grids. EVERYTHING runs on a locally installed MF - despite the wild temperatures. The whole planet literally depends on it.

The solution becomes to take a step back.

The basic theory of economics is the accounting for costs. There is, until this point, no one paying for the cost of the heat generation, so no one is doing anything about it. Step one is going to involve regulation and taxation. Heat is the new global warming demon. Money will change hands.

Now what we need to do is rebuild the grid, but probably not the WHOLE grid. This thing is perfect for what we in the power industry call islanding - smaller chunks of the grid that can be operated separately without problems. If we rebuild the local distribution, it turns out a Megawatt of electrical power goes a long way. Lots of units would get mothballed or recycled, and Fusion Industries bails out with its golden parachute after the designs are appropriated by pretty much every government in the world, at gunpoint if necessary.

Now we need to modify the way we USE that power. Here we have box that makes heat and electricity in response to being fed water. The winning design will lock this thing inside of a boiler and just let it run. 80% of this thing's output is pure, high quality heat, and it produces four times more heat than it does electricity. I can safely say recovering and delivering 40% of that is doable. I have then turned a 1 MW unit into, potentially, a 2.6 MW system, with an overall efficiency of around 52%. Limiting this to smaller scale systems and piping steam to local heating loads can further increase system efficiency, and regardless of price, would get very popular as people get absolutely disgusted every time they walk into their backyards past the little box that emits no less than 40 kilowatts into free air whenever it runs.

Demonstrably, the planet has an absolutely insane cooling capacity as-is. Once we get this heat output under control, things will pretty reliably return to normal.

  • $\begingroup$ I think the underlying point is encapsulated by an observation (I read on the internet somewhere) that with a 2.3% annual growth in world energy usage we'd reach planetary boiling point in 400 years due to waste heat (but would all be dead long before that). So at some point the issue of being an energy constrained society bites, regardless of how much extra efficiency you can squeeze out of an MF - that is just a can-kicking exercise. Jevons Paradox states that energy efficiency gains tend to translate into more energy usage, not less. $\endgroup$
    – rumguff
    Sep 10, 2015 at 18:37
  • $\begingroup$ @rumguff, 2 percent growth in waste heat has a 3 degree increase lower limit by the year 2300. Which is awful. But in this extreme scenario, cooling the planet by is being done by efficiency gains and eliminating the vast majority of the units. With greenhouse gases you affect release rate - this problem concerns addition rate, equivalent to about 2 percent increase in heat from the sun. And overall energy use from all sources went down from 2004 to 2011 in the US. The order of magnitude on an industrial process makes these gains absurd. I promise. $\endgroup$
    – user8827
    Sep 10, 2015 at 18:51

Besides encouraging people to go into space or building fantastical radiator schemes, the best way to control the heat is to look at reducing the other source of heat energy; the Sun.

With the amazingly cheap energy of Mr Fusion, mega engineering projects will be quite simple and low cost in today's terms, so building a "sunshade" at the Earth Sun L1 point will be affordable in terms of what resources we need. Putting a sunshade up and reducing the incoming insolation will cool the Earth, and the design of the sunshade could be adjusted to vary the amount of insolation depending on the cooling scenario you desire. Note this isn't a total blockage of the Sun like a solar eclipse, but a reduction of incoming solar energy by 1 or 2%. A translucent shade will work well in this scenario.

As a bonus, the engineering work and resource development will encourage people to move off the Earth to join the Lunar or asteroidal mines or the building crews at L1, bringing people and their Mr Fusion's away from the planet and reducing the heat burden on the Earth.

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    $\begingroup$ The lost starlight is a direct hit to photosynthesis. Plants can't utilise the waste heat from the MFs. The MFs can only be accommodated by a solar shade at the expense of all living things on the planet, including the users of the MF. Basically the low grade waste heat of the MF is accommodated by blocking high grade free energy from the sun, a bad trade. $\endgroup$
    – rumguff
    Sep 10, 2015 at 22:30
  • $\begingroup$ @rumguff What about filtering specifically the 500-650 nm band? Maybe even use it to generate additional electricity in space. $\endgroup$
    – timuzhti
    Oct 14, 2015 at 9:35

1) If you're talking about the Mr. Fusion from Back to the Future I toss the report in the closest trash can. There's no way something that size can dissipate 4MW of heat. It's actual efficiency has to be upwards of 99%

2) If the report is accurate I laugh at radiators. We don't need any fancy schemes to get rid of heat, we just need to turn down the fire a bit. Solar incident radiation is about 1kw/m^2. (Yes, the raw number is higher but some is reflected away. I'm looking at what gets through to warm the planet.) Since the MF is producing 5MW (the amount it produces as power almost all ends up in the environment eventually.) that translates to 5,000 m^2 of solar energy. I impose a tax on all new MF units sufficient to pay for the launch of 5,000 m^2 of sunshade. This sunshade is in the form of a large solar sail craft (or likely multiple craft) that is hovering somewhere to the sunward of the L1 point. It keep station on Earth but is not actually in orbit, it's using the energy being reflected off the sail to hold position. (If it were in orbit the radiation pressure would push it off position. Thus you set it up so the radiation pressure pushes it into position instead.)

You might need to increase this tax somewhat to make up for the decreased agricultural production. Expect yields to drop by something more than the % of the sun that's blocked.

  • $\begingroup$ Of course the actual Mr. Fusion as seen in the movie is an impossible device -- consider that it an amusing literary device that represents a practical and insanely cheap power source. In reality even if were magically 100% efficient, nothing changes at all except that the timeframe for global warming shifts by 33 years. $\endgroup$ Sep 11, 2015 at 6:00

To begin with, a very fast and affordable solution is not to use coal and concentrate on submarine volcanoes, to move the turbines into the ocean. It's a simple solution available with today's technology levels. Free heat and cooling in one.

  • $\begingroup$ Welcome to Worldbuilding, N3znicitelny! We generally try to give a bit more detail in our answers, but you've got some great ideas here- could you expand on some of those theories and add links or references to help others understand what exactly you're proposing? $\endgroup$
    – Dubukay
    Jan 11, 2018 at 6:50
  • $\begingroup$ If we find stable underwater volcanoes we can use Stirling Engines, but work underwater it's more safety and these motors can float, but there is heat 350° C, i'm not sure with temperature? google.cz/… $\endgroup$ Jan 11, 2018 at 10:31
  • $\begingroup$ But i mean if its possible put there floating turbine or some pipes? $\endgroup$ Jan 11, 2018 at 10:56
  • $\begingroup$ The widespread use of MF means that global CO2 emissions have already plummeted. There is no significant use of any carbon fuel any more because it is not cost effective. While charcoal grills, etc. are still used, the global CO2 output is negligible. $\endgroup$ Jan 11, 2018 at 13:47
  • $\begingroup$ Maybe this Nano-spike catalysts convert carbon dioxide directly into ethanol ? $\endgroup$ Jan 12, 2018 at 19:18

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