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For context: I'm writing a story where the main antagonist is an empire that builds Dyson swarms around every star they can get their hands on. The protagonist lives on a planet around one such star, and over the course of several years, he watches as the planet's climate changes from one resembling Earth today, to one resembling Earth during the last Ice Age. If the Dyson swarm is completed, his home planet will be rendered utterly uninhabitable, and the protagonist's desire to keep that from happening that is what motivates him throughout the story.

I've heard that blocking roughly 2% of the sun's light would be enough to lower Earth's temperature from where it is now to pre-industrial levels. So what percentage would you need to block to lower the temperature to uninhabitably cold levels?

Also, some related questions: could a Dyson swarm block out enough light to do this, or are Dyson spheres needed? At what point does the dimming of the sun become noticeable to naked eyes on the ground? How would the cooling be affected if only one side of the sun was being partially blocked?

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    $\begingroup$ If the protagonist's society is only a mere one century more advanced than present day Earth, they should be able to build enormous amounts of fusion generators to heat up their planet, and launch sun satellites, giant fusion powered satellites which direct artifical sunlight at the sunward side of their planet to make up for diminished sunlight. Or they could ask to join the society of the Dyson Swarmers. $\endgroup$ Jan 1 at 22:04
  • $\begingroup$ How long have they got? There will be differences between cold enough so that everything dies of hyperthermia, and cold enough that everyone starves because enough cold-sensitive plants and animals have died off. $\endgroup$
    – Mary
    Jan 2 at 15:17
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Thermodynamics will give you an approximate answer here.

A planet without an atmosphere can be reasonably approximated as a black body radiator. The energy thus radiated goes at the 4th power of the temperature--cut the sunlight 16 fold and you halve the temperature--note that the units are Kelvin (or if you're die-hard imperialist, Rankine), not Celsius! Earth's average temperature is 287K. Cut the sunlight 10% and you lower the Earth's temperature by about 5 degrees.

It gets more complex when you add an atmosphere, the Earth gets about 32 degrees of warming from the greenhouse effect. I suspect this will remain basically constant until the cooling starts trashing the ecology.

There's also the matter of time. That 10% would certainly trigger an ice age but it wouldn't happen all at once--the ice would just accumulate faster than it melted and slowly grow, reflecting the sun back and cooling the planet even more.

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It depends a bit on what you mean by uninhabitable.

But having said that, your best bet would be to find some analogue to measure against. Probably the best in this case is to consider the decrease in energy in going north or south from the equator.

According to this source

The Arctic and Antarctic circles only receive 40% of the energy of the equator. So as a very rough guide I would suggest a Dyson sphere blocking 60% of Earths light would eventually make Earth uninhabitable.

But it does depend on the exact habitation conditions and it also depends on the speed of onset and the time frame. A few days would be less of a problem, months and years would be deadly.

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  • $\begingroup$ I'm not sure Arctic/Antarctic climates are a valid parallel here, even for a rough estimate, because reduced insolation at the poles is only part of the reason for their coldness. Polar cells caused by warmer temperatures at smaller latitudes play a big part in dumping frigid air on the poles from high altitudes; if you dimmed the Sun by 60%, even though the equator would now be getting the same amount of sunlight as the poles were pre-dimming, it probably wouldn't get as cold because it won't have the same convection effects. $\endgroup$ Jan 2 at 7:21
  • $\begingroup$ @Geoffrey Brent why not? Hadley cells also operate in the tropics $\endgroup$
    – Slarty
    Jan 2 at 10:34
  • $\begingroup$ Yes, Hadley cells do indeed operate in the tropics, but their effect at the equator is not the same as the effect of polar cells at the poles. The equator is at the "warm low-altitude air about to convect upwards" part of that cycle; the poles are at the "cold high-altitude air descends" part of the cycle. $\endgroup$ Jan 2 at 12:33
  • $\begingroup$ @GeoffreyBrent - tbh, I'd argue it the other way. The poles are X-temperature - which comes from a low level of sunlight but augmented by warmer air from the tropics heating it up. Lowering the sun's energy by 60% to make the equator match the current poles' light amount means it'll be colder: it'll have the same level of sunlight, but have even frostier air from the poles blowing into it. $\endgroup$
    – Kevin
    Jan 2 at 16:07
  • $\begingroup$ Well I can't claim to be an expert so I will not pursue this too far, but I think that polar, Ferrel and Hadley cells all involve hot air raising and cold air descending. $\endgroup$
    – Slarty
    Jan 2 at 20:54
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Happy New Year, and welcome to WBSE. I expect someone will have a better answer than me, soon. But, for now:

Sunlight on a bright day has something like 120,000 lux. Many crops require something like 5 to 10,000 lux a day to grow at a decent rate. So, as you get down to 10% of the sun's light... that means even a bright day is barely enough to manage the crops, so cloud cover could be enough make them weak. Most plants need about 500 lux to survive in general.

So, maybe life could scrounge by with as little as half a percent of light, as far as plants go.

Of course... everything will like die of cold, before you get to that fraction of sunlight. You might want to check out the question and my answer about what would happen if the sun disappeared, for comparison: https://worldbuilding.stackexchange.com/a/100223/35649

Unfortunately, I can only guess at what rate reducing light reduces heat. Maybe halving the light would reduce the world to -10 Fahrenheit? That'd be really devastating, but not unlivable. If it is a linear scale, you'll need to take out a lot of heat before you can get rid of all life. If you just want to get rid of humans... well, my answer shows that it's possible for humans to survive even without the sun, at least for a couple of centuries (under optimal conditions).

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    $\begingroup$ −10 °F is very unlivable, because of starvation. $\endgroup$
    – AlexP
    Jan 1 at 19:53
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    $\begingroup$ @AlexP It often gets as low as -50F in northern China, Mongolia, and Siberia. But people have been living in these places since the dawn of history. Farming might disappear, and you may be forced into a hunter-gatherer lifestyle, but it wouldn't kill all humans, much less all life. $\endgroup$
    – Johnny
    Jan 2 at 5:17
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    $\begingroup$ There is no place on Earth where humans live and the average temperature is −10 °F (−23 °C). "It gets to" and "average temperature" are very different things. Among others, at an average temperature of −10 °F (−23 °C) water is frozen, there is very little if any precipitation, the place is a desert and nothing grows. $\endgroup$
    – AlexP
    Jan 2 at 7:07
  • $\begingroup$ @AlexP Yes... if average Earth temperature is -10F... then the hottest places on Earth, particularly those near the equator, will be considerably more than this? As for how hot it would need to be... the winters average at about -25F, for Yakutsk, and only a little more for Inuit communities in Canada. It commonly hits -30F in the winters for northern China. If you meant average for the year, then that's at about -16.5C or 2.3F, for Grise Fjord, where the summer scarcely gets above 0C. That should be doable with average equatorial temperature, let alone the hottest places on Earth. $\endgroup$
    – Johnny
    Jan 2 at 8:15

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