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We are developing a game setting and, while discussing certain aspects of the it, have stumbled upon our inability to adequately account for some phenomena due to our lack of knowledge. The thing is, in wake of a catastrophe the lower layers of the world’s atmosphere are filled with a dark almost opaque substance, thus an expansive region of the planet hardly gets any sunlight. We are uncertain to what extent such a hindrance to natural light would cool the surface and whether it is conceivably possible for a substance to only block the visible spectrum, while still allowing some portion of the heat to come through (as infrared rays or whatever it is that normally transmits it). We would like to arrive at a bearable science-ish explanation, so your comments on the relevant physical, environmental and chemical processes are welcome.

Basically, we want for climate to stay relatively the same, somehow, even with constant dusk.

EDIT

I think we may have asked the question in a slightly misleading way. The world we're trying to describe is not a 100% alternative Earth, as many of the suggestions seem to assume, but more of a fantasy/steampunk alternative world. So many of the problems may be solved by saying "because magic". On the other hand, we wanted to have enough understanding of the science behind the relevant natural processes so that we could plausibly explain how the population desperately attempted to at least partially fix the effects of the calamity by magical means (e.g., based on your suggestions, magically modifying wheat and some other plants, which turned dark grey/black in order to be able to absorb the remnants of the sunlight; red tint of the light coming through the substance in the sky also seems appealing for providing a more gothic undertone).

The substance blocking the sunlight is basically a result of a failed magical experiment and, as we plan it to be right now, mostly immaterial (basically, as we understand based on your suggestions, it should block 80-90% of all light sans the orange and red spectrum and above, so that the plants can somehow still photosynthesize after having been tampered with).

In the end, we want the world to be a dark and a slightly gothic one, but at the same time with populace more on the optimistic and pragmatic side (even if in the first, say 10-100 years after the calamity there were many losses and conflicts), which has found ways to influence their surrounding's bad conditions (adaptation of the flora; possibly coping with drops in temperature?). Basically, we're not very fond of desperation and decadence.

Also, the substance in question will cover approximately half of the planet, gradually thinning as we move away from the epicenter.

EDIT

And the world is supposed to have two suns.

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    $\begingroup$ If your atmosphere is opaque, it will absorb sunlight and make your surface a burning hot hell probably hotter than the surface of Venus. If your atmosphere is reflecting all the light back before reaching the surface, you would be looking at a surface temperature comparable with that of Pluto. $\endgroup$ – A. C. A. C. Nov 17 '17 at 20:40
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    $\begingroup$ So what you want is to add an opaque cover over the planet but not change the albedo (en.wikipedia.org/wiki/Albedo) with your long term low level cloud. This would depend on the albedo of the substance in the lower atmosphere. For this to be temperature neutral sounds unlikely but not impossible. Lack of surface sunlight produces other problems but that is not in scope. $\endgroup$ – P Chapman Nov 17 '17 at 20:56
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    $\begingroup$ @PChapman could you please also elaborate on the effect of the lack of sunlight, please? We're complete newbies in this, and would still like to make the description of the setting plausible enough. $\endgroup$ – Exander Nov 17 '17 at 21:10
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    $\begingroup$ @Exander I was thinking specifically of the lack of sunlight for plants. $\endgroup$ – P Chapman Nov 17 '17 at 21:12
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    $\begingroup$ In my experience with laser cutting, different materials absorb or reflect different wavelengths. Mirrors don't reflect all wavelengths and clear acrylic absorbs (invisible) CO2 laser light really well. You might need to bother a materials science type to get a good answer on this one. Good luck! $\endgroup$ – computercarguy Nov 17 '17 at 22:05
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It's definitely conceivable.

Light, as we see it, is in the wavelength band of 390 to 700 nm. This is a rather narrow band of light, but it's the only band you need to reflect to make your world dark. All the rest can come in as normal to heat the planet. And you wouldn't even need to completely reflect all the visible light. You wanted constant dusk, so you only need to stop 70-90%(ish) of the light from reaching Earth. The rest could be reflected... Or absorbed. (For more about that, see articles describing global warming.) When volcanoes erupt, they spew a bunch of reflective stuff into the atmosphere. In big eruptions, this usually lowers the global temperature for a year or so.

Great, there's some background. Now how do I do it?

The first thing you'd wanna do is set up a big black cloud in the lower atmosphere, preferably of a greenhouse gas. This will absorb light from the sun and heat up. Unfortunately, as A. C. A. C. pointed out in comments, this absorbs a lot of heat, so much that your planet will cook. Well, how do you fix that? Mirrors. Right above the black cloud, if you set up another cloud, but this time use something like sulfur to block the light This will allow only a set percentage of the light to hit your black fog.

So... You've successfully destroyed our beautiful sky. What's that gonna do to the planet? (I swear, I plead NPOV.) For one, your plants will change. If they don't die outright, they'll turn black. Since visible light is in such short supply, they'll try to capture as much of it as possible. Let's assume that your plants survived without a significant hit to the $O_2$ in the atmosphere. Great, what's next? Wind. As your cloud stops most of the light from reaching Earth, but traps heat, the ground won't radiate heat like it does today. This will alter or stop the cyclical motion of the lowest atmosphere, causing weird effects.

In summary

While it is definitely possible to selectively alter the climate (indeed, we can do it today!) be careful of unintended effects that result from it.

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  • $\begingroup$ Your big black could be black on the bottom, like big black clouds are. But viewed from the top these clouds are fluffy and white, reflecting the sun. It looks so sunny and nice when you are in a plane descending into one of these clouds and then on the bottom it is dark and raining. Like that, but more. Also, the warm rain gives a mechanism to bring warmth down from the cloud to the surface. $\endgroup$ – Willk Nov 18 '17 at 2:10
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Google "Nuclear Winter" including the quotes. The concensus was that a dust cloud heats at the top, creating a thermal inversion. The bulk of the material was soot from all the fires started by the bombs. Some heat gets down by radiation, but the number of interactions is sufficient that overall it is a cooling effect.

Practical tests: Mount Pinutubo dropped global temperatures for a degree for a year. Krakatoa was followed by "The year without a summer"

I've been downwind from serious forest fires -- sufficient that the sky was overcast to the point the sun wasn't visible. No heating during the day. No cooling at night.

Note: You can choose any combination of twilight and fog. For fog -- stuff disappearing in the distant (or nearby) haze you the particles are in the surface atmosphere. Give them a permanent static charge to keep them from settling. To have dusk but clear air the murk has to be higher up.

The climate below is going to be much more even -- The layer blocks outgoing as well as incoming light and infrared. Since surfaces don't absorb as much light, there is goign to be a lot less evaporation. You will get a much drier climate.

Since mass transport of parcels of hot air rising is greatly reduced, wind speed will be far less. Sailing ships will take much longer to get anywhere.

If you arne't blocking all the light, I'd expect temperatures to drop on the average 5-10 degrees -- half that of a nuclear winter. You can mitigate that by adding some extra greenhouse gasses.

But your world has no agriculture. Drop the sunlight below about 30% and crops won't mature. Oxygen productioon is way down. CO2 climbs as the bacteria keep decomposing. Don't know how long it would take for the CO2 to climb to lethal levels.

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Let's combine a few different effects. You want dark fog? You got dark fog. Great for "atmosphere" (cue psychotic laughter). So you've blocked most visible light, but IR can still come through, and any visible light that makes it will be toward the deep red. Scary place!

So, what about the heat effect? Not to worry, planet was just swinging into a glacial period anyway. Phew!

Still, there's a problem. If this change came on suddenly, plants may not be happy. There'd've been some huge die-off; only those plants lucky enough to be able to cope with missing the blue wavelengths will survive. But this sounds like a pretty dystopian world anyway.

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What you describe is like the particulate matter suspended after a volcanic eruption. The substance itself need not be black for it to scatter or reflect light. Darkness is confered by the absence of light at the eye. Your climate will be affected by the amount of energy that is directly reflected or scattered away. You should expect a severe cooling of the surface temperature with no visible light making it there to be reemitted as infra red radiation for anyone to feel. The analogy to this is the drop of temperature at night time despite the presence of greenhouse gases in our atmosphere. Greenhouse gases contribute to heating by trapping IR radiation, which doesn't happen as much if the sunlight never reaches the Earth for the visible light to be dissipated as IR radiation in the first place. The IR portion of sunlight would still have atmospheric heating effects and UV would still reach the surface. You may be able to retain more of that energy with additional greenhouse gasses, but your surface climate will not be the sames as before.

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Depends on the way the light gets "extinguished". The extreme scenarios:

  1. absorption - the stuff absorbs the light and "store" the entire energy - think "black body". Gets hotter (the absorbed energy is converted lower frequency spectrum + increased thermal agitation). Cannot get hotter than the source of the radiation (the planet's star) without violating the 2nd law of thermodynamics - otherwise you'd get heat transfer from a colder body to a hotter one.**

  2. diffusion - i.e. reflective scattering. For a perfect scattering (no energy absorbed by the particles), you'll have an extremely cold planet and experience a drop in the radiation you can observe as you descend into the stuff (very likely an exponential decay; perfect but random-direction reflection on the particles, the light passing through a thickness of D into the stuff will result in x% of the radiation being reflected back. The next D will reflect back another x% and so on).

Pick any combination of the two phenomena and you can get quite a big range of possible temperatures.


** an interesting thing about "can't get hotter than the source of radiation" is the laser. Yeap, you can cu steel with it and the laser won't melt. That's because, from the point of view of thermodynamics, the laser have a negative absolute temperature. Even more weird is that a negative absolute temperature is higher than the infinite positive temperature. And the highest temperature one can asymptotically approach is negative zero Kelvin. https://en.wikipedia.org/wiki/Negative_temperature#Lasers

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