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I had posted a question a couple of days ago asking if it would be possible for an artificial satellite/cosmic body at the L1 orbit to block out the Sun, but the logistics of that is a little too extreme. So I'm changing tacks now, and handwaving the actual blocking of the sun by different means.

In my initial inquiry, the celestial body was a Lovecraftian entity from another world and something similar will be occurring here too, but in this case, the rift between worlds will be letting out a strange black mist from the other side that floats up into the atmosphere and effectively starts blocking out sunlight and darkening the sky.

The desired effects: start with the sky getting darker, akin to what it looks like during sunset/twilight hours. As the mist spreads, I'd assume that it gets colder as well, which would lead to disruptions in the ecosystem, and psychologically affect human beings. I want the light to be cut to a level where it would hit the biopsphere at a fundamental level and cause mass extinctions around the world to all but the hardiest animals. Let's say that the mist is some kind of supercharged version of a reflective aerosol (sulfur dioxide maybe?)

So essentially, I don't want to block out the sun entirely, but drastically reduce the energy arriving to Earth to lower worldwide temperatures, permanent dark skies and ecosystem getting messed up.

What I'm looking for is - how much would the global average temperature need to be reduced to in order to achieve these effects?

EDIT: Adding some details. I want it cold enough where even sub-tropical areas start to receive snow (Algeria, Israel, El Salvador etc.) and temperatures drop to the single digits at the Equator. Waterways start freezing over, and both terrestrial and aquatic plants start to die off, creating a devastating chain of death throughout the food chain.

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  • $\begingroup$ without details about how cold you want to make it this is unanswerable there is a very wide range of ways to alter he temprature, it has even happened naturally in earths history. $\endgroup$
    – John
    Commented Apr 8, 2020 at 19:52
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    $\begingroup$ @John, the OP is not asking for ways to alter the temperature but rather how much the temperature needs to chaing to snow at the equator and kill of almost all terrestrial and aquatic life. I don't have the science to answer that, but I fear he won't like the answer since long before cold could wipe out all but the most rugged of the aquatic life, everything above the surface (including us despite all our technology) would be frozen dead. I don't get the idea that he is planning to write the story with whales and giant squid as the main characters. $\endgroup$
    – Henry Taylor
    Commented Apr 8, 2020 at 20:13
  • $\begingroup$ You will have a devastating chain of death long before you get to single digit equatorial temperatures, basically you are asking how to get another snowball earth. Which has already been asked before worldbuilding.stackexchange.com/questions/61101/… $\endgroup$
    – John
    Commented Apr 8, 2020 at 20:17
  • $\begingroup$ This does not sound as bad as "Snowball Earth" to me, but definitely more intense than the last Ice Age, which seen about 6°C average temperature decrease. $\endgroup$
    – Alexander
    Commented Apr 8, 2020 at 20:21
  • $\begingroup$ @HenryTaylor you're definitely on the right track. I want a state of winter and darkness, but warm enough where small bands of human survivors are present, barely getting by $\endgroup$
    – Space_Cadet
    Commented Apr 8, 2020 at 20:30

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You probably want to look at the data for the eruption of Mount Tambora. Also known as "Year Without a Summer" and "Eighteen Hundred and Froze To Death." In July and August, Pennsylvania had lake and river ice, and Taiwan -- not just subtropical, but tropical -- had snow. Famine occurred all over the earth.

This was caused by a drop estimated at 0.4–0.7 °C. Also it was caused by volcanic ash in the air, which would add other similarities. (Though it sounds like you want to omit its spectacular sunsets and sunrises.)

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