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So essentially in my story, aliens put something around the sun that doesn't stop all the light from it hitting Earth, but only some of it, effectively dimming the light without outright extinguishing it.

What I'm looking for is a level of reduced brightness that will kill off most crops but leave the plants vital for oxygen still alive. What would this brightness level be, and what would it look like?

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  • $\begingroup$ It is somewhat difficult to predict, but decrease as little as 1% in solar irradiance is capable of pushing the Earth into an ice age. This won't happen overnight, and not even in one year. For quick "winter" effect you'll need a decrease of 10% or more. The supply of oxygen in the atmosphere will not be a concern for at least 100 years. $\endgroup$ – Alexander Mar 28 '18 at 23:23
  • $\begingroup$ In support of @Alexander's comment, in the northern hemisphere, our winter occurs at Earth's perihelion - when it's closest to the Sun. Basically, our nearly circular orbit is irrelevant to the seasons. It's the axial tilt: shorter days + sun's lower in the sky = cold. The amount of energy hitting Earth hasn't changed significantly, but it's cold. Drop sunlight by 1% and what do you get? Honkin' cold. Drop it 10% Blue nosed moose cold. You can't get a survivable dark earth without a thicker atmosphere (hold heat) and different biology (more efficient or no photosynthesis). $\endgroup$ – JBH Mar 28 '18 at 23:52
  • $\begingroup$ It's going to be a lot easier to build something that's near the Earth and does what you want rather than something near the Sun, because (surprise) the Sun is huge compared to the Earth. But with such a tech you could build an Orbital (like I M Bank's Culture novels), so why bother with Earth at all ? $\endgroup$ – StephenG Mar 29 '18 at 0:41
  • $\begingroup$ I am writing a story like that too! Remind me to award a bounty to the best answer for this question :) $\endgroup$ – Renan Mar 29 '18 at 12:20
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About 2200 lumens, of 550nm light would be the cut-off point after which absorbance goes below levels that would allow plant life. Now, if you go a little below this(and are assuming in all cases that the dimming mechanism doesn't affect the wavelength of sun's light) then only certain oxygen generating algae will survive. But I think the aliens will find that desirable, cos now all land crops are dead, (unless supported by artificial lights in grow roomsin which case they're thriving), but the humans are still surviving on oxygen from algae, and food from the oceans.

Gradually, natural selection will promote mutations that allow for lower intensity light to be able to support new plants, and life might keep going on normally, albeit after a couple millennia of trajectory changing.

(Un)fortunately, there's not much research on this, as it's not a very practical question for institutions to use their funding on. Most photosynthesis-light intensity based studies are on finding the maximum intensity after which more increases don't yield proportional gains in yield.

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Guten Morgen ziehr eirthlings. (for purpose of this answer we assume that Nazis on moon are those aliens). We've put a photo filter between earth and sun, an we blozked ze red light.

They block wavelength form 500 to 650 nm. You cannot block entire visible light as, unfortunately, chlorophyll production is in the same place as growth and fruit production. But when lowering the amount of said wavelength you would force plants to focus rather on surviving than giving fruits.

Simple experiment I made last summer, My balcony was on sunset side of building with extra red blocker on rail. Tomatoes I've planted grown quite tall but gave around 5 flowers per bush and resulted in 2 tomatoes per bush. Tomatoes where the size of large olive.

The result would be a world more blueish, with almost no orange sunset.

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