[Calvin]

There's a book by Ian M. Banks called Feersum Ennjin which has an incoming interplanetary dust cloud as one of the major plot points for the book. The idea being that it's substantial enough to block a substantial amount of sunlight, which does all sorts of bad things to crops.

If you wanted a hard(ish) sci-fi way of having a thick enough dust cloud for your needs, you might set the campaign during such an event. The dust might not stick around very long on a stellar level, but that could still mean several generations of people living with the dust cloud as a reality.

Granted, being thick enough to make agriculture much more difficult probably goes against your desire to avoid "unforeseen/undesirable consequences", but you could tone it down a little.

As for a scientific basis for reducing the modifiers to detect something, I couldn't really give you a proper answer.