I started to write a story that is about a huge and catastrophic climate change. The story is narrated as a series of unconnected episodes (in large part) that have different protagonists from around the world who testify to the consequences of such a cataclysm. I do not want a 100% scientifically correct story, but I would like, in principle, to have everything in the narrative is not completely impossible but only very unlikely.
As the trigger for this climatic cataclysm I first hypothesized the classic shutdown of Thermohaline Circulation then a huge methane eruption but in the end these two events would not work fast enough, and would not be quite catastrophic enough for the scenario I imagine (yes, I like to exaggerate). Meteorites and super-eruptions intense enough to cause the change I imagine would have to be more catastrophic than climate change they cause. So I chose a more exotic and dramatic idea: a drastic change in the Earth's orbit. I imagine a retreat of the Earth from the Sun by about 20 million kilometers.
Now I wonder, what could cause such a change in orbit (without notice and without completely expelling the earth from the solar system if at all possible) and how fast it could happen (I would like to see events take less than a decade, but I do not know if this is possible).
I hypothesized that the passage of a wandering black hole or a cluster of high-density dark matter near the solar system could cause this perturbation of the orbit (which I know, though infinitely improbable, are not impossible events) but I would like more clarifications.
Lastly, I wonder, given that at that distance from the sun the Earth's temperature would drop by about 15°C and that the poles would cool much faster than the rest of the planet, what would be the effects on weather? I expect (at least in the short and medium term) a large increase in storms and in general extreme weather caused by the response of the atmosphere to this massive imbalance (among the various events I would like to include massive Arctic storms similar but scientifically more accurate to those portrayed in the film "The Day After Tomorrow").
I know it's a long and articulated question and my English is not the best (I'm Italian) so I apologize.