Actually I was going to ask you that: Why would you care about scientific accuracy when you see what’s out there? Remember the answer to the question:
”Are you smarter than a fifth grader?” Is usually, sadly: no!
If you are limiting the cause to either use solar or greenhouse AND you don’t need to actually combust the entire atmosphere you can come up with something at least passing plausible.
You can have a very large corona mass ejection, better yet, a series of them (spaced at the WORST possible time for the Earth). You can then heat the atmosphere at your leisure. You could take the atmosphere to say 1000 degrees at max. This will allow your story to breath, your other scenario would have been: we are here/we are gone (or saved). Frankly it’s hard to work in the requisite gratuitous sex scenes. Trust me; bad things happen with out the entire atmosphere going up. I believe someone found Fahrenheit 451 to be plenty (and that for only very localized areas). The first one throws out part of the power grid the second the entire Earth's power grid, etc.
In this scenario you can have the atmosphere close to the surface stay cooler so that the menace can be seen descending, for example when planes cruising at an altitude of 34,000 feet suddenly start to fall from the sky. They don’t need to be flying through a fire-filled sky -- aircraft and the pilots in them don’t generally do well in 500 degree heat for any length of time. Set the aircraft on fire but SAVE THE ATMOSPHERE (if not the whales).
Think about it: “Last week/yesterday/whatever we saw a flock of geese way up just catch on fire, today it was a hawk soaring a lot closer to the ground!” [As he gently pulls her down by her auburn hair and…”]
By the way, I get credit for any Large Wooden Badger in the story.