I need some sort of event that will cause immediate global warming in the course of one summer/fall - to the point that it will not freeze come winter in the arctic and it will happen so fast it will surprise the world. What sort of event can you think of, outside of deus ex machina?
Umm. What kind of story is this? You could always blame it on space aliens with superior technology.
Barring that, basically you'd have to find a way to dump a whole lot of carbon dioxide into the atmosphere. A whole lot. Enough to overwhelm all the balances that are presently in place.
I used to have a book, Out of Thin Air by Peter Ward, that explained how the Mesozoic got to be as warm as it did. I no longer have it, so going entirely by memory here, but basically it was because all the accumulated plant detritus during the Carboniferous (which was called that for a reason...) suddenly released their stored carbon into the atmosphere. I think he said it was a really big fire.
So... maybe in your case, find a way to burn every single plant in the entire world?