My bet's on climate change if anything, because all the evidence I've seen points to us having ruined the climate too much to be able to hold back the big bitch slap that's coming (with global warming, you hit a certain "point of no return" where warming becomes a positive feedback loop, and we are either rapidly approaching that point, or have already passed it depending on whose paper you're reading).
I don't think it'll be fast, mass extiction events rarely are. We'll see a lot of key species going extinct and a lot of ecosystems going tits up, especially marine systems due to ocean acidification (I've got some great papers on that if anyone's interested in reading). We're going to have to get a lot more conscious of how we use our resources, especially when it comes to overfishing, because our current consumption rates aren't sustainable. It's going to get really hard to keep feeding people at the rates we do now, and as the climate continues to change, we're going to have to rework where we're cultivating specific crops, likely with big population relocation to suit rising food and transportation costs of said food. I'm personally imagining a big flight out of California as it becomes too arid for productive agriculture.
As food gets harder and more expensive to grow, costs will rise and people will inevitably go hungry. Maybe we'll see political unrest over things like food scarcity or fresh water reserves (which are also being consumed at a very unsustainable rate). Richer nations will obviously be able to float over these sorts of issues for slightly longer, but I think it'll start getting to everyone in the end.
Not sure if we'll actually see human extinction out of this in particular, but I wouldn't rule it out. Don't bother apocalypse prepping, we're going to go out with a whimper, not a blockbuster-worthy bang.
I'm not an atmospheric scientist, but my studies have overlapped into the field quite a bit, and this was my takeaway from all of that. Make of that what you will.
