I study this kind of stuff pretty heavily, because I have a teenage son who could (should!) be alive in the last quarter of the 21st century.
The chance that humans will go extinct is close to zero over the next 100 years. Even in the worst case scenarios, thousands of people with access to huge resources will do what it takes to survive in the long-term.
The biggest natural threat I see to society and civilization isn't sea level rise, even though that's going to become a huge problem.
I think there is a good chance that within the next 50 years, it will be very difficult to grow sufficient food for billions of humans. If my concern comes to pass, then billions of humans will die of starvation, and our society and civilization will be utterly changed, though I would not hazard to guess how.
The scenario is straightforward: in the past decade especially, global weather has gotten a lot more wild. We know that it is getting wilder, and many climatologists believe that the rate at which it is getting wild is increasing.
How does wild weather starve billions? Too much rain, too little rain, too much heat, too little heat and too much wind. Everywhere.
I think there's a chance that in the next 50 years, it will be difficult to find plots of earth that aren't exposed to crop killing weather over the course of a growing season.
We're already seeing the beginnings of this, all over the world, though the total human impact hasn't yet been extreme. Indeed, we live in extraordinarily stable times. I believe it's safe to say that the average level of food safety today is higher than it has ever been.
Hopefully I'm wrong, and as the overall global temperature warms, the weather's intensity won't increase, increasingly quickly.
PS: Most of the people reading this will be alive past 2050. Think carefully about how many '100 year' and '500 year' weather events have come about in the past few years. Also note that, 30 years ago when I was a teenager, a '100 year' or '500 year' weather event anywhere in the world would have been big news, because of how uncommon those events were. Last I checked, we've had something like six or seven '100 year' or '500 year' weather events just in the United States, just in the year 2016. My point is that unless you're digging, it's quite likely that you aren't aware of even a majority of such weather events worldwide. They just don't get consistent mainstream coverage anymore.
PPS: I didn't vet this too closely, but it's interesting: https://www.google.com/#q=frequency+of+100+year+storms&tbs=q...