Is 2015's record average temperature evidence that we are now experiencing runaway climate change?
Runaway climate change is, given our current state of knowledge, only something that could be confirmed in historic context - in the rear-view mirror. Inconveniently, there's likely to be a much-diminished version of human civilisation around to observe it, if and when it does happen.

In other words, it's too early to tell if we've passed a catastrophic tipping point.

We don't know what the equilibrium state of the Earth's atmosphere is. It will take 30 years for our current stock of GHG emissions to fully show its effects: and we'll be continuing to release GHGs for a while, so the equilibrium will continue to change.

But it's worth bearing in mind what we can do, when we really have to; World War II saw massive realignments both in industrial production and in expectations within a very short space of time: after decades of delay, it now seems likely that decarbonisation will require at least a similarly fast and large industrial realignment; and it is possible.