One aspect about the possibility of runaway climate change needs to be clarified. The basic mechanism through which it could take place is akin to a feedback loop in a sound system: a small initial warming gets amplified through a feedback, producing more warming in a manner that itself generates even more warming. For such a loop to occur, the feedback effect needs to be quite strong.
Stefan–Boltzmann’s law expresses this mathematically. For an intuitive appreciation, consider the difference between bank lending and a nuclear chain reaction. In an idealized case, a bank would draw from the savings of customers to make a loan. The recipient of that loan might then put part of it in the bank, and the bank may then make additional loans on the basis of that. The total lending of the bank becomes larger than the original loan, but to a non-infinite extent. By contrast, each time an atom of uranium splits in a runaway chain reaction, it releases neutrons that cause more than one other atom to split as well. The result is a reaction occurring at an ever-increasing rate.
It is quite possible that genuine runaway climate change is not possible on Earth – that the existing feedbacks are of the bank lending rather than the nuclear blast variety. That being said, the possibility of warming itself producing further warming remains extremely worrisome. It wouldn’t require ever-escalating temperatures for climate change to be globally devastating. Quite probably, any warming of more than 5˚C would deserve the adjective. The most credible climatic models project approximately that level of warming by 2100, if emissions continue to increase at the present rate.