Isis getting a nuke is quite far fetched. What is a bigger danger is one of the terrorist groups in Pakistan (which actually has nuclear weapons) getting a nuke. Or India & Pakistan having a nuclear exchange. Or one of the wars in the Middle East leading to Israel or the USA to use the nuclear weapons they have ready there (which they continually threaten). Nuclear war is one of the scariest things we face and could destroy us instantly, it's a small chance but a possibility.
However climate change is definitely already happening and is potentially also devastating for the survival of the species. It could itself ignite a number of conflicts, we have no idea what will happen, but we know it could be very bad.
Actually since Israel's nuclear weapons are a secret they are not continually threatening to use them, however the US is a different story. There are US nuclear weapons present in every war and the threats are made with statements like, "no option is off the table", and by looking at official policy, which maintains the option of a first nuclear strike.
However climate change is definitely already happening and is potentially also devastating for the survival of the species. It could itself ignite a number of conflicts, we have no idea what will happen, but we know it could be very bad.