You can read it like this: "With the exception of... (list of disasters)" -- almost the same meaning. Alternatively, the text may be rephrased like this: "Global warming may be the single largest treat to our planet, if you exclude possibility of another world war, giant asteroid, or an incurable plague."

An exception is like a contradiction. It's saying that global warming is the largest threat, but it's also possible that a world war, asteroid or plague could be the largest threat, which would be an exception to the first statement. This "possible exception" phrasing is simply used as a way to compare the severity and danger of global warming to other obviously terrible things like plagues and World Wars.