[soko]

He is essentially saying:

He is either saying there is a 50% chance global warming will be significant enough to eradicate the human race. and a 50% chance that nothing bad will happen.

Or even the smallest incalculable fraction of a chance of human extinction warrants an entire overhaul of the world policies to stop climate change at any expense that is less catastrophic than t6he potential climate change even though the chance of it happening could be less than 1 in a million.

He is using the same logic George Bush used to get us in to Iraq. Taking preemptive action without all the facts in order to avoid any potential danger.