Summary: We can't ever know for sure what the future holds. What we can know is the worst case scenario. If that scenario is bad enough, then we MUST take action to reduce that risk, regardless of what the chances are.
Let's say you're handed a gun with six chambers. You have no clue how many bullets are in the chambers. You are told we have to put the gun to your head and pull the trigger. You have the choice to either (A) do it now, or (B) first pull the trigger once while pointing at the wall, then spin the chamber and point the gun at your head. Which do you do? Of course you should fire at the wall first - if it was an empty chamber you spin it and then point it at yourself, losing nothing; if it was a full chamber you have emptied it, spin it, and then point it at yourself and have gained one additional empty chamber.
What if you were told you could either (A) pull the trigger now, or (B) take 10 lashes, empty one chamber, and then point it at your head? I would still take choice (B). 10 lashes will not kill me. I have no clue how many bullets are in those six chambers, it could be that they're ALL full and my only chance of survival is emptying one of them in exchange for the lashes. It's not worth that risk of my life, so long as the cost (10 lashes) is not enough to kill me either.
That's what this guy is arguing about climate change - even if we had no clue about whether it was happening, the cost of trying to reduce it just in case is so much less than the potential consequences, that we must take that choice.
And there's his various replies here.