Honestly, climate change was a bad way to go on the "we're all going to die" front.
Ocean acidification is a much juicier target. As carbon dioxide increases in the atmosphere, more of it is dissolved in the oceans. As more carbon dioxide is dissolved in the oceans, the pH of the oceans drops and the acidity increases.
And then everything in the ocean dies and then we all die.
Well, the last couple of parts are extrapolation and up for argument, but the great thing about ocean acidification (so to speak) is that it's indisputable up to a point. We know we're increasing the amount of CO2 in the atmosphere and from there it is high school chemistry that the pH of the ocean will decrease by a certain amount. Granted, most of us didn't pay enough attention to high school chemistry to do the maths ourselves, but it's like two equations.
From there the only question is how bad the rapid acidification of the oceans actually is. We know that corals are probably screwed, because the calcium carbonate reacts poorly to the acidification, but on the other hand the single-celled organisms are probably fine, just because they have tens of thousands of generations to react to the change. Everything in the middle is fair game.
Oh, and ocean acidification is a much more rapid process than climate change, because it takes much less time for the ocean to reach a new equilibrium with the carbon in the atmosphere than the atmosphere to reach a new temperature equilibrium. (Recall that CO2 isn't supposed to increase temperatures directly, it increases the amount of heat retained in the atmosphere from year to year.)
In my opinion, ocean acidification is a much better reason to worry about atmospheric CO2 than climate change