It is perfectly reasonable use of the word for public discourse, and it's explicitly outlines.

It's not? Our externalities might literally put some nations completely underwater. How much would it cost to raise the entire Maldives by 1 meter?

Might is a big word. Climate change might cause the Earth to turn into Venus, or it might just involve nation's doubling thir seawall budgets.

Climate studies have wide error bars.

And you might die tomorrow, so stop saving for retirement?

I would imagine the error bars on a 1m sea rise to be a lot narrower than the error bars for Earth turning into Venus.

You almost had a good point, but we know for a fact that simply doubling sea wall budgets is a dramatically inadequate response to climate change.

For example, obviously that won't do anything to mitigate the temperature increases, agricultural problems, or ecological damage.

What is your point? In this case "might" is used quite responsibly and arguably, understates the likely-hood. While the error bars on how much ocean level rise we will see are fairly large, the chances that some (small island) nations are going to be put completely underwater are fairly close to guaranteed.