Also, the sea ice itself is much fresher than the ocean, so it's melting temperature is much closer to 0C than you might expect. And even the distinction between freezing seawater and freshwater at the surface is less then 2C, so it's not a huge difference.
I can't think of an obvious way that acidification factors in, unless there are important connections between sea ice and sensitive biological factors (I'm not aware of any, but I'll wait to be corrected).
> Also, the sea ice itself is much fresher than the ocean, so it's melting temperature is much closer to 0C than you might expect
It's not just about the sea ice melting, but also the sea ice forming. How does sea ice become "fresher" than the salt water that it formed from? How is that processing being affected?
> ... is less then 2C, so it's not a huge difference.
That's really context dependent.
> can't think of an obvious way that acidification factors in
When you dissolve substances in a solution, it changes the melting/freezing points. This is how salt works to de-ice roads during the winter. The ocean is becoming more acidic because the concentrations of other substances in it are increasing. This can affect the freezing point required to form sea ice (or for existing sea ice to melt).
I'm not sure how, or to what extent, which is why I posed it more as food for thought.
I can't think of an obvious way that acidification factors in, unless there are important connections between sea ice and sensitive biological factors (I'm not aware of any, but I'll wait to be corrected).