If the NW passage is relatively open, some cities on Earth are flooded, some areas are uninhabitably hot... And as it has been pointed out, swampy ground isn't really useful for major economic activity, either.
Not at 1-1.5 degrees C. There is no major sea level rise. There is minimal, if any, uninhabitably hot areas. Now, yes, 1-1.5 degrees C is an unrealistically low amount of warming compared to what will probably be happening, but it's an amount we'd want to keep the climate at.
One effect is an increase in wheat yields in places like Canada, which can be exported through the now more open port of Churchill.