The thing about farming in the Arctic is you're gonna have a short growing season no matter what the temperature is. However, you're not going to get that far,

The thing about farming in the Arctic is you're gonna have a short growing season no matter what the temperature is. However, you're not going to get that far, because you're talking about an area that is substantially underlaid by permafrost. When that melts, it subsides and usually fills with water, which is why there are millions of boggy lakes on the North Slope of Alaska. Fertile topsoil is not just dirt, it's a complex ecosystem of its own, and it takes a great deal of time to produce. It's probably slightly easier to start farming in the melted Arctic than in your average desert, but it's a pretty near thing, and your 5C difference is going to wreak havoc on current farming areas. The odds of success on this one are not good. I think there's probably some limit to how much Earth-resources should be used to terraform other planets, but with what we spend on bombs I don't think it'd be a big deal. Terraforming Earth is a spectacularly bad idea, so it would take a lot of spending to be in any sense worse than that.