That former frozen wasteland will become one huge swamp. It'll take a whole lot of work to make it habitable by modern standards. That's assuming it doesn't just dry out and start a decade long peat fire that suffocates the area.
Who said anything about volunteering? Europe couldn't stop ~1 million people from entering in 2015, so what makes you think they can stop 50 to 100 times that number?
> Greenland, Siberia etc. Former frozen wasteland will become habitable
"Billions of Acres of Cropland Lie Within a New Frontier. So Do 100 Years of Carbon Emissions: Crops grown on this land could feed a growing population. But if these soils are plowed, they could unleash as much carbon as the U.S. emits in more than a century." https://insideclimatenews.org/news/12022020/agricultural-fro...