What's the top soil look like in the US NE? It's been a while since I've been to, say, Maine. But I can tell you that the other new farmland folks propose is British Columbia, and it's obvious that they've never been to mid-to-northern BC and looked down toward their feet. You'd be better off trying to grow corn in sand that the stuff that passes for top soil in BC.
This also ignores the fact that when one starts heading north, that Sun in the sky tends to get pretty weak-ass, even in the summer. As a Seattle resident, I about go blind getting off the plane in Miami until I can get those sunglasses on. And I can tell you how much corn enjoys the lack of good sun in comparison to my home state of Indiana: it doesn't.
The northeast US was a huge agricultural region before the great plains. The entire area is covered with the remains of former farms. It is not as cost effective and there were other issues (rocky soil, for one) but it was once an agricultural center and can become one again if the economics shift.
That an over generalization. The more temperate parts of southern Siberia would have a thawing that could potentially open up more agricultural space and increase Siberia's food production capacity.
Russia benefits, I think.