But it's not just warming, which of course chilly Canadians and half-frozen Russians would probably enjoy in itself.  Warming causes drought,  and the widening of disease vectors. Wild fires become more common and the fire season expands. Paradoxically, when rain does come in a warmer world, it all comes at once, and flooding is not particularly conducive to agriculture . . . or human habitation for that matter.

Also, bugs love heat, including agricultural and arboreal pests. There are thousands of acres of dead forest in BC, for instance, because winters there are no longer cold enough and long enough to kill off  pine bark beetle infestations. Eventually, those forests will burn, expelling more GHG into the atmosphere.

Finally, a lot of the Canadian and Russian land that would supposedly become arable in a warmer climate consists of thin, rocky, impoverished subarctic soils, that, simply put, are never going to be farmland, at least not on a human timescale.