Logistics of coping
One of my heroes, Otto Jespersen, suggested that a problem is best canvassed from all sides. One approach not yet noted in the comments is the need for a larger brain to handle the 'logistics' of coping with climate extremes. Much like the argument that complex social relationships required larger brains it can be argued that temperate climates require a larger brain to cope with maintaining a healthy body temperature over a broad, seasonal climate change.
If wide temperature variations promote bigger brains to handle the logistics then peoples of temperate climates might show increased brain size compared to people of extreme but less fluctuating climates. Just a thought, but it's worth noting I once attempted to develop a "fat-assed" theory of increased brain size in our species by postulating our big brains evolved as a counter weight allowing us to walk upright. In light of our plight seeing our brains as big ass ballast seems fitting.