The problem is not that we couldn't improve cities to sustain -30 Celsius winters. This is doable. The problem is that places suitable for agriculture would vanish in the upper half of the continent. Canadians need to import many of their food and they are like 40 million people, Europe would need to find a way to feed 500 million people.
Cities resilience to war will be a concern much before climate resilience.
Good thing we've been working on beating photosynthesis efficiency of turning sunlight into carbohydrates (e.g., sugar) by using special electrolysis cells to synthesize plant-accessible carbon-nutrients (one kinda-successful (in beating natural leaves with PV based sunlight electricity and their electrochemistry)) attempt focused on generating a nutrient solution rich in acetate from CO2 feed (and electricity); that's one of the energy carrier molecules that existing plant biology can efficiently utilize.
And note that by exploiting winter time solar yield and the grow houses with such technology not using sunlight directly and thus allowing easy and efficient thermal insulation, production could run year-round just with the amount of grow houses "online" varying with the average solar yield. Also gives convenient off-times (if they rotate which building runs through the winter) for maintenance.
And eating less meat or at least feeding the meat animals efficiently produced (but not as "tasty/varied/textured") plant/algae(/possibly-microbiological with bacteria or yeast) feed instead of things like Alfalfa that are very resource intensive to farm, would also help a lot with these issues.
Probably we should get back to doing grain bunkers as strategic reserves, though, lasting long enough to realize the problem, slaughter 80% of livestock, and plant low-failure-risk human suitable food crops on farmland that previously had feed crops for those no longer living 80% and land that had things like corn dedicated for ethanol that's then added to gasoline.
Ideally ofc. we'd not need to cull as much life stock and could instead just slightly reduce new breeding and switch those corn fields to human food.
We have beaten it but only for an output that's approximately "green smoothie".
We're already better than just LED grow lights for e.g. lettuce, though, from how I recall the linked paper.
Unsure how you conclude that cold outdoors necessitate more meat though; aside from some micronutrients vegan diet works just fine so reverting back to "meat only on Sundays" 1910 style household dinner menu structures would work fine...
Most people could, for example, do with replacing the caloric intake from half the portion of any non-vegan food they consume, with potato.
If outdoor farming breaks from climate meat will get more expensive...
Vertical solar panels mounted like fence panels between fence posts high enough to not get covered by snowdrifts will produce electricity year-round; and we only need around 3x as much solar farm currently to power sealed grow "basements" than the same plant output would need if just farmed on an open field (assuming climate is matched and weather is average). The benefit of it is just that the panels can be other places (like noise barrier walls next to an urban highway) than the plants and the climate can be freely picked. Also no unlucky correlated dry spell weather events that can cut 100 million people's worth of harvest yield down to half or worse.
Grow houses won't fail collectively as much as farming weather is correlated across a region.
Pretty much just need to keep measures around to prevent the plants from freezing just because the electric grid falls over for 3 days in winter.
Some foods yes, there are huge fisheries, but not agriculture. (I admit that should had been more precise in my post)
After the British Columbia University web, Canada still seems to import like a 75% of the fruits and a 50% of the vegetables eaten by Canadians (excluding potatoes). Apparently, this is one of the problems with the tariffs.
The problem is not that we couldn't improve cities to sustain -30 Celsius winters. This is doable. The problem is that places suitable for agriculture would vanish in the upper half of the continent. Canadians need to import many of their food and they are like 40 million people, Europe would need to find a way to feed 500 million people.
Cities resilience to war will be a concern much before climate resilience.