Building a Super Huge city in an Arctic Wasteland is perfectly believable ... if it happened to be the intersection where all nations/cities did trade.
An artic wasteland is essentially an eternally cold desert, a desert without sun. Some of the biggest Metropolis's have been out in the desert .... although I'll admit that most large cities had some sort of river nearby.
Im fairly certain that a LARGE percentage of Rome's food (like 30-60%) came from the colonies along the outer mediteranean, and not Italy herself.
In Elemental, you cannot build cities in remote Wasteland, only revived earth. I think its a silly point of realism to make Global Food only available to Cities that are built along a river/ in a river valley.
One possibly interesting approach is that each Food resource has a three tier Radius. Any cities within 15 tile radius can use maximum food, any cities within a 30 tile radius can use 70% food. Any Cities within a 50 tile radius can use 30% food.
This wouldn't make using food less efficient if its farther away ... merely changes where the Global Food is available.
I would call this style "Mapable Food" as opposed to Global food. Say your in third tier and can only use 3 food. So you build a city in tier 3 of Food Source X (which has 10 food) and you build 3 houses (which use 1 food each). Then ... the global food available in that region is lowered, however all tiers are lowered in that amount, as its transported directly from the source.
What I mean is, if 3 food is used in tier 3, then 4 food would be still available in tier 2 (instead of 7), and 7 food would be available in Tier 1 (instead of 10).
Another example, would be two food sources of 10 food each, and their tier 3's intersect. The food is subtracted in a round robin, starting with the nearest city (if equal space, one is assigned as the primary city in the RNG). So where the tier 3's intersect 6 food is available, otherwise its 3 food, and tier 2 is 7 food, and tier 1 is 10 food. If the city used 3 food, then 2 food would be reduced from all of City Prime's tiers, and 1 food would be reduced from all of City Secondary's tiers.
However, Tiers and Rivers is all bollox to me. I'd much prefer a simply system of global food ... but here is my idea for you purists
(and for you River purists out there, any city along a river would have access to ALL food, in a round robin as if it was in a criss-cross of all the food sources tier 1) ... meaning that a River city using 5 food, if there are 5 food sources ... is using 1 food from each food source.
However, again as I have said ... all terrain in Elemental is (currently) essentially the same, and all the in-game cases I have seen will have much more believable applications than "giant city in the tundra" .... and honestly I would LOVE to see a giant city in the tundra ... Avatar:Air Bender (water tribe), and FFH Doviello or Illians would probably love to see that. It IS a fantasy game after all.