magic, but loki doesn't put much stock in that

i don't know how much of this is a spoiler. None of this should be material that the show hasn't had a chance to explain, but its been long enough that I don't remember completely.
Spoiler:

Their summer has lasted ~10 years, and their last winter/summer cycle was really short and mild. so short that the summer before the current one was called a false spring. I think the kingdoms agreed that 12 years of wheat minimum is what people should be storing. Winters are hard, but not north of the wall hard. I got the impression that the winters are hard enough to kill fields, but not all the greens and thus not all of the animals, and the winters weren't so hard that green houses didn't work (they are mentioned as life savers in the book). The Stark place was on a thermal vent and had hot springs too.

The veterans say that the new recruits still smell of summer, which I think is Martins way of saying that fewer and fewer people are in power that remember and have the ability to plan for winter as they need to be.