Is aggression correlated to the climate? Researchers from the Vrije Universiteit Amsterdam and Ohio State University seem to think so (read the press release and the preprint of the paper).
They put forward a supposedly new model titled CLimate, Aggression, and Self control in Humans (CLASH). I can't comment on the merits and novelty of this approach, but it reminds me of Montesquieu's remarks in The Spirit of the Laws (book XIV, chapter II):
In northern countries we meet with a people who have few vices, many virtues, a great share of frankness and sincerity. If we draw near the south we fancy ourselves removed from all morality; the strongest passions multiply all manner of crimes, every one endeavouring to take what advantage he can over his neighbour, in order to encourage those passions.