Ah yes, those Haitians and Congolese

Your medieval-colonial theory of violence holds false against evidence.

Brazil's two mega-cities, Rio de Janeiro and Sao Paulo, have distinct winter seasons, clearly less warm than their summers. Brazil's Northeast has only summer, year-round - and a very hot one.

You are much more likely to be shot on the streets of Rio and Sao Paulo than on the streets of Recife and Pernambuco.

And what about Singapore? It is probably the safest country on Earth. Care to guess its weather? Hint: It lies almost on the Equator.

And what about the Polynesian Pacific Islands? Every single Polynesian society has been described as quite, erm, pacific - except for one, where warriors were honored and privileged. Funny that the Maori are the Polynesians with the coldest climate

Of course, if we're speaking history, maybe I should mention that Scandinavia before the 20th century was better known for its Viking raiders (who took away Sicily and southern Italy from Greeks and Arabs, not exactly known for their skiing skills), than for its fluffy welfare state. But that might be too much

But yeah whatever, those tropical Barbarians are destruction-bent hooligans

And you can apply it in reverse too. If Scandinavia's record suicide rates were due to cold, Canada should be even more suicide prone. Canada (except BC and a small patch next to the Great Lakes) has longer and colder winters than Scandinavia. Yet suicide in Canada is certainly not that high.

Russia also works as a counterexample: Both crime and suicide exploded right after the demise of the Soviet Union, peaking during Yeltsin's last period. Both have steadily descended during Putin and Medvedev, but both remain considerably higher than during the USSR. Not even Al Gore would argue that the Russian climate has undergone the rollercoaster you are proposing. Yet it is colder than Scandinavia in most places, and will remain so, no matter how much or how little climate changes.