Slavery was abolished in England in 1772, in Pennsylvania in 1780, in Canada in 1793 (Upper) and 1803 (Lower), and throughout the British Empire in 1833. The large, politically active advocacy groups against it in the English-speaking world were predominantly religious — Quakers and evangelicals — although secular philosophers such as Mill were opponents of slavery as well.

Slavery in Latin America started downhill when Chile declared no more children would be born slaves ("freedom of wombs") in 1811. Revolutionary France abolished slavery in its New World colonies in 1793-1795, after the Haitian slave uprising of 1791. However, slavery was reestablished by Napoleon; and had to be abolished again in 1848.

Notably, the "cold places abolished slavery first because they didn't have long growing seasons" idea falls flat — most of the tropical New World colonies had abolished slavery well before the U.S. did in 1865.

Some more competent historian would need to do that.