Hemispheric considerations.
The Southern Hemisphere is mostly water, and the Antarctic icecap is miles thick. In the event of a runaway greenhouse phenomenon, these factors would tend to moderate weather and climate in the Southern Ocean just when conditions were becoming intolerable in the Northern land masses.
Suddenly, Down Under would be attracting a great deal of unwanted immigration from the North. It is interesting to speculate on what effect this would have on global politics.