Imagine that Terra Australis was real, that there really was a vast continent in the Southern Hemisphere like that shown on old maps. It's massive, encompassing what in our world would be Antarctica, Tierra del Fuego, the Kerguelen Plateau, Australia, New Zealand, and most of the Pacific Ocean south of the Tropic of Capricorn.
I envision the existence of Terra Australis creating a kind of alternate history. Although the actual point of divergence, if there is one, would have to be far back in Earth's past, I'd like to have European history be virtually identical up until the Age of Exploration. From a European perspective, the divergence would occur when explorers start finding land which doesn't exist in our world (This would technically be in 1526, but nothing really changes until 1616.).
I know, I know, the butterfly effect: Make one change, especially one this massive, and you can change everything. But I'm not overly concerned about that. This isn't a time travel story after all; it's about a wholly separate constructed world. What I am concerned about is making this world realistic on its own merits.
And what I'm having the most trouble with is the effects of Terra Australis's existence on the climate in the rest of the world. All of the extra land in the south means no Circumpolar Current and Roaring Forties winds to trap cold water and air in the south, which might make the rest of the planet colder. On the other hand, more land and less ocean means less surface water to absorb heat from the sun, which should make the planet hotter (I don't really care about higher temperatures increasing sea levels; I can just decrease the amount of water on Earth.).
Is there a way for me to have these and other climatic effects balance out so that regional climates in the rest of the world are more or less the same as they are in reality? If this can work, then I think that it makes for a very exciting setting for alternate history stories. Thank you.