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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.

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    $\begingroup$ In a first order approximation the two hemispheres have separate climatic systems. Of course, if we go into second order effects things change in interesting and unpredictable ways, but by and large it would not be completely unbelievable that a large-ish southern continent would lurk with no ill effects on history. $\endgroup$ – AlexP Jul 27 '20 at 11:39
  • $\begingroup$ I think this would also shut down the Gulf Stream, which would drop European temperatures quite significantly. $\endgroup$ – Michael Richardson Jul 27 '20 at 15:39
  • $\begingroup$ @Michael Richardson Not sure it would. The North Atlantic gyre doesn't seem to go much further South than the West African bulge. $\endgroup$ – Bloke Down The Pub Jul 27 '20 at 21:38
  • $\begingroup$ @BlokeDownThePub A significant amount of cold, briny, water sinks in the North Atlantic and then makes it way all the way south to Antarctica, where most of it goes to the Pacific, with a smaller amount split off to the basin of the Indian Ocean. The massive heat sink that is the Pacific Ocean is heated by the Sun and expands, which causes its surface to flow out of the Pacific basin, mostly through the Philippines. Water along the surface eventually flows back into the Southern Atlantic, which is the source of the Gulf Stream current. Splitting the ocean basins stops all this. $\endgroup$ – Michael Richardson Jul 28 '20 at 14:24
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I think that increasing the size of Antarctica and linking it with Australia (which I think is what you have planned) would have major ramifications for the Earths climate due to the lack of circumpolar circulation.

However in "god mode" most of the effects could be counteracted by introducing counterbalancing factors, which may or may not be of interest to you. As god you have some fairly sizeable levers that can be pulled for example the Earth's axial tilt could be adjusted slightly or the Earth's distance from the sun by a few thousand miles. The density of the atmosphere could be 1% greater or 1% less, CO2 concentration different or the intensity of the sun could be a little greater or a little less. The solar cycle could operate over a different period than 11 years with a greater or lesser effect. Any of these things or a combination of them could conspire to mitigate the effect of such a large continent.

The system is so complex and there are so many variables that you will never come to a definitive answer. Using a climate model might help but even those are only gross approximations. I wonder how many climate models would allow you to change the solar cycle, the axial tilt of the Earth and the pressure of the atmosphere? And even if they could would the answer to accurate?

How much detail do you want to go into and do the mitigating circumstances have to be obvious for the story to work?

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  • $\begingroup$ I don't need to explain it in-story. I was just wondering if the situation which I described was scientifically plausible. I don't want my Europeans to be aware of anything which would set their world apart from ours until they start finding the extra land. So the less obvious the better. :) $\endgroup$ – Mark Morales II Jul 28 '20 at 7:01
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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?

Climate is a chaotic system with complex and interlinked feedbacks and feedforwards. The way climatologists do their studies is basically:

  • create a model containing the elements they want to analyze and those which they think are related/influenced
  • run that model several times with slight changes of the initial parameters
  • analyze the trends they observe coming out from the simulations
  • if possible compare those with experimental observations

That already gives a wide range of outcomes.

What you are asking is basically the above for a new planet where you don't have any historical data nor the possibility of verifying the model.

Unless you want to venture into a PhD in climate science, I would say that getting a sound understanding of the detailed climate consequences of having a large meridional continent is kind of overshooting, unless the climate is of paramount importance for your story.

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    $\begingroup$ Also worth noting that as climate is such a complex subject you can ignore any changes and nobody but the aforementioned climate scientists will be able to definitively say you’re wrong. $\endgroup$ – Joe Bloggs Jul 27 '20 at 8:24

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