Having found a setting for my story, I am thinking of a country which encompasses a giant New Zealand, basically a partially surfaced Zealandia, and having studied a little science, I have noticed that sea levels and polar, and non-polar ice sheets are often intertwined.
When you think of it, ice actually is solid water, frozen away from the ocean, when it melts, it turns into water and rejoins the ocean, but is unlikely to evaporate since the climate has yet never reached beyond 30° beyond the Arctic and Antarctic circles.
If ice accumulates at the poles, then that means water turns into ice and is not water anymore, so sea levels get lower, if the ice melts, it turns into water, now there's more water than before, sea levels are now higher.
Now to the plan, I am thinking of throwing a satellite in space to shield the southern hemisphere, shielding everything at least 1 kilometer below New Guinea, so that the ice sheet in Antarctica get bigger and expand further.
My ideal would be to resurface parts of Zealandia and other parts of the South hemisphere still underwater, at least 1,500 meters of water turned into ice, that means a sea level 1,500 meters lower than now. The Earth is also not a perfect sphere, and sea levels are influenced principally by natural forces, such as continents moving, currents and temperatures.
My hypothetical and desired results would be that the Australian subtropical ridge gets pushed away over higher latitudes, the Australian ridge being the reason most of Australia gets so dry since it passes over in the middle, so Australia becomes more fertile.
The increase of the Antarctic ice sheets would lower sea levels, thus increasing the amount of land in the south hemisphere, that would resurface huge parts of Oceania, southern Africa as a whole and the southern cone.
My last desired effect would be a much cooler south hemisphere, Australia and southern Africa become more oceanic and Zealandia and Patagonia become either more continental or subpolar, although a form of agriculture would be worked on in my scenario.
I still don't know a lot about this kind of thing, so I couldn't provide all the infos, but I think I explained what was necessary. Lastly, this question ignores the effects on the Northern hemisphere, and the supposed or real climate change has been reversed.
My question is: How would the change (The south hemisphere 5 degrees colder), if successful, influence the climate, landscape and geography of the whole entire South hemisphere, more particularly the temperate regions?