Before 1900... hmmm.
A series of global conflicts in the 1800s leads to the atom bomb being developed by 1880, at which point it starts being used on everyone, until half the world is radiated and Antarctica is miraculously a cool, temperate paradise.
Oh right, you cannot change the climate yada yada.
Or, humans evolve the ability to breathe antarctic temperature air, grow enormous oily coats of fur, and at the same time, develop a hybrid of corn that grows optimally at -100 degrees F.
Oh yeah, no PODs before 1000BC.
So at 1000BC, King Solomon or whoever says, "Somebody please invent me a steam engine! These human workers are not cutting it!" So they do. By 1900, several Fusion reactors are up and running on the Antarctic continent, providing heat, light, and power to thousands of people each in nice climate controlled domes. They are connected by high speed underground trains to each other and to some offshore artificial islands that act as ports.
Or yeah, island hopping del Fuegans build a tiny colony there... thus Colonizing Antarctica!