Without enough forests, Plankton and diatoms humans won't be able to breathe

Or more precisely said, "We would be able to breathe but wouldn't get enough oxygen to prevent us from passing out and then dying."

So, this is one of the problems of Global Warming in that if all the plankton and the diatoms in the oceans die (especially starting around the equator as the oceans heat up) then people will start to die around the equator too unless they live near forests.

A secondary problem regarding oceans is that EVERYTHING alive in the oceans comes from the smallest things eating plankton and diatoms. So, without plankton and diatoms everything in the ocean from the smallest to the largest will die too. It's just like with people. The things that eat plants are the way everything else stays alive. Without plants in the oceans (plankton) everything dies in the ocean. Without plants and trees on land, everything dies on land. And this is also regarding both food and oxygen.

So, I'm thinking the last place people likely will still be alive with enough oxygen to breathe likely would be closer to the north pole (Russia, Mongolia, Canada, Alaska) and the South pole on Antarctica. However, this likely would be later this century or the next caused by global warming. So, wherever there still are enough forests still alive and making oxygen some people will still be able to breathe.

However, technologically, almost anything could be possible but this may or may not include Trees, diatoms, plankton, trees humans, animals birds etc.

In other words it is possible to terraform earth into just about anything humans (or whatever other species might be here in the future) might want it to be.

So, in the meantime as Antarctica melts off I think you will see people of all nations beginning to plant forests there so they can stay alive on into the future there.

Remember, 1 million years ago the average temperature at the north pole was 76 degrees Fahrenheit.
So, it is possible that also was the average temperature at the South pole around then too.

And learning about this isn't necessarily depressing because: "Imagine you are alive in World War II and Hiroshima and Nagasaki just got nuked." What would you think then? You would think that humans are just about done on earth maybe by the late 1940s or early 1950s just like most people in the U.S. thought then too. IF you had grown up in the 1950s like I did it was pretty depressing by the way when adults thought like this. Many many people couldn't deal with the Cold War after World War II and just committed suicide instead between 1945 and 1960 or 1970 by the way. So, difficult futures were seen then too. But, somehow we made it this far. So, it's important to be innovative in your thinking about how we will all solve this problem too.

However, here we are now over 70 years later and still haven't nuked ourselves out of existence (at least on this timeline).