will the southern usa become tropical because of gobal warming?

It's plausible. Most of the South is already a "humid subtropical climate"; as you would know, if you've ever been there, it already has quite a few hot months out of the year, and quite a lot of rainfall and humidity. One of my most vivid memories of Birmingham, Alabama, was walking into the desert exhibit at the botanical gardens on a hot and humid day, and feeling cooler because of the dehumidifiers they use.

As for the remainder of the South? That would be the tip of the Florida peninsula, which actually has a tropical climate already.

To move from the definition of subtropical climate to tropical climate, an area's coolest winter months would have to average warmer than 18 C (64 F). With global warming, it's possible for the tropical section of the South to expand northward over more of the Florida peninsula.

Maybe in a few thousand years. I don't think us today will live to see it..but I can't say for sure. I'm no scientist.

ALl the same, I don't really believe in Global Warming either, so..

No. climate change will most likely case the south to become colder like the north will the north becomes warmer.

Global Warming wont turn one ecosystem into another. Perhaps after a prolonged period of time it is possible if nature will bounce back but not in our lifetimes.

I don't know about the southern us but the tropical zones of the world are certainly widening

http://www.inscc.utah.edu/~reichler/publications/p...

http://www.atmos.washington.edu/~qfu/Publications/...

http://www.nature.com/ngeo/journal/v1/n1/pdf/ngeo....

I hope so!