50 million hectares

That is the same amount as currently planted with GM crops! 

www.europabio.org/articles/article_111_EN.pdf

South Australia is 100 000 000 square kilometers, and mostly desert. So if you grow algae and pump into into the ocean sediments you can sequester the amount of CO2 that humans produced historically in 100 years.

And hooray for that.

(they maybe a few other consideration outside of land....)