ABOUT 500 BILLION SHOULD DO IT
Scientists at the Carnegie Institution have found that if we removed 100 billion tons of carbon-dioxide from the atmosphere the average global temperature would drop only 0.16 degrees Celsius.
Which means that to reverse the damage we have done to the planet we would have to remove about 500 billion tons.
Even if we knew how to do that it would take many lifetimes.
Full report at ScienceDaily.