990
We're not going to terraform Mars, but we're doing a good job of venusforming Earth.
(fediscience.org)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. A showerthought should offer a unique perspective on an ordinary part of life.
The limit is our distance from the Sun. After a certain point the greenhouse gasses can't make up for the fact that we just get less radiation than Venus does. The maximum potential I've seen is 10°C - almost all life would go extinct and we'd have to live on the tropical Antarctic archipelago, but not Venus.