Let's Hope That Never Happens...Mars is, apart from our Moon, our nearest celestial neighbour. It is also a feasible option for terraforming [despite plenty of massive hurdles]. So if something did go massively wrong on Earth [man-made or natural], we would probably have to make the best of things on Mars.
The current plan from NASA [I think we can probably ignore the MarsOne venture] is to attempt a manned mission to mars by 2030. We dont think we will see expansive terraforming of Mars until at least 2050 and probably 2100, depending on the state of space business and international cooperation. Sealed cities?… maybe by 2300.
We advocate getting a proper framework for safe space commerce and exploration … a New Space Treaty and UN trusteeship.