It won't be wise - would it ? - to use the EARTH as a laboratory to prove up the concept of what a runaway greenhouse effect can do to a planet.
We don't have to - not that the libertarians/deniers/SF fans among us aren't doing their darnedest to try anyway.
Instead we can just consider the latest science on Venus, a planet somewhat close to us in features, where a classic case of runaway greenhouse gases destroyed any hopes of Life on a planet somewhat similar to ours.