if everyone in the world reduced their polluting output by 90% it will take only a few hundred years to be in exactly the same situation. This is why if you look at a person's long term environmental footprint you must include their offspring in the equation. If we follow the "carbon credits" model, each person would be born with a lifetime pollution budget. Those without offspring pretty much get a free pass to do anything. Those with two or more children would have to buy pollution credits to stay above zero. I know this would not be a popular policy, but it is the only one that realistically accounts for the damage done.