Do you think humans will ever be willing to adopt a global birth limit?

I think the problem with such a thing would be that it would be really hard to enforce and would create a free-rider problem at the country level. That is to say, even though everyone might be better off if all countries tried to do such a thing, each country would reap benefits by violating or failing to enforce the policy within their own borders if everyone else was following it. Another problem is that the economic hardships associated with such policies would be unevenly distributed globally. It would be a lot tougher for poorer countries, especially those that have largely agricultural economies.