If I were to give a speach to the United Nations, this is what I would say.
Our world's overpopulation problem evolves over time. Now, it's becoming less of a problem of birth rates and more of a problem of migration.
Birthrates are still high in a handful of countries, like Nigeria, but overall, world birthrates are declining. Today's big problem is that there are many countries which are run by authoritarian governments so an overwhelming number of people are desperately trying to take refuge in the countries, such as the US and Canada, who are still relatively safe to live in.
Religious bigotry, war, oppression, climate change and famine are making much of the world into hellholes that more and more people are striving to escape.
There is quite a bit of fear, in more livable places like the US and Europe, about immigration as large numbers of refugees overwhelm existing resources; thus leading some native born folks to resort to rightwing, zenophobic politics.
The study of population is sometimes called "demographics." While world population is over 8 billion, demographics is still sending us warning messages about declining populations in some countries. Population decline can lead to economic problems as in, "who's going to pay for the retirement of older generations if there is a shortage of young workers?"
Russia is a prime example of this problem with it's rapidly declining population. Who wants to live in that country with it's anti gay bigotry, war and censorship? The best and brightest people are fleeing, for their lives, from Russia; inventors, entrepreneurs and all trying to get out.
They are fleeing to other former Soviet Republics, such as the Republic of Georgia. They are fleeing the war and trying to get into Europe, the US, Canada and Australia.
Talented folks are fleeing from many Third World countries as well, for similar reasons and to escape poverty. When it comes to entroprenurism and innovation, many countries of the world are experiencing "brain drain."
If the US had more open borders, we would have plenty of workers to pay into our retirements, but there are devastating shortages of housing. The American way of life would lead to even more tremendous automobile gridlock of traffic. There would be more shortages of fresh water and more impact on the environment.
Meanwhile, around the world, famine, war and oppression is becoming so severe that the work of aid agencies is becoming more futile; like trying to bail out the ocean with a tea cup.