People make decisions; not populations.
People are having less kids because they are very expensive and infant mortality is a sad story in our past that isn’t (thank God) happening any longer. The birthrate is going down even in the least developed countries.
In any case, the decision to have kids, or one kid, or no kids is mostly in the hands of the parents (except in China, which has truly screwed over the country with the older one child enforcement).
BUT—it’s not the atmosphere, nature, lack of resources or global warming... It’s hundreds of millions of people deciding.
Interesting to note that people are not having kids in the developed world while they are having them in poor nations.
“...less kids...”
Fewer?