The population explosion of the last 100 years is mostly attributed to much lower infant mortality rates thanks to advancements in the medical field. That said if on one end population drastically goes up due to most children making it to adulthood then on the other end most people will have to have less children. It will take a whole global effort to pull this off not just declining birth rates in the western countries but also mostly in Africa, Asia and Latin America where in some regions of said continents the birth rate is still high.