It's multifactorial, fertility is tied to a looooot of things, including but not limited to quality of life, religion, economic prosperity, personal wealth, education, access to contraception, view on having kids,...

Online you'll find many accounts of people not wanting kids because it's too painful/will ruin your body, it's too expensive, it won't let you live your lifestyle, the future will be shit anyways, &c.

Global warming will play a role later but for now it's probably more about our modern lifestyle and individualism. Obesity &co are mechanically reducing fertility but it's a drop in the ocean compared to the fact that people don't want (as many) kids anymore