> We need to reduce the number of humans on Earth.
I'm happy to point out that this is not the case.
We're already peak child and the next two billion will not come from population growth but from population replacement[0] (Thanks, Hans. RIP, I loved you), where children replace the people in previous cohorts that never made it to reproductive age.
The number of humans will reduce rather drastically in the next fifty years. Many countries on earth already and most countries on earth in about twenty years will have problems maintaining their population as it is. Fertility has been dropping all over the globe, with a few exceptions in sub-Saharan Africa.
No, really. There's nothing we can do about the people we already have and there are barely any new babies arriving.
What we do need to worry about is population migrations. We need to make it easier for people to migrate. We need to set up political and social infrastructure to welcome people from places that climate change has made inhospitable.
Since most of the world will be suffering from over aging and from a lack of births, these migrants will be dearly needed.
So, no need to talk about birth control, let's talk about immigration reform instead!