You'd have to take-on organised religion to get this one through.
I agree it's important. In China, these days, the sky is pink, the sea yellow, and the lakes bright green due to pollution. Imagine what the place would be like if they had NOT limited the population with their much-criticised "one child" policy. Without that, there would be an extra 200-300 million mouths to feed today. India has NOT done the same thing and in a little while will become the most populous and miserable country in the world.
Most developed countries are seeing natural population decline due to the individual choices of people not to have large families (or indeed, sometimes not to have children at all). What is required for the world to survive is to pass-on this desire for fewer children to the third world, and THAT requires education and support, imho.
Space/Science » in reply to Best solution I can think of.
Go forth and DON'T multiply...