The birth rate is below replacement level in pretty nearly every developed country and a bunch of the developing countries. We are in a bit of a race. Who knows what minerals are available in space. Getting out there is tough, but getting them back to earth is somewhat easier (if a bit hazardous, as we'd be dropping rocks into a well while standing at the bottom of said well). Educate women and explore space. Does our population fall before or after we run out of resources?