It should be interesting to see when the world achieves below replacement fertility levels and how soon/ when the total world population begins to decline and stabilises at what level, if it does. Will that level be sustainable? Will we have , then, already degraded the environment / overused our resources beyond repair or renewal? Or is there some hope that population will stabilise at levels of high incomes, high education and awareness about sustainable living and general good quality of life? Higher happiness quotients?