Number are relative. There were fashion websites, a couple of years ago (I assume they still exist), that were supposed to calculate approximatively your ecological footprint. As a result, you usually got a message telling you that if every human on earth had the same way of life as yours (meaning living at less than 4 people in a 20m² flat, eating meat once a week, using public transports instead of riding a bike or walking, etc) it would require four or five planets to provide the necessary resources (and I let you guess what it's like if you frequently travel by plane). Of course western countries have developed too greedy habits and will have to change them. But if we consider how the consumption habits of a small part of the world population have damaged ecosystems, it seems we have little choice. We can try to keep these habits and impede other peoples to reach correct living standards (because they have a smaller footprint if they starve and die as soon as possible); or, if we are not comfortable with using the majority of human beings as slaves for providing cheap products for Westerners, and if we accept that people of China and India (and of other countries used as manufactures) have a right to live comfortably and even to drive a car and wear decent clothes and use computers, then we clearly have to reduce drastically the exploitation of natural resources and the amount of our rubbish production. And this can be achieved by reducing the population, and by this way reducing its global needs.
How to reduce global population? According to Malthus, disease and wars are natural unpleasant checks; I don't feel like praising them, and unfortunately they keep doing their horrid job. If China's previous one-child policy was too authoritative to my mind, stopping encouraging large families would be a good start. Instead of paying child benefit that sometimes encourages people to produce children while they can't afford their education, making education free of charge for children already here seems fairer to me. We could imagine some allowance for one-child families only, or for women who don't have children (since it can be harder to measure it for men).
There are plenty of numbers in Bartlett's conference, and he is definitely a better mathematician than I am. I know it's quite long but it's a conscience raiser. I can't help to cite it once more, even if I get boring:
https://www.youtube.com/watch?v=sI1C9DyIi_8