There is a huge inequality in the world that we know. Where we are blessed by the many chances and opportunities we get, whilst others are severely limited in their opportunities. Well, how does this relate to sustainability?
We all strive for equal living chances in the world. If we then take a look at our current ecological footprint, and multiply this style of living according to the number of inhabitants of planet earth, we can see that the current ecological basis will never be able to suffice. If everybody on earth had the same ecological footprint as the average North-American, we would need 4.1 planet earth's just to supply everyone with this life quality.
(http://www.popsci.com/environment/article/2012-10/daily-infographic-if-everyone-lived-american-how-many-earths-would-we-need)
If we all keep striving for equality and maintaining the current living standard, then we should drastically improve our ecological footprints. How can we do this? First, must be willing to change the focus of our economic development and world capitalism. Production and consumption are key in this change. "The challenge is to improve the overall environmental performance of products throughout their life-cycle, to boost the demand for better products and production technologies and to help consumers in making informed choices."
(http://ec.europa.eu/environment/eussd/escp_en.htm)
Even with lowered ecological footprints, we can seriously doubt if mankind can be entirely sustainable with the amount of human inhabitants the world has now. Therefore, the second change, would be to focus on the reduction (preventing growth) of the human population. China, for example, enforced a one child policy. Aside from the many problems that this induced, the fault here is that population growth does not only relate to the amount of births. It also has to do with how much the babies are spaced apart.
If every woman has a child on her 20th birthday, in a 100 years, there will be five babies. With increasing life expectancy, this means that the population will grow anyway, even with this one-child policy. Delaying the age at with a woman gives birth however, does seem to be a more effective way of controlling population. If a woman chooses to give birth on her 25th birthday, it means that in a 100 years, 4 babies would be born. Effectively, this would mean that one person could have one extra baby, whilst not affecting the population growth directly.