Although population growth rates are falling, the population size has actually remained to be increasing. This is due to the exponential rate of size increasement which occurs as the size of the population gets larger. This can be easily demonstrated using a number sequence. If we assume a constant population growth rate of 2X, the number sequence is: 1 - 2 - 4 - 8 - 16 - 32 - 64 - 128 - ...

In practice a population increase by 2X means that a (monogamous) couple (hence consisting of 2 people) would have 4 children (we hence assume that the parents die over time). A population growth rate of 1X would mean 2 children in practice. A sustainable population number is estimated by many environmental organisation at about 2 billion people. Although this number is discutable, there is a general concensus the population number should be (allot) lower than what it is now (7 billion people). As such, in practice, this would mean that would mean that the population size should be falling (which isn't the case now, at all, see the table at this wikipedia article) and the amount of children per (monogamous) couple should be 2 or less (ie 1 child). This, as the population growth rate needs to become 1X or less than 1X. If we assume a 1X population rate (2 children) starting at the population size we currently attained (7 billion, but let's assume 8 for convenience), the sequence would be: 1 - 2 - 4 - 8 - 8 - ...

If we assume a 0,75X population rate (1,5 children) starting at the population size we currently attained (7 billion, but let's assume 8 for convenience), the sequence would be: 1 - 2 - 4 - 8 - 6 - 4,5 - ...

If we assume a 0,5X population rate (1 child) starting at the population size we currently attained (7 billion, but let's assume 8 for convenience), the sequence would be: 1 - 2 - 4 - 8 - 4 - ...