Suppose that 6.5 billion people on the planet did as they were asked and reduced their carbon production. The claim is that human CO2 production is contributing to disastrous global warming.
Let's suppose that each individual person's contribution to the earth's CO2 is cut in half on average. That is excellent! It sounds almost impossible, but suppose anyway.
And now suppose that the number of people living on earth doubles to 13 billion. -- Half the carbon footprint on average per person, but twice as many people. Unfortunately, we are then right back where we started with the same amount of carbon loading to the earth by humans.
According to the United Nations, we could actually be at double the present population in eighty years, although leveling off at 7 or 9 billion are seen as possible alternative scenarios. Success will require that the population numbers come in on the lower side. We can hope!
Reference: http://www.carbonrationing.org.uk/file/images/act-on-co2-logo