Atmospheric Co2 reduction of 50% by the year 2050.?

Is this reduction based on where we are today with population levels, or where we'll be in 2050 with twice the population of today. If it's the former does this mean that because of the energy demands of the increased population Co2 pollution will actually remain at todays' level? If the latter are they saying that we will need a reduction of 75% on todays' usage to have reduced by 50% in 2050?