The mechanics of reducing global population to below 50 million in 70 years.

     Step 1) make everyone reversibly sterile (new technology to be developed)
     Step 2) issue 350,000 birth permits (sterility reversals) each year by lottery

As a result of steps one and two
     a) 100 million people a year will die of old age each year for 70 years
          (that equals 7 billion)
     b) 350,000 will be born each year. that equals about 25 million in 70 years.

The population will rapidly decline from 7 billion to a little greater than 25 million
     in 70 years, because the average life span will increase above 70.

If we started today 2081 could be the end of population decline.

The actual decline will be steeper in the beginning of the 70 yr. period.
     There will be additional early deaths from starvation and conflict
          as the rich and powerful attempt to maintain present consumption levels --
     I see a billion deaths before 2020.
                    (These deaths will happen on either the present path or the 350,000 plan.)
     I see 2-3 billion more killed in starvation and conflict before 2030.
          (These deaths will happen on either the present path or the 350,000 plan.)

Step 0) The implementation of "350,000 plan" depends on recruiting half the people
          who live on the earth, to democratically implement a law that implements:
     a) universal reversible sterility.

     b) the lottery for the sterility reversal distribution
          The alternative to the lottery is eugenics. (I am not going there.)

In all scenarios, we exhaust liquid fuels fossil water and
     the human experiment will be forced into a smaller footprint.
          Even, if we find new energy sources there are other resource metrics that
               will encourage us to have a smaller footprint.

Some side aspects of the 350,000 plan.
While TFR at the start of the the "350,000 plan" is very low <.01
it rises every year until in 2080 it will reach 2.0
     It will only rise to 2.0 if enough women want the available permits.
Curtailed fertility will last only 2.5 generations. 70 years.
Pretty much after that everyone can have the number of children they want.
     I expect that in the new civilization, like today's developed world,
          people will tend toward smaller families (1.6 TFR) to facilitate today's life styles.
          There will be more permits available than there are women that want them.

So the challenge is, how do we get half the people to see the 350,000 plan is better for the human experiment (and their kids) than the next best solution. How do we get them to vote for it and campaign for it

If people are afflicted by an unbreakable bias to stay on the present course, then it is time to go drink some merlot. However, if the bias is breakable let us charge forward with all manner of cognitive pitchforks.