1) The source doesn't combine Guangzhou, Shenzhen, and surrounding cities like citypopualtion.de does. Otherwise Guangzhou would be here.
2) We can be pretty confident about projections made for the next 30 years.
3) However, on this time scale, projections will probably be very inaccurate. 80 years out, population growth matters a lot. Blantyre currently has a population of around 1 million, and is projected to grow to 40 million.
The second big aspect is the oil supply : even optimistic estimates don't think the supplies will last for more than 50 years (and it is probable we are currently nearing peak oil). Without more oil supply global trade will diminish (or even collapse) thus leading to stravation and wars for resources. I'd say we can go on with business as usual for 10 years but after that nothing's too certain. But a population of 20B people is just impossible, whatever happens something will stop us to grow that big (be it ourselves, shortage on food or diseases)...
"Projecting city growth, typified by Lagos growing from 10.6 million in 2010 to 88.3 million in 2100, calls for a healthy measure of scepticism; many variables could change. Projections used here are linked to the 2050 WUP city estimates that, especially in Africa, are suspect, and here these projections are extended for yet another 50 years, suggesting caution in their use. . . . Niamey, Niger for example is projected to grow from under 1 million inhabitants today to 56 million in 2100. Growth rates like these appear overstated, especially in one of the world’s poorest countries with an insufficient economic base to support this rate of urbanization."
(The updated paper is here: https://journals.sagepub.com/doi/full/10.1177/0956247816663557)
I mean seriously these numbers must assume political or economic changes, with which would probably come reduced natural increase.
It reminds me of end of 19th century predictions of the amount of horse manure for the upcoming 20th century...
Lagos (2100): Hold my beer
But seriously, it would be awesome to look at this quiz again in 2100 and compare.
yes I am from kabul