Note the massive increase in Africa. By 2050, there will likely be over a billion Christians in Africa, due in large part to the increase in population there. Over half of those will likely be in East Africa.
Why? In AD 2000, Christianity in Africa was estimated at about 382 million, or 46% of the continent's population. Christianity is currently growing as a percentage—but at the same time, the population is growing too. By 2050, the population is likely to be 2.5 billion—and any % Christian north of 50% will well exceed a billion people.
Note also the relative size of North American Christianity to the other bars.
The implications are immense.
(Source: Status of Global Mission 2018, Operation World, World Christian Encyclopedia, etc)