It's no secret that the world's population growth is heavily skewed by region -- so much so that the changes expected for the next century will radically change the world as we know it.

But according to projections recently revised by the United Nations Population Program, those changes will come even faster than we thought.
A data set titled "World Population Prospects, the 2012 Revision" was released this month, and the new figures point to explosive growth in Africa based on new findings on higher fertility rates. The UN has dispensed with the old estimate that Africa's population will triple within 90 years -- instead, it could very well quadruple.
Developed countries, especially in Europe, are more liable to shrink -- an alarming prospect since low fertility rates mean that young, working citizens will be in short supply, and thereby less able to support aging societies. 
The U.S. can rest a little easier; its population is projected to continue to grow at a steady and sustainable rate. But the world's richest country, which now boasts the third-highest population after China and India, is on track to be surpassed by Nigeria as soon as 2050.
The Nigerian story is an incredible one; its population, currently around 170 million, is already Africa's largest and is expected to be eight times greater within a scant 100 years. This presents a massive challenge for the central government, which is keen to promote family planning and already faces poverty rates of more than 60 percent and rising.
Other sub-Saharan African countries -- especially Tanzania, Ethiopia and the Democratic Republic of the Congo -- will face the same challenge, albeit on a smaller scale. Their population estimates for the end of this century hover around 250 million each.
China's population is currently growing, but looks liable to crest in the 2020s due in part to the state's strict reproduction policies and to a slowdown in economic growth. Overall, the number of Chinese citizens is expected to decrease steadily over the next century. That decline won't be enough to see Nigeria surpass it by 2100 -- but it'll be close. Nigeria has a land area of 356,669 square miles (923,768 square km), ten times smaller than China and the U.S.
The winner in terms of sheer numbers will be India, whose population is the second largest now but is projected to exceed China's by about 500 million at the century's end, taking first place.
All told, the revised data brings the projection for the total world population up to 11 billion by 2100, an increase of 800 million from previous estimates.