World Population Reaches 8 Billion, U.N. Says

Until 1804, fewer than one billion people roamed our planet. More than a century later, in 1927, we crossed two billion.

Since then, the world population has shot up in the shape of a hockey stick, boosted by the triumphs of modern medicine and public health.

The latest marker was passed on Tuesday, when the United Nations said the world population had reached eight billion, just 11 years after it passed seven billion. (It is an inexact number, since there is no official count, but the international organization said its projections crossed the line on Tuesday.)

The growth rate, which is expected to slow globally over the coming decades, has been uneven around the world. Slowing growth rates in populous nations like China and the United States have caused some alarm, threatening to upend their societies. Rising birthrates in poorer nations threaten to strain systems that are already struggling.

Here are a few of the challenges ahead.

Much of the population growth comes from the poorest countries, especially in sub-Saharan Africa.

About 70 percent of the growth to eight billion from seven billion happened in low- and lower-middle-income countries, most of which are in sub-Saharan Africa, the United Nations said. The trend is expected to become even more pronounced in the years ahead.

“When the next billion is added between 2022 and 2037, these two groups of countries are expected to account for more than 90 percent of global growth,” the organization said.