Speculation on population numbers raise questions about whether the planet can sustain us all.
The world's population is expected to hit seven billion this year.
By 2100, there will likely be more than 10 billion people living on Earth.
After 2100, the population level will probably plateau.
The world's population is on track to hit seven billion this year, which is double the number of people that lived on Earth in the 1960s, but far from what the future holds. By 2100, according to recent projections by the United Nations, we'll hit the 10 billion mark.
Those numbers, which are drastically larger than anything the Earth has experienced before, have sparked concerns about how all of those people will impact the world. They have also raised questions about whether the planet can sustain us all in the first place.
There may, however, be at least some end in sight to the relentless swelling of population pressure. Around the end of the century, many demographers believe, the global population will gradually level off.
Researchers can't predict with certainty exactly when that will happen and at what level. Also up for debate is how the current level of population growth will impact the environment, the economy and quality of life.
Overall, though, the level of rapid population growth we are experiencing today cannot be considered a good thing, said John Bongaarts, a demographer at the Population Council, a research organization in New York City.
In some parts of Africa, for example, population is doubling every 20 years, making it impossible for communities there to keep up with the growing demand for housing, roads, schools and health clinics. To many experts, those kinds of issues highlight the need for a global-wide investment in family planning programs that provide women education and access to contraception.
"Every billion people we add to the planet makes life more difficult for everyone and will do more damage to the environment," Bongaarts said. "Can we support 10 billion people? Probably. But we would all be better off with a smaller population."
The multiplication of people on the planet wasn't always so explosive, according to a series of papers in this week's Science. Growth started to accelerate with industrialization around 1750, said Ron Lee, a demographer and economist at the University of California, Berkeley.
By 1800, global population reached one billion for the first time. It took another 125 years to reach two billion. After that, though, numbers rose from three to seven billion in just the last 50 years. The population growth rate reached a peak of two percent per year in the mid 1960s, before declining to today's annual growth rate of 1.1 percent.
No one can predict the future, but the U.N. has done a good job in the past of estimating population sizes for several decades forward. By 2050, its predictions range from 8.1 billion to 10.6 billion. For 2100, projections range from 6.2 billion to 15.8 billion.
There's nothing magic about the 10 billion number," Lee said. "On the other hand, there's pretty good agreement to expect something like this leveling off."
Longer life spans and lower death rates help explain why population size is growing at its current pace. But the variable that will make the biggest difference in how many people will live on Earth 100 years from now is fertility rate, or the number of babies that women give birth to.
If every woman had two babies, the world's population would remain stable. Today, there is a global average of 2.5 births per woman -- down from five in 1950. That comes with huge geographical variation.
In Japan, China and Europe, women are having fewer than two babies, while women in many developing countries are still having five or more. Ninety-seven percent of the projected population increase over the next century is expected to happen in developing nations, according to a review article in Science by David Bloom, of the Harvard School of Public Health in Boston. Nearly half of the growth will be in Africa.
Women who matter more to society and are given access to education, according to previous research, end up having fewer children. Informing people about contraception and making it available also make a big difference, and not just in places like Africa, where a disproportionately large population of young people is exacerbating problems like school overcrowding and unemployment. In the United States, Bongaarts said, about 15 percent of births are unwanted.
Even as the global population begins to level off in the coming decades, experts are already expressing concern about the environmental and economic consequences of stuffing so many people onto the planet. Parts of the world are running out of water. Prices of food and energy continue to rise.
"It's not clear how this is going to sort out," Lee said. "That's what I'm worried about."