How can we understand a world with 7 billion people? Population expert Joel Cohen explains the top population trends on a planet with 7 billion inhabitants.

According to figures released by the United Nations in May of 2011, Earth’s human population will cross the seven billion mark on October 31 of 2011.

Revkin said, “A new way of thinking is required. The old messages of ‘woe is me’ and ‘shame on you’ are so 20th century.”

He said three major factors contributing to the decline are lower pregnancy rates, more education for women, and higher income.

How does the 2010 U.S. Census count homeless people and immigrants who might be afraid of being deported?

The U.S. census it a great resource to look at population change,” said Mather, “not only at the national level, but the regional level, state level, and to see what’s happening in local communities all the way down to the city block level.”

Potts said he agrees with the many studies which suggest that empowering women through increased access to education and contraceptives – plus delaying marriage – is key to 21st century peace and stability.

Sachs underscored the complexity of the global systems driving humans’ great and competing needs, and said the quality of our lives – and our children’s lives – depends on making the human population on the planet more sustainable.

Environmental scientist Gadgil is perhaps best known for his inventions, which include a UV water purifier that can provide clean, cheap water for a thousand people a day, and an inexpensive, ultra-efficient cooking stove for families in Darfur.

Listen to Hania Zlotnik, director of the UN population division, talk about the economic challenges for a future with an aging population, and why an aging population is an encouraging sign.

Alex deSherbinin of Columbia University talks about the potential impacts of climate change on humans. Scientists believe climate change may force people in environmentally stressed regions to migrate.

Every year, millions of people migrate from one country or continent to another. Joel Cohen’s mathematical models are used to map patterns in human migration.

Biologist and author Paul Ehrlich talks about the single biggest step forward in all of human history.

Paul Ehrlich of Stanford explains his ideas on why we humans dominate Earth. He says that our genetic evolution gave us things like dextrous fingers and binocular vision, but what put us over the top was our cultural evolution.

A few stone tools and two front teeth, found decades ago, were recently dated by paleoanthropologist Rick Potts and his colleagues. It’s evidence, said Potts, that humans arrived in China from Africa about 1.7 million years ago.

“Even if we’ve made really serious progress in the fight against AIDS, the glass is half full. We still have a long way to go,” said UNAID’s Peter Piot. Piot’s been on the front lines of the fight against AIDS in Africa for three decades.

Microcredit – loans to the poor made without collateral – is catching on in Latin America with the help of the Whole Planet Foundation, according to Muhammad Yunus.

Engineers without Borders works to build things such as systems to collect rainwater and photovoltaic solar cells to provide power in health clinics.

As Earth becomes more urbanized, it be better off, according to George Martine, lead author of the 2007 State of World Population report.