The damming of a river that feeds the world’s largest desert lake could lead not only to less drinking water for thousands of Kenyans, but international conflict between tribes for what little water remains.

Population projections undergird many important policy decisions, from the U.S. government’s Feed the Future program to the Sustainable Development Goals. But they’re not as straightforward as they appear. Demographers often base their estimates on complicated assumptions that aren’t obvious to the end user.

Much of the time, discussion about climate change is focused on the future – How bad will it be? Will it lead to more conflict? Who will be most vulnerable? But it is in fact a current phenomenon. The climate system is already, for all intents and purposes, irrevocably changed and millions of lives have been changed along with it.

“Valuing women, paying greater attention to women’s rights – that’s the solution to our population growth issues and, I would argue, it’s also the solution to our food security challenges,” said Suzanne Petroni at the Thought for Food Summit in Berlin last year.

An estimated 1.8 billion people today are between the ages of 10 and 24 and 85 percent of them live in developing economies and/or fragile states. Such youthful age structures can lead to a number of challenges, including increased potential for instability, and countries with large numbers of young people must find ways to address their unique needs.

As presidents, prime ministers, and other policymakers from across the continent gather in Washington, DC, this week for the first-ever U.S.-Africa Leaders Summit, what are the issues to watch?

World Population Day began in 1987 after public imagination was sparked by the idea that there could be 5 billion people on Earth. Today, we’re well past 7 billion and according to the latest UN projections, headed north of 9 billion by mid-century.