The unity in diversity of the United States is one of the country's greatest strengths.

Climate change...is, simply, the greatest collective challenge we face as a human family.

Our world is one of terrible contradictions. Plenty of food, but one billion people go hungry. Lavish lifestyles for a few, but poverty for too many others. Huge advances in medicine while mothers die every day in childbirth, and children die every day from drinking dirty water. Billions spent on weapons to kill people instead of keeping them safe.

We have seen in the past where leaders have not been open-minded or have not been flexible. That's why the people came out to the streets and shouted and chanted for more reforms and more freedoms.

One of my earliest memories is walking up a muddy road into the mountains. It was raining. Behind me, my village was burning. When there was school, it was under a tree. Then the United Nations came. They fed me, my family, my community.

On this International Day for the Abolition of Slavery let us reaffirm the inherent dignity of all men, women and children. And let us redouble our efforts to build societies in which slavery truly is a term for the history books.

You are here to step into the shoes of UN ambassadors - to draft resolutions, to plot strategy, to negotiate with your allies as well as your adversaries. Your goal may be to resolve a conflict, to cope with a natural disaster or to bring nations together on an issue like climate change. You may be playing a role, but you are also preparing for life. You are acting as global citizens.