A view of the city skyline from the Beijing Yintai Centre building amid heavy haze and smog in the morning October 29, 2011. The world's population will reach seven billion on October 31, 2011 according to projections by the United Nations, which says this global milestone presents both an opportunity and a challenge for the planet. Picture taken October 29, 2011. REUTERS/Jason Lee (CHINA - Tags: ENVIRONMENT CITYSPACE)