rate is overzation 70%. in china, it's still 50%.t bigger or there will be more of them. i think a lot of emerging markets, especially those with a large population are going to experience trends like that in the next 50 years. >> this incredible rate of growth makes the challenges of managing a large city even more unmanageable. the biggest risks are the same risks that challenge all of us, politically governing, climate change, economic inequality, productivity, education, transportation. those issues that face cities are the same that face everyone except in a much more concentrated way. narrator: one city battling with these problems is rio de janeiro in brazil. she believes the world's biggest cities are in danger of sinking under a tide of poverty, infrastructure and citizen apathy. unless we do something about it billions will suffer the consequences. >> the kind of urbanization we have today can only go so far. if we do not change the way we design our cities and make cities change with us we will have serious limits. they wi