By 2050, the world's major cities will be crowded by six and a half billion people. Pollution, inequality and changing climates threaten the higher living standards that cities can offer. But people designed these problems, so people can design their solutions. CNN profiles the cutting-edge projects that combat some of the pressures that modern cities face. China's economic miracle has made it the world's worst polluter, but small-scale innovations and large-scale government initiatives are working to bring back blue skies over smoggy cities. In Seoul, transport innovation is keeping its massive population moving through a capital that wasn't ready to be a megacity. In Detroit and in Colombia, low-income housing and public library parks respectively are proving that innovation can be harnessed to boost social inclusion.