This piece originally appeared on Forbes.Between meetings with President Obama this week, China’s vice president and leader-in-waiting Xi Jinping will make time to visit Iowa farm country. Back at home, cities– not the countryside– will likely dominate Xi’s domestic agenda.In a momentous shift, more people in China now live in cities and towns than in rural areas. Forty years ago, eight in ten people in the world’s most populous country were peasant farmers, living off the land. Today, 51 percent of its 1.35 billion people live in sprawling cities, with high-rise skylines.China surpassed this milestone in a fraction of the time it took Western Europe to shift from rural to urban societies. Nor is it alone. A similar exodus is taking place across Africa and Asia, prompting the United Nations Population Fund to estimate that almost 5 billion people worldwide will live in cities and towns by 2030, up from around 3.5 billion in 2010. This transformative shift in human society offers both big challenges and great promise for sustainable development.On the one hand, emerging cities must avoid the poverty, over-crowding, pollution, and congestion that have plagued many 20th century cities. Beijing’s air pollution problems, now so severe as to be regularly closing down the airport, provide a stark illustration of the downside of breakneck urbanization.On the other hand, tomorrow’s cities offer the tantalizing prospect of improving how we live, work, travel, and consume, while protecting our planet’s scarce resources. But fulfilling this promise requires approaching sustainable development with fresh eyes. Traditionally, we have thought of protecting the environment as a mainly rural challenge, with efforts focused on greening agriculture, and preserving forests and rivers.As people increasingly move to urban centers, national planners, civic leaders, and environmentalists need to turn their attention to different issues and questions. Questions such as: How can city dwellers get around most cleanly and efficiently? What types of buildings are most energy efficient? What kinds of low-carbon energy strategies are best for cities?While the idea of smart cities is not new, creating sustainable cities for the booming global population requires scaling up on a whole new level. And there is no better place to start than in China.In Europe and North America, accommodating growing urban populations will largely mean adapting existing cities, many of which are centuries old. New cities in China, by contrast, are being built from the ground up, offering the potential to act as incubators of leading-edge, urban sustainability that could serve as a global model.By 2030, 221 Chinese cities will have at least 1 million residents. This is a truly startling statistic. It underlines the extent to which the battle to achieve sustainable growth will in no small part be won or lost in China’s cities. By comparison, Europe boasts only 35 million-plus cities today.Fortunately, China’s leaders see the merit of building sustainable cities. Anxious to ensure that urbanization and economic growth are not undermined by pollution and other environmental ills, the government’s 12th Five-Year Plan focuses heavily on sustainable urban planning and low-carbon development.Translating this into practical policies and projects, of course, is easier said than done. That’s why the World Resources Institute is collaborating with China’s National Development and Reform Commission (NDRC) to design models for low-carbon urban development, focusing on key issues like efficient energy and land use, sustainable transport, and reliable, clean water supply. WRI will also be expanding into major cities in India and Brazil to help them design similar low-carbon models in the months ahead.Cities are here to stay. They hold the promise of lifting millions out of poverty and becoming powerful engines for social progress and sustainable lifestyles. Let’s realize that potential.