More than half of the world's population lives in an urban setting. People in cities run a 20 and 40 percent higher risk of both anxiety and mood disorders than people in rural areas. And we're spending more and more time away from nature. Researchers estimate that if every city dweller spent just 30 minutes per week in nature, depression cases could be reduced by 7 percent. Globally, that's a whopping 21 million people. The answer lies in incorporating green space into urban planning, weaving nature into the fabric of everyday city life.