There's a lot of scepticism going around about what will come out of the climate summit in Paris (COP 21). After 21 COPs and countless calls to action over the past two decades, the international community has yet to come up with a viable agreement on climate protection. However, despite the lack of progress at the state level over the years, a number of cities around the world have been leading the way and making significant strides when it comes to the implementation of eco-friendly policies.The Green City Index, which measures the environmental performance of over 120 cities across the globe, lists San Francisco (USA) as the top performer in North America, Curitiba (Brazil) in Latin America, Copenhagen (Denmark) in Europe, the city-state of Singapore in Asia, and Cape Town (South Africa) as one of the "above average" performers in Africa.So how did these cities do it? And what can we learn from their experience? Join our conversation at 1930GMT.