Copenhagen wants to become the world's first carbon-neutral capital by 2025. Since 1995, it has reduced its carbon emissions by half. It stands out for its efforts on sustainable mobility, with large car-free zones, high quality public transport and impressive cycling facilities. District heating and cooling systems – some of which use cold seawater – do their bit to reduce emissions, too.

The Icelandic capital already has a renewable supply of heat and electricity – mainly from hydropower and geothermal. An impressive 95 percent of homes are connected to the district heating network. The city is also aiming to make all public transport fossil-free by 2040 and strongly encourages residents to do without their cars.

In Brazil's eighth biggest city, around 60 percent of the population relies on the urban bus network. They also have 250 kilometers of bike lines at their disposal, as well as the country's first major pedestrianized street, Rua das Flores. Curitiba's green belt provides natural protection against flooding. But its rapid population growth is putting its green ambitions under pressure.

In 2016, San Francisco passed a law that all new buildings must set aside space for rooftop photovoltaic systems – the first major US city to do so. Plastic bags have been banned since 2007, and it introduced an urban food waste program in 2009. Now, it plans to go waste-free by 2020. Plus, the majority of its buses and light rails are zero-emission.

Germany's financial center was one of the first cities to adopt a roadmap towards a 100 percent renewable energy supply by 2050. New buildings must follow strict guidelines on energy efficiency. Controversial materials like PVC are forbidden, and it has drastically reduced its waste, thanks to a modern waste management system. Frankfurt also has ambitious plans for e-mobility.

Vancouver is trying to become the world's greenest city by 2020. By then, it seeks to reduce carbon emissions by 33 percent compared to 2007. The city's electricity comes almost entirely from hydroelectric dams, but it still needs to move away from natural gas and oil for heating and transportation. The goal is to reduce per-capita ecological footprint by 33 percent.

Kigali has been described as Africa's cleanest city. It's planning to develop pedestrian and cycling corridors. Plastic bags are banned and citizens spend a day each month cleaning up the city, where it's rare to find litter. However, human rights groups have denounced the high price of this "cleanliness," which they say is as an excuse to impose a discriminatory control over the population.

The European Green Capital 2016 gets all its electricity from hydropower. It has a strong focus on public transport, pedestrian and cycling networks, and has banned cars from its city center. It was the first European city to aim for zero waste, and already recycles over 60 percent – one of the highest rates in Europe.