Happily, Nature still abhors a vacuum, and into the vacuum created by governments failing to meet their responsibilities on climate change are now stepping more and more cities – as part and parcel of their overall ‘greening’ strategies.

In the USA, it’s often city mayors who are making the running, sometimes with the active support of state governors. It was Mayor Bloomberg of New York who succeeded in forcing climate change into last year’s presidential election, endorsing Barack Obama in the immediate aftermath of Hurricane Sandy. In China, there’s an extremely active network of eco cities, enthusiastically competing amongst themselves for government support and new industry investments.

Cities like Bogotá in Columbia and Curitiba in Brazil have provided inspiring case studies, not just for South America but also for the rest of the world.

And there are now umpteen European cities setting the bar at a far higher level than their national governments, particularly in the Nordic countries. (Just check out the ambition level for cities like Copenhagen, Gothenburg and Germany.)

I was inspired to read recently that Berlin’s legislature is working closely with the biggest energy cooperative in Berlin (BürgerEnergie Berlin – Bürger being the German word for ‘citizens’, in case anyone is confused!) to take over control of the electricity grid for the entire city, serving 3.5 million people. Citizens are being invited to invest a minimum of €100 in the project – for the far better rate of return (around 4% or 5%) than they can get from the average savings account. They’ve got a long way to go to reach the necessary €200 million, but energy cooperatives of this kind (of which there are now more than 600 with 80,000 members) have a phenomenal track record in Germany.

And it wouldn’t surprise me one little bit if the new Independent Mayor of Bristol, George Ferguson, found time to visit Berlin and see how they do it. It’s so important that cities get better at learning from each other – even from Copenhagen, which just squeezed Bristol into second place in last year’s Green Capital competition.

But this year the title could be Bristol’s!

If there’s one all-important success criterion for Green Cities, it’s engagement. Cities don’t go green just because their mayors or political leaders want them to. The more people there are involved, the more sustainable these ‘transformation journeys’ are – and not just on energy. Some of the most successful urban projects are food-related, with a huge increase in community-supported agriculture schemes, ‘farm to fork’ initiatives, and campaigns to combat the problems of food waste or urban ‘food deserts’. Just look at the success the Incredible Edible Todmorden project has had in getting towns and cities around the world to rethink their own food strategies.

There are many who now believe that it’s at the level of the city (with its surrounding hinterland) that sustainability will really take root. This in no way lets our national politicians off the hook, but it gives them an amazing opportunity to empower and incentivise cities to drive forward in this way, rather than forever holding them back.

Jonathon Porritt is Founder Director of Forum for the Future. www.forumforthefuture.org