Big cities have something of a mixed relationship with climate change. On one hand, higher density living is clearly better for the environment than lower density living: New Yorkers are less likely to drive to work in an SUV than the residents of, say, Houston.

On the other hand, the world’s megacities are still basically the source of most of the nasty stuff we’re pumping out into the atmosphere; people who live in those cities are more likely to travel internationally or demand the latest bit of consumer electronics, and so have an enormous carbon footprint; and no amount of smugly telling people, “Oh, I don’t even own a car” is ever going to change that fact.

Add to that the fact that something like 54 per cent of the world’s population now live in urbanised areas, and you get a growing consensus that, if we’re ever going to stop the Earth from turning into a great big dust ball, then making cities work better for the environment will have to be part of the solution.

So it is that yesterday the mayors of 26 European cities, including London, Vienna and Rome, came together in Paris, at which they released a rather high-fallutin’ statement, which was published over at the Guardian:

…we, the European capitals and metropolises that represent more than 60 million inhabitants and have significant investment capacity (€2tn GDP), have decided to join forces and strengthen the instruments that will lead us toward the energy and environmental transition.

We are addressing the major causes of greenhouse gas emissions: polluting transport, old and/or poorly isolated buildings and energy supply.

The letter goes on to promise “ambitious projects” in a number of areas: reducing urban sprawl, getting biodiversity back into cities, improving recycling, “increas[ing] electrical mobility”. Which we assume means stuff like the “Autolib” electric cars that the mayors, presumably with a certain degree of smugness, used to get to Paris city hall to announce all this.

The big one, though, is (sorry, this is an off-putting word, but hold my hand and we’ll get through it together) “procurement”. These cities between them buy €10bn worth of stuff from the private sector each year. That means that if they all start refusing to buy anything that isn’t sustainable, they can single-handedly have a pretty big impact on the sustainability of the things the private sector actually makes.

We’ve included a complete list of the mayors and cities that signed the pact at the end of this story, but we also like maps so here they are:

Population density map adapted from Wikimedia Commons.

Why no one in the Netherlands, Germany or Poland wanted to play, we’re not exactly sure.

This is not the first pact of this sort. Last September, the UN partnered with the C40 group of cities to launch its “Compact of Mayors”, which it promised would “enable cities to publically commit to deep emissions reductions, make existing targets and plans public and report on their progress annually”.

The C40 itself traces its roots to as far back as 2005, when then London mayor Ken Livingstone convened a meeting of 17 other mayors to discuss how they could all reduce greenhouse gas emissions in their city.

But 2005, you’ll note, was 10 years ago. Emissions are continued to rise. London’s air quality has declined. And last week Paris was declared the most polluted city on the planet.

Cities may be key to tackling climate change – but as ever, it’s easier to talk about it than to actually do it.

The letter was signed by:

Anne Hidalgo, mayor of Paris; Michael Häupl, mayor of Vienna ; Yvan Mayeur, mayor of Brussels; Jordanka Fandakova, mayor of Sofia; Constantinos Yiorkadjis, mayor of Nicosia; Frank Jensen, mayor of Copenhagen; Jussi Pajunen, mayor of Helsinki ; Alain Juppé, mayor of Bordeaux ; Yiorgos Kaminis, mayor of Athens ; István Tarlós, mayor of Budapest; Christy Burke, mayor of Dublin; Giuliano Pisapia, mayor of Milan; Ignazio Marino, mayor of Rome; Arturas Zuokas, mayor of Vilnius; Antonio Costa, mayor of Lisbon; Sorin Oprescu, mayor of Bucarest; Boris Johnson, mayor of London; Zoran Jankovic, mayor of Ljubljana; Karin Wanngård, mayor of Stockholm; Sami Kanaan, mayor of Geneva; Ana María Botella Serrano, mayor of Madrid; Alexiei Dingli, mayor of Valletta; Dario Nardella, mayor of Florence; Edgar Savisaar, mayor of Tallinn; Gérard Collomb, mayor of Lyon; Roland Ries, mayor of Strasbourg.