Climate change poses a unique and growing threat to every city on Earth. This uncontroversial truth is precisely why more than 190 nations committed to the Paris Agreement on Climate Change, pledging to cut greenhouse gas emissions in order to keep global warming to below 1.5 degrees.

Yet today it is mayors, rather than national leaders, who are proving to be the protectors of the Paris Agreement.

The executive order signed by President Trump, which rolls back many of the environmental and climate change regulations introduced under President Obama, signals the US federal government’s retreat from leadership on the climate change issue. By contrast, mayors from cities across the United States pledged to deliver on their commitments to the Paris Agreement, describing climate change as “both the greatest single threat we face, and our greatest economic opportunity for our nation”.

Miguel Arias Cañete, the European Union’s climate and energy commissioner, struck a defiantly positive tone last week by declaring that “a new climate era has begun, and the EU and China are ready to lead the way”. It will be wonderful if he is right.

Domestically, China is certainly striding ahead towards a ‘new normal’ of environmentally sustainable economic growth. China’s investment in renewable energy last year was nearly double that of the next largest country, the US. Chinese cities are setting 2020 peak emissions targets, rolling out fleets of electric buses and taxis at a startling rate and, just last month, Beijing closed the city’s last coal power plant.

China’s leader, President Xi, has shown signs that his nation may be ready to step up to provide global leadership. In his speech to the World Economic Forum in Davos, he urged that “all signatories must stick to” the terms of the Paris Agreement.

Yet it is unrealistic to think that this is going happen with the speed necessary to galvanise international delivery on emission reductions commensurate with what climate science demands.

In Europe, nations have embarked on two years of complex, expensive and painful negotiations on Britain’s exit from the European Union. The Brexit debate will inevitably draw focus from the urgent challenges facing our cities, especially climate change. German, French and other leaders are too focused on saving the European Union to think about saving the world. So whilst national leaders grapple with such issues and continue to build barriers, walls and tariffs, mayors focus on what the world needs and are getting the job done.

In every part of the world, it is mayors that are concretely tackling climate change with immediate and bold measures. This week, representatives of 12 cities from five continents met at London’s City Hall with leaders from major global financial institutions and urban infrastructure investors, at the C40 Financing Sustainable Cities Forum, supported by the Citi Foundation and WRI Ross Center. Their ambition is nothing less than creating a model of co-operation between the private sector and cities that has the potential to transform every city on Earth. Just as it always has been, London is open to this truly global collaboration.

The key now is to unlock the potential of cities. Research by the New Climate Economy found that almost $1trn of investment will be needed each year between now and 2020, to help deliver on the ambition of the Paris Agreement. This is a formidable amount of investment. Yet the data shows that investing in sustainable infrastructure would actually provide an economic benefit to global cities worth $16.6trn by 2050, due to savings made in energy bills and other costs.

The urgency of the climate crisis facing our cities means there isn’t a moment to lose. Mayors and city halls are working together, sharing ideas and harnessing the expertise of business, global financial institutions and citizens. They are committed to delivering on the ambition of the Paris Agreement and creating green, sustainable, healthy and prosperous cities.

Mark Watts is executive director C40 Cities.

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