Nation states have failed to tackle climate change. What if a network of cities paved the way to a more sustainable world? Mark Watts is executive director of C40, a network of 91 of the world’s major cities, committed to bold action on climate change. Here is his perspective on this global issue.

Samuel Roumeau: While Donald Trump is moving the U.S. out of the Paris Agreement, what should be the role of cities in the fight against climate change?

Mark Watts: The decision by President Trump to withdraw the U.S.A. — the world’s biggest per capita polluter — from the Paris Agreement on climate change is undoubtedly a setback to a unified global response to climate change.

However, the response by U.S. cities, along with states, businesses and citizens, has been truly inspiring. More than 300 American "Climate Mayors" have committed to "adopt, honor and uphold Paris Climate Agreement goals." Michael Bloomberg, the former mayor of New York City and current C40 board president, generously pledged to give $15 million to the United Nations Climate Change Secretariat to compensate for the U.S. federal government withdrawing its share of the U.N. budget. Under his leadership, hundreds of U.S. cities, states, universities and businesses declared "We Are Still In" and committed to "pursue ambitious climate goals, working together to take forceful action and to ensure that the U.S. remains a global leader in reducing emissions." C40 is now supporting this group to create "America’s pledge," an unprecedented effort to aggregate carbon reductions by cities, regions, businesses and other social actors to ensure that the U.S. achieves its Paris Agreement pledge. Citizens and businesses should continue to 'vote with their feet,' and choose to live, work and operate in those cities that are protecting our climate.

Moreover, President Trump’s backward step has stimulated almost 50 mayors from cities around the world to sign a petition to G20 leaders urging them to work with them to "Save Our Planet." Tens of thousands of citizens have shown their support for this message by signing a global petition. I would urge everyone reading this to sign and share the petition.

We still really need the world’s presidents and prime ministers to work together to tackle climate change and turn the historic Paris Agreement from a fantastic aspiration into a real roadmap to a low carbon future. But leadership by mayors, governors, businesses and ordinary citizens has shown that we don’t need to wait for national governments to act.

C40’s own research, Deadline 2020 (PDF), has shown both the urgency of the current situation — global emissions really have to stop rising by 2020 and then come down very steeply if we are to prevent global warming turning into global catastrophe; but also that action at a city level can really make a difference. Our calculation is that action in cities can deliver 40 percent of all the emissions cuts that need to happen to keep temperature rises within a level that scientists believe is manageable.

Roumeau: We can observe that some cities are launching concrete action to mitigate climate change at the local level. At the global level, what could the modus operandi to create more successful collaboration between cities look like on this specific topic?

Watts: Former New York mayor Fiorello La Guardia once said, "There is no Democratic or Republican way of fixing a sewer." Mayors tend to be far more collaborative and open to new ideas than other politicians, because they are judged on their ability to get stuff done.

Mayors recognized far earlier than most politicians the risk that climate change posed to their citizens. They saw the damage caused on the streets of their cities by floods, heatwaves and other climate-related disasters. They realized the risks of inaction and crucially the many benefits and opportunities that taking action on climate change would deliver for their citizens. They also realized that the transformation of their cities could be delivered faster, at lower cost to their tax payers, by looking at what had worked in other cities around the world. Mayors recognized far earlier than most politicians the risk that climate change posed to their citizens.

The best person to convince a mayor of the benefits of a policy is another mayor. The C40 network model brings together mayors, city officials and urban planners to share lessons, ideas and inspiration through peer-to-peer learning. This model has brought incredible benefits to cities and massive emissions savings and it has the potential to deliver transformational change in these crucial years ahead for our planet. To give just one example, when Paris launched the Velib bike share scheme, just six cities in the C40 network had such a scheme. Today, 43 C40 cities have bike share schemes and Chinese cities are now taking cycle hire to entirely new levels, with hundreds of thousands of bikes transforming travel in Beijing and Shanghai. That represents hundreds of millions of bike journeys in cities each year, not generating any greenhouse gas emissions.

Roumeau: How do you think we can collectively raise awareness and encourage action around climate change beyond megacities, with a network of smaller cities?

Watts: C40 is a leadership group, which means we work with the world’s largest and most innovative cities that have the potential to deliver the largest emissions savings. These cities also set the agenda for every other city around the world. If a policy or project can be shown to work in London, Paris, New York City, Rio de Janeiro, Seoul and Lagos, then other mayors of cities big and small will sit up and take notice.

Our research has shown that if all U.S. cities follow the lead shown by the 12 American C40 cities and pursue deep, rapid emissions reductions, by 2025 cities would contribute more than one-third of the emissions reductions needed to meet the United States’ commitments to the Paris Agreement.

The best way to make sure that cities of every size follow the lead of the most ambitious mayors is to recognize and celebrate the benefits that these pioneer cities are enjoying. Those cities which transform to become sustainable and low-carbon most quickly will also be the healthiest, wealthiest, most livable cities of the future. Citizens and businesses should continue to "vote with their feet," and choose to live, work and operate in those cities that are protecting our climate.

Roumeau: What is your thinking around and what actions have you taken related to citizen engagement?

Watts: In 2016, C40 carried out a major piece of research on the barriers that mayors face in delivering climate action in their cities. By understanding them, we will be better equipped to help mayors overcome these barriers. Amongst the key challenges that mayors face is securing public support for sustainable projects, policies or investments.

Therefore, it is essential that we get better at communicating with citizens on the benefits of action against climate change. That doesn’t mean scaring people about the risks of catastrophic global warming that will occur decades in the future, which can actually have the opposite effect of demoralizing individuals from wanting to act. Instead mayors, campaigners and groups like C40 must start talking to citizens about the benefits that climate action can bring to their lives today. From cleaner air and healthier citizens to more jobs and less inequality, this is the space where we need to engage with citizens.

Roumeau: What role do global companies have to play in this and how can they collaborate more with cities?

Watts: Alongside mayors, corporate leaders have emerged as some of the key partners in the global fight against climate change. Market forces are driving some of the major shifts towards sustainable business, from the plunging costs of using renewable energy, compared with expensive and polluting fossil fuels and the growing demand from consumers for electric vehicles, locally sourced food and sustainable clothes.

Paul Polman, CEO of Unilever and a genuine pioneer in the world of climate-focused business, has said, "Most CEOs … know that their companies cannot prosper in a world with runaway climate change. This is increasingly evident. They understand the need to work together with political leaders to address these challenges." Ultimately, the battle against climate change will be won by a global collaboration between cities, businesses and citizens.

That collaboration is underway in cities as never before. Crucially, cities are beginning to realize their own potential to shift markets. The C40 Clean Bus Declaration brought together 26 cities across 20 countries to commit to roll out fleets of low and ultimately zero emission buses on their streets. This public declaration of support demonstrated the scale of the potential market and therefore incentivized bus manufacturers to increase investment in these technologies. Within months, cities found that the price of electric buses were falling as bus manufacturers competed for their business in what had become an established and growing global market.

Through the C40 Cities Solution Platform, based out of our Copenhagen office, the private sector and leading cities are working together to find solutions to urgent climate challenges. Cities, including Seattle, Melbourne and Rio de Janeiro, bring complex challenges, and technology companies compete and collaborate to create viable solutions.

Ultimately, the battle against climate change will be won by a global collaboration between cities, businesses and citizens, all determined to secure a sustainable world for future generations.

Roumeau: "Cities of the world, unite" is this year's OuiShare Fest theme, with the aim of gathering cities and citizens to regain power to tackle global challenges: What do you think of it? What would you expect from an event with this title?

Watts: The urban philosopher, activist and visionary Benjamin Barber — who sadly died earlier this year — argued in his seminal 2013 book, "If Mayors Ruled the World," that cities are "the primary incubator of the cultural, social, and political innovations which shape our planet. And most importantly, they are unburdened with the issues of borders and sovereignty which hobble the capacity of nation-states to work with one another."

More than half the people on earth already live in cities — a figure expected to reach 70 percent by 2050. The future is undoubtedly urban, and I’m confident that thanks to the leadership of cities, that future can be made safe from climate change. Events like OuiShare Fest offer an opportunity for citizens of the world’s great cities to consider just how much power they have in shaping the future and how vital it is to act now, before it is too late.