This is the defining decade for humanity.

What we do today will determine whether or not there is a thriving future for our communities, our cities and the natural world around us. We know that climate breakdown and rising inequality, compounded by the COVID-19 pandemic, require an unprecedented response to match the scale of the crises. That’s why C40’s mission is to halve the emissions of its member cities within a decade, while improving equity, building resilience, and creating the conditions for everyone, everywhere to thrive.

Mayors of C40 cities are on the leading edge of climate action, and are deploying a science-based and collaborative approach to help the world limit global heating to 1.5°C and build healthy, equitable and resilient communities.

C40 member cities earn their membership through action. C40’s most distinguishing feature is that it operates on performance-based requirements, not membership fees. C40’s Leadership Standards set the minimum requirements for all member cities and ensure the integrity of C40 as a network of climate leaders.

The COVID-19 pandemic continues to present extraordinary challenges for people everywhere and our cities and communities may never again look the same. Cities and the mayors who lead them have been on the frontlines of responding to the crisis. C40 is working side-by-side with our mayors to secure a green and just recovery from the pandemic, one which both reduces city emissions and builds a more resilient and equitable future for all. 

The pandemic has also shown us how quickly things can change: the way things are is not the way things have to be. Science tells us that we must cut emissions in half by 2030 if we are to avoid runaway climate change. Nothing short of transformational change on a global scale is needed within the decade, but such a shift has never seemed more possible than it does right now. 

Ours is a vision of climate action that is rooted in equity, because we know that climate, social and economic justice can only be achieved together.

The change we need can be delivered through our Chair’s vision for a Global Green New Deal, where mayors are working alongside a broad coalition of representatives from labor, business, the youth climate movement and civil society to go further and faster than ever before. The Global Green New Deal is our essential blueprint to delivering climate justice and strong, fairer economies that serve everyone. An integral part of building thriving communities is ensuring that we have a sustainable and just recovery from the COVID-19 pandemic. Collaboration has always been in C40’s DNA – in a crisis it is needed more than ever. 

Our History

C40 mayors have been at the forefront of climate leadership for over 15 years, driving the conversation around climate action and environmental justice to place these issues front and centre in our local policies and on the international agenda.

#Together4Climate in Paris 2017
© Sarah Bastin / C40 Cities


2005 – C40 is founded. Mayor of London Ken Livingstone convened representatives from 18 megacities to forge an agreement on cooperatively reducing climate pollution and created the ‘C20’.

2006 – By 2006, the C40 Steering Committee invited a further 22 mayors to join, ensuring balance from the Global South, creating an organisation of 40 cities, and thus the name C40. That year, President Bill Clinton’s Climate Initiative (CCI) became our implementing partner on world-class climate action projects.

2007 – Mayor of New York City Michael R. Bloomberg hosted the second C40 Summit, bringing together 36 mayors from major global cities, business, and civil society.

2008 – Mayor of Toronto David Miller was unanimously elected Chair by C40’s Steering Committee and led broad efforts to build climate action through peer-to-peer knowledge sharing, including climate action planning, buildings, transport, energy and adaptation. Over the next two years, C40 hosted significant events in Berlin, Rotterdam, Tokyo and Hong Kong, and including the C40 Summit in Seoul, where member cities formally agreed on C40’s unique peer-to-peer learning model.

2009 – Mayor of Toronto and C40 Chair David Miller convened C40 member cities at the 2009 UN climate talks in Copenhagen, where cities gained lasting global recognition for their leadership in real action on climate change.

2010 – Mayor of New York City Michael R. Bloomberg was elected Chair of C40. During his tenure, he pioneered C40’s data driven approach – and he has continued working to expand C40’s impact in the global climate fight.


2011 – C40 announced a formal merger between C40 and CCI’s Cities Program, forged by President Clinton and C40 Chair Michael R. Bloomberg, with funding from Bloomberg Philanthropies. The merger has brought significant resources and infrastructure to create a preeminent global climate action organisation.

2014 – At the 2014 C40 Mayors Summit in Johannesburg, Mayor of Rio de Janeiro Eduardo Paes became the first C40 Chair from the Global South. During his tenure, cities from the Global South grew to represent more than 50% of our membership.

2015 – C40 celebrated its 10th anniversary. At the pivotal UN Climate Change Conference in Paris (COP21), cities were crucial voices in shaping and advocating for a strong Paris Agreement.


2016 – At the C40 Mayors Summit in Mexico City, C40 made it a standard of membership that all C40 cities commit to deliver an inclusive and resilient climate action plan in line with the 1.5°C ambition of the Paris Agreement by the end of 2020. At the Summit, Mayor of Paris Anne Hidalgo became C40 Chair – the first woman to hold this position. As outgoing and incoming Chairs, Mayor Paes and Mayor Hidalgo introduced C40’s flagship Deadline 2020 programme. 

2017 – Mayor Hidalgo, recognising the critical role of women in the climate movement and the need for equal participation at the decision-making table, launched C40’s successful Women4Climate programme, helping to bring new voices and perspectives into the climate conversation.   

2018 – C40 and our cities played an important role at the Global Climate Action Summit in San Francisco, where world leaders shared their successes on delivering on the Paris Agreement goals so far—and announced new initiatives to raise the level of ambition. C40 mayors made a series of bold pledges to reduce emissions and deliver on 2030 targets through green and healthy transport, zero waste, net zero carbon buildings, and equitable climate action. More than 70 mayors committed to develop and begin implementing ambitious climate action plans by 2020, in line with C40’s Deadline 2020 programme. 

2019 – The C40 World Mayors Summit took place in Copenhagen with 80 mayors in attendance, where Mayor of Los Angeles Eric Garcetti became C40 Chair and launched a Global Green New Deal alongside a coalition including youth climate activists and representatives from labour unions, the business community and civil society. Other major announcements from the Summit included: 30 cities from around the world had peaked their emissions; Mayor Garcetti launched the C40 Global Youth Initiative; and mayors made new commitments to 2030 targets for clean air and sustainable, healthy food systems.

2020 – 54 C40 cities representing nearly 10 percent of the world’s economy completed climate action plans in line with averting catastrophic climate change, compared with only two countries to have similar plans in the lead up to the 2021 UN Climate Change Conference (COP26). In the face of the global COVID-19 pandemic and its profound impacts on our communities and people everywhere, C40 convened the Global Mayors COVID-19 Recovery Task Force, chaired by Mayor of Milan Giuseppe Sala, to develop a plan to help our cities rebuild their economies in a way that improves public health, reduces inequality and addresses the climate crisis, resulting in the Mayors’ Agenda for a Green and Just Recovery. 2020 also saw 12 cities commit to divest from fossil fuels companies and advocate for greater sustainable investment.


2021 – C40’s Leadership Standards for 2021-2024 went into effect, setting an ambitious standard for C40 cities to ensure they are on a path to a zero-carbon future. Two new collaborative groups launched this year; the Global Mayors Task Force on Climate and Migration to address the impact of the climate crisis on migration in cities, and the first-of-its-kind Global Youth and Mayors Forum, which brings together youth climate leaders and mayors to work together on shaping the vision of a Global Green New Deal. C40 and our cities also played an important role at the UN Climate Change Conference (COP26), where Mayor Eric Garcetti passed the C40 Chair baton to Mayor of London Sadiq Khan and it was announced that more than 1,000 city and local governments around the world have joined Cities Race to Zero, committing to limit global temperature rise to 1.5°C.

C40’s Annual Reports