20 years of climate leadership
C40 began in 2005 when 18 mayors met in London to talk about cutting emissions. What started as a conversation became a coalition, now a global network of nearly 100 cities representing 920 million people.
C40 mayors have tripled climate policies in their cities since joining the network, cut per capita emissions by 11.5% in the last 10 years, and 73% of member cities’ emissions (where there is enough data) have now peaked, meaning emissions are now falling year on year.
Our history is built on 20 years of mayors coming together to share solutions, challenge each other to go further, and show the world what climate action looks like when cities lead.
Our history at a glance
2005-2010: A conversation becomes a coalition
What began as a meeting of 18 mayors in London quickly took shape as a first-of-its-kind coalition of cities determined to act on the climate crisis. Within five years, the group had doubled, partnered with the Clinton Climate Initiative, and developed a way for cities to learn from each other’s successes and failures.
2011-2015: A more representative, more influential C40
Rio de Janeiro’s mayor became Chair, the first from the Global South, and by 2014, most C40 cities were in Africa, Asia and Latin America. When governments negotiated the Paris Agreement, mayors were in the room pushing for the suggested targets.
2016-2020: Aligning cities with the Paris Agreement
C40 set a requirement: every member city had to deliver a Paris-aligned climate action plan by 2020. 54 cities met that deadline, at a time when only two national governments had done the same. The network also launched Women4Climate and responded to COVID-19 with a green recovery plan.
2020-2025: Shaping a green and just future
C40’s focus expanded beyond cutting emissions to creating good green jobs, supporting frontline communities, and making sure the benefits of climate action are shared fairly. Cities have now delivered more than 1,200 climate actions, and the network turned 20 with a World Mayors Summit ahead of COP30.
What comes next
C40’s three missions guide the work ahead:
- Help the world halt climate breakdown by halving fossil fuel use by 2030.
- Increase resilience so cities and people are protected as they face the impacts of climate breakdown.
- Ensure a transition to a clean, green world that is fair, inclusive, and addresses the injustices of climate breakdown.
C40 is pushing for direct access to funding and a formal role for mayors in global negotiations, so cities can deliver climate action at the pace and scale required.
A timeline of city climate leadership
2005-2010: A conversation becomes a coalition
2005: 18 mayors meet in London
Ken Livingstone, Mayor of London, convened representatives from 18 megacities to forge an agreement on reducing climate pollution together, creating the ‘C20’.
2006: The network doubles
The Steering Committee invited 22 more mayors, ensuring representation from the Global South, and renamed the organisation C40. President Bill Clinton’s Climate Initiative became C40’s first implementing partner.
2007: New York hosts the second summit
Michael R. Bloomberg, Mayor of New York City, brought together 36 mayors alongside business and civil society leaders.
2008: Peer-to-peer learning takes shape
David Miller, Mayor of Toronto, was elected Chair and led efforts to build climate action through knowledge sharing on buildings, transport, energy and adaptation. Summits in Seoul, Berlin, Rotterdam, Tokyo and Hong Kong established C40’s peer-to-peer model.
2009: Cities gain global recognition
Mayor David Miller convened C40 cities at the UN climate talks in Copenhagen. For the first time, cities were recognised as leaders in climate action on the world stage.
2010: A data-driven approach begins
Michael R. Bloomberg became Chair and pioneered C40’s focus on measuring and tracking emissions, an approach that continues to shape the network’s work.
2011–2015: A more representative, more influential C40
2011: C40 becomes a global organisation
C40 merged with the Clinton Climate Initiative’s Cities Program, with funding from Bloomberg Philanthropies, bringing significant resources and infrastructure to accelerate urban climate action.
2014: Leadership shifts to the Global South
At the Johannesburg Summit, Eduardo Paes, Mayor of Rio de Janeiro, became the first Chair from the Global South. During his tenure, Global South cities grew to represent more than half of C40’s membership.
2015: Mayors help shape the Paris Agreement
C40 marked its 10th anniversary. At COP21, mayors were crucial voices in shaping and advocating for the Paris Agreement targets.
2016–2020: Aligning cities with the Paris Agreement
2016: Membership means commitment
At the Mexico City Summit, C40 made it a condition of membership that all cities deliver a climate action plan aligned with the targets laid out in the Paris Agreement by 2020. Anne Hidalgo, Mayor of Paris, became Chair, the first woman in the role, and launched the Deadline 2020 programme.
2017: Women4Climate launches
Mayor Anne Hidalgo launched Women4Climate to bring new voices and perspectives into climate leadership.
2018: Bold pledges in San Francisco
At the Global Climate Action Summit, C40 mayors committed to 2030 targets on green transport, zero waste, net-zero buildings, and equitable climate action. More than 70 mayors committed to deliver climate action plans by 2020.
2019: A Global Green New Deal
At the Copenhagen Summit, Eric Garcetti, Mayor of Los Angeles, became Chair and launched a Global Green New Deal with youth activists, labour unions, and civil society. 30 cities announced they had peaked their emissions.
2020: Cities outpace nations
54 C40 cities completed Paris-aligned climate action plans, compared with two countries. The Global Mayors COVID-19 Recovery Task Force developed a plan for green and just recovery. 12 cities committed to divest from fossil fuels.
2020–2025: Shaping a green and just future
2021: New standards, new partnerships
At COP26, Sir Sadiq Khan, Mayor of London, became C40 Chair. C40’s Leadership Standards for 2021–2024 set an ambitious path to zero-carbon cities.
Over 1,000 cities joined Cities Race to Zero, and new initiatives launched including the Global Mayors Task Force on Climate and Migration and the Global Youth and Mayors Forum.
2022: Record investment in the Global South
C40 expanded support for Global South cities, with ten African cities joining the Clean Air Accelerator and projected investments of more than USD 1 billion announced. The Buenos Aires Summit was C40’s largest ever, with 75 C40 mayors and 143 non-C40 cities participating.
2023: A new model for shared leadership
Mayor Sadiq Khan introduced a new Co-Chair model requiring one mayor from the Global North and one from the Global South lead the organisation.
Mayor Yvonne Aki-Sawyerr of Freetown became C40’s first female Co-Chair and is the first woman leader from the Global South to hold the position under the new inclusive Co-Chair model, which was designed to give greater prominence to Global South voices in climate leadership.
2025: 20 years of mayors working together
C40 marked two decades of city climate leadership with a World Mayors Summit in Rio, hosted by Mayor Eduardo Paes and convened by Co-Chairs Mayor Sadiq Khan and Mayor Yvonne Aki-Sawyerr.
Nearly 300 local leaders from five continents gathered days before COP30, delivering a joint statement to UN Secretary-General António Guterres backed by more than 14,000 cities, towns, and regions.
At COP30 in Belém, cities were recognised as a formal part of the process for the first time, with mayors calling for direct access to climate finance and a stronger role in turning global commitments into local action.
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