From London in 2005 to 97 of the world’s largest cities in 2025, C40 Cities marks two decades of unwavering climate leadership.
Launched in 2005 by then-Mayor of London, Ken Livingstone, C40 Cities was born from a simple idea: that cities, where most people live and where emissions are concentrated, must lead the fight against climate breakdown.
Twenty years on, C40 Cities connects 97 of the world’s largest and most influential cities across 51 countries, representing around 920 million people and 23% of the global economy. C40 cities and mayors are committed to using an inclusive, science-based, and collaborative approach to cut their fair share of emissions in half by 2030, help the world limit global heating to 1.5°C, and build healthy, equitable, and resilient communities. C40 membership operates on cities’ performance against the organisation’s Global Leadership Standards, never fees.
A proven track record of urban climate action
C40 Cities has built a track record of driving measurable climate results:
- Three-quarters of C40 cities are cutting emissions faster than their national governments
- More than 1200 transformative climate policies delivered across waste, buildings, energy, and transport, a figure that has tripled over the last decade
- Air quality improved for 107.8 million people between 2017 and 2023, as PM2.5 concentrations fell by 7% across C40 cities.
- Average per capita emissions fell 7.5% across C40 cities (2015–2024), compared to a global average of just 1.5%.
- 16 million green jobs created in C40 cities, with a goal of reaching 50 million by 2030.
- Nearly all C40 Cities now have Paris Agreement-aligned Climate Action Plans (up from only one in 2017)
- 26 million people benefiting from 30+ C40-backed adaptation programmes, with 78% of those beneficiaries in the Global South.
(Source: C40 Cities Annual Report 2024)
Milestones
2005 – 2008: Founded in London as a network of 20 cities, based on the largest cities in the G20, establishing a global voice for city climate cooperation.
2008 – 2010: C20 expands to become C40. Under the leadership of Mayor of Toronto, David Miller, C40’s unique peer-to-peer learning model was formally established, allowing first-mover cities to rapidly spread best practices on decarbonising buildings, transport, and energy with other cities. The C40 Knowledge Hub now supports 2.5 million visitors in 28,000 municipalities worldwide every year, spreading solutions far beyond the C40 network.
2010 – 2013: Data-driven emissions tracking was introduced to all C40 Cities under Mayor of New York, Chair Mike Bloomberg.
2013 – 2016: Mayor of Rio de Janeiro, Eduardo Paes, becomes the first Chair from the Global South. C40 introduces ‘participation standards’ for members, meaning each city must prove they are delivering science-based climate action. Member cities begin to use their alliances to push international action. C40’s Climate Finance Facility launched – the CFF is set to unlock $1 billion in investment across 30 cities by 2026.
2016 – 2019: After the historic Paris Agreement, Mayor of Paris, Chair Anne Hidalgo, encourages cities to be trailblazers in turning what science demands into actionable plans. Today, almost every C40 city has a 1.5°C action plan, whilst most countries still don’t. C40 launched its first ‘Accelerators’, science-based pledges by mayors to slash emissions in key sectors.
2019 – 2021: Global Green New Deal launched by Mayor of Los Angeles, Chair Eric Garcetti, embedding equity and justice in climate action. In the wake of COVID-19, C40 mayors united behind a Green and Just Recovery, championing investments in public health, clean air, and green jobs.
2021 – 2025: To elevate Global South representation, Mayor of London, Chair Sir Sadiq Khan, introduced the C40 Cities co-chair model; one mayor from the Global North and one from the Global South. As the first Co-Chairs, Sir Sadiq Khan and Mayor of Freetown, Yvonne Aki-Sawyerr, push for faster, fairer delivery and elevate Global South leadership. Nation states recognised the importance of working with subnational governments to meet their climate goals, with 75 national governments pledging to work with cities to deliver on their national climate plans through the Coalition for High Ambition Multilevel Partnership (CHAMP).
What’s next?
City-led climate action in defiance of the delayers and deniers
The next chapter for C40 is defined by:
- Halving fossil fuel use in cities by 2030 through clean transport, building retrofits, and urban redesign.
- Strengthening city resilience to extreme weather and climate impacts with green infrastructure and adaptive solutions.
- Improving air quality and creating millions of green jobs to support an inclusive, healthy, and sustainable urban economy.
- Combating climate disinformation by empowering mayors with science-based, transparent, and inclusive communication strategies.
- Strengthening global climate governance by pushing for COPs and international agreements to focus on annual, action-oriented delivery.
At the 2025 World Mayors Summit in Rio de Janeiro, on the eve of COP30, C40 will celebrate 20 years of achievement and challenge the world to follow cities’ lead.
As global climate leaders, we are committed to protecting the people and places we love. The next 20 years must be about delivery, with cities on the front line of the heart of global progress.
C40 Cities Executive Director Mark Watts