C40 is an abbreviation for the C40 Cities Climate Leadership Group, a key global network that leads climate action among major cities around the world. The history of C40 dates back to October 2005, when Ken Livingstone, then Mayor of London, felt the limitations of G20 discussions that proceeded at the national level and invited representatives of 18 megacities to form a working group for cooperation on climate change and air pollution reduction. Nicky Gavron, Deputy Mayor of London and another key figure in C40's founding, stated that "if nation-states are at the heart of international climate agreements, implementation and innovation will happen at the city level," highlighting the need for direct action at the city level. Starting with 18 cities, the group grew to 40 participating cities in its early days, and the name C40 became established.
Member cities are broadly classified into four categories: Megacity, Innovator, Observer, and Inactive.
[C40 — Cities Climate Leadership Group ⓒESG.ONL]
// Megacity
Large cities with a population of over 3 million that demonstrate outstanding climate leadership at a global level.
// Innovator
Cities that demonstrate outstanding climate leadership at a global level but do not meet the scale of a Megacity.
// Observer
Cities that meet the qualifications of a Megacity or Innovator based on climate leadership, but require approval of full membership due to regulatory or procedural reasons.
// Inactive
Cities that have not met C40's climate leadership standards for 12 months or more.
As of 2025, C40 has grown into a global network of 97 member cities, leading global climate action. Participating cities account for approximately one-twelfth of the world's population and more than 22% of the global economy. In 2022, C40 hosted the largest-ever World Mayors Summit in Buenos Aires, developing concrete city-level responses to climate change — including the creation of 50 million green jobs by 2030, investment of over USD 1 billion in cities in the Global South, and recognition of climate leadership cities through the C40-Bloomberg Philanthropies Award.
As awareness grows that cities are central to solving climate change, C40 is poised to play a pivotal role not only in building a sustainable urban future, but also in driving climate action at the national level for the countries in which these cities are located.
by Editor O