C40, officially known as the C40 Cities Climate Leadership Group, is an international organization that connects mayors from nearly 100 major cities to develop collaborative approaches to address climate change. The group was launched in 2005, following an initiative by then-Mayor of London Ken Livingstone, who brought together leaders from 18 megacities to forge a collective climate agreement. The organization quickly expanded, reaching 40 member cities by 2006 and adopting its current name. That year, a partnership was established with President Bill Clinton’s Climate Initiative, bringing additional resources and practical know-how.

C40’s headquarters are located in New York, with significant operations in London as well. The group works on reducing greenhouse gas emissions, enhancing urban resilience, and promoting equitable climate policies. Regular membership reviews ensure that cities maintain ambitious climate action standards, including measurable targets for emissions reduction.

The organization played a significant role in increasing urban participation during key events such as COP21 in Paris. Projects range from supporting cities in implementing clean transport systems, developing urban adaptation strategies for extreme weather, to promoting green job investments. For instance, C40 has facilitated peer-to-peer learning in areas such as building efficiency and clean energy.

Mayoral leadership rotates and is fundamental to the group’s direction. Current and former chairs have included the mayors of London, New York, Los Angeles, and Rio de Janeiro. As of 2025, Sadiq Khan, the Mayor of London, serves as the chair.

C40’s work is anchored in data-driven analysis, international advocacy, and city-led action. Its research efforts help cities prioritize effective mitigation and adaptation strategies, with over 98% of member cities reporting that climate change poses significant risks to their populations.