C40 Cities

C40 Cities (the C40 Cities Climate Leadership Group) is a network of the world's megacities committed to addressing climate change. It supports cities to collaborate effectively, share knowledge, and drive meaningful, measurable, and sustainable climate action.

The organization was launched in October 2005 when then Mayor of London Ken Livingstone convened representatives from 18 megacities to cooperatively reduce climate pollution. By 2006 it had grown to 40 cities, giving rise to the name C40. President Bill Clinton's Climate Initiative became C40's first implementing partner, and the network expanded to ensure representation from the Global South.

At its 20-year milestone, C40 connects 97 of the world's largest and most influential cities across 51 countries, representing around 920 million people and roughly 23% of the global economy. Member cities include megacities of more than 10 million people such as Tokyo, Delhi, and New York. The Chair of C40 is the Mayor of London, Sadiq Khan; Michael R. Bloomberg serves as President of the Board; and Mark Watts has been executive director since 2013. Its work is supported by strategic funders including Bloomberg Philanthropies and the Children's Investment Fund Foundation (CIFF).

Mayors of C40 cities commit to a science-based and people-focused approach to help the world limit global heating, making innovation in urban climate solutions, measurement, and collaboration directly relevant to the network's mission.