More than half the world’s people live in cities, and cities are responsible for more than 70 percent of all energy-related carbon dioxide emissions on Earth. These dramatic statistics mean cities have a critical role to play in addressing climate change.

Until now, there was no universal, comprehensive methodology for cities around the world to measure their emissions. One of the tool's creators explains its power in the fight against climate change.

A coalition of the world's biggest cities has launched an international tool that will for the first time allow many local authorities to measure their greenhouse gas emissions, providing them with a baseline...