Urban areas are responsible for more than 75 percent of all greenhouse gas emissions, making reduced energy crucial in the effort to slow the pace of global warming.

Energy-efficient traffic lights, street lighting, the use of biofuels for city transport, and traffic congestion schemes were some of the practical steps that cities are expected to take to reduce greenhouse gases.

"The world's largest cities can have a major impact on this. Already they are at the center of developing the technologies and innovative new practices that provide hope that we can radically reduce carbon emissions," said London Mayor Ken Livingstone, who launched the initiative in Los Angeles with Clinton and British Prime Minister Tony Blair.

The Clinton Foundation said it hoped that coordination between major cities will boost efforts now being made by some areas on an individual basis.