A coalition of 19 mayors from the C40 group signed a pledge in August to make all new buildings net-zero carbon by 2030.

The mayors also agreed that all buildings in their cities, old and new, will be net-zero by 2050. U.S. mayors in Los Angeles, New York City, Portland, San Francisco, San Jose, Santa Monica, and Washington D.C., signed the agreement. Homes, offices, and other buildings where people work are among the greatest contributors to climate change.

For many cities, transportation is an even bigger contributor to greenhouse gas emissions. Thus, dense, walkable neighborhoods will be as crucial as net-zero construction to reduce atmospheric carbon.

The C40 cities will work with the World Green Building Council to achieve the goals.