(Bloomberg)—Chicago is the greenest U.S. city to work in for the second year running, with about 70 percent of its office space now certified for environmental efficiency, a new study shows.

The Windy City’s percentage of green offices is up from about 66 percent last year, placing it ahead of San Francisco, Atlanta, Los Angeles and Minneapolis/St. Paul, according to an annual ranking by CBRE Group Inc.and Maastricht University. The study tracks buildings in 30 of the biggest U.S. real estate markets that are LEED or Energy Star certified.

Across the 30 areas, about 4,700 buildings are now green certified—or a record 41 percent of the total square footage of office space in those cities, the study shows.

Green certification has “become a proxy for good building management,” said David Pogue, a senior vice president at CBRE. “Particularly in the services sector, a lot of their energy use and environmental impact is in the buildings they occupy, so the tenant base is demanding this, and institutional financiers also believe a sustainable building is a better building.”

Commercial properties are often the biggest sources of carbon emissions in any one city, representing about 19 percent of total U.S. energy use, according to the Environmental Protection Agency. The Green Building Council’s Leadership in Energy and Environmental Design, or LEED, program rates buildings on environmentally friendly features, such as efficient energy and water systems, while the EPA’s Energy Star certification shows a building has met certain energy-performance standards.

Chicago has been working toward its goal of 100 percent renewable energy in all city-run properties by 2025, and created its own four-star energy rating system for large buildings last year.

The city’s hold on the top spot could be shortened, however. The recently opened Salesforce Tower in San Francisco—the tallest office skyscraper west of the Mississippi River, at 1,070 feet (326 meters)—could put the California city back on top next year. San Francisco’s percentage of green-certified square footage rose to almost 64 percent in the new ranking, up from 62 percent last year.

The EPA has also toughened the rules for Energy Star certification starting this year, so some properties could lose their certifications. Chicago has about 339 buildings with the Energy Star designation, according to the EPA. Los Angeles, with 716, has the most.

“So many buildings have become dependent on this as a way of demonstrating quality to investors and corporate tenants,” Pogue said. “It’s going to cause every building to go back and reconsider what their energy use is -- and that’s not necessarily a bad thing.”