OAKLAND, Calif. — Highly dense cities emit the least carbon dioxide, according to a report looking at emissions from the 100 largest metropolitan areas in the United States.

“Shrinking the Carbon Footprint of Metropolitan America” looks at the carbon emissions from residential energy use and fuel used by cars and trucks, leaving out emissions from commercial buildings, industry and other transportation such as planes, transit and trains. The report is part of the Brookings Institute's Blueprint for American Prosperity initiative.

How much carbon a metro area emits is related mostly to long-standing practices or factors that can't be changed.

The least-emitting areas are mostly older, densely-developed, located near rail lines and are in regions of the country that require less heating or cooling than others. Areas that can take advantage of non-carbon emitting energy sources, such as Seattle does with hydropower, also emit less, and cities with higher energy prices typically use less energy, the report found.

Dense cities emit less carbon for a number of reasons, the report points out. They have smaller homes or more multi-unit buildings, require shorter trips to get places, have public transit or layout that encourages walking and bicycling, and facilitate reuse of existing structures.

The report also examines what can be done to lower emissions. Along with looking at what states are doing right, it calls out the federal government for what it is doing wrong or not doing at all.

Technical innovation is greatly needed, according to the report, and high efficiency building practices needed to be adopted on a widespread basis. While that provides a broad area for innovative businesses to take advantage of, the lack of national standards for green building and renewable energy portfolios is hindering efforts.

The report also rails against federal incentives for energy efficient building investments that are biased to new developments and households that can afford high-cost systems like solar installations.