Health care, a giant in the U.S. economy, may be a gentle giant when it comes to greenhouse gases.

According to the first estimate of the sector's carbon footprint, the health care industry emits less than its share of the gases that promote global warming, compared to its size in the economy.

Hospitals are the biggest offenders, a finding that may motivate more of them to audit their energy usage and plant rooftop gardens _ as one big Chicago hospital has done.

Hospitals, nursing homes, drug companies and the rest of the sector contributed 8 percent of U.S. emissions, according to an analysis in Wednesday's Journal of the American Medical Association. Health care makes up 16 percent of U.S. gross domestic product.

The University of Chicago analysis, based on federal data, takes into account emissions from the manufacture of goods used by the industry and the power needed to run hospitals and other health facilities. The primary cause of global warming is the buildup of greenhouse gases from the burning of fossil fuels, such as oil and coal.

The analysis used 2007 health care spending data, and a model that breaks down the supplies used by the sector. The 1997 model was the most current available to the researchers, but was a weakness they acknowledged. Other experts said the supply chain probably hasn't changed much since then.

"It was important to generate at least a first estimate of emissions," said lead author Jeanette Chung of University of Chicago's Department of Medicine.

Hospitals are responsible for more than a third of health care's greenhouse gases, the analysis found.

U.S. hospitals are trying to reduce emissions. Chicago's Rush University Medical Center planted rooftop gardens that help insulate the building and reflect heat. The hospital wants to reduce its emissions by 8 percent by 2012 by scrapping three old mechanical systems and moving to one central energy plant for its 27-building campus.

Such efforts can save money that can be used to improve patient care, said Brent Kim of Johns Hopkins Center for a Livable Future in Baltimore, who wasn't involved in the new study.

"Health care has an opportunity and a responsibility to lead the way," Kim said.