While industry in the U.S. has made solid strides in reducing greenhouse gas emissions over the past 25 years, the transportation sector has increased its carbon footprint, say University of Michigan researchers.

Industry, still the nation's largest emitter, accounts for about 29% of all emissions — down from nearly 36% in 1990. However, transportation — the country's second-largest contributor to greenhouse emissions — has increased its share from 24% in 1990 to 27% in 2014.

The relative contributions of the commercial, residential and agricultural sectors have also increased since 1990. Emissions are now about 17% for the commercial sector, 17% for the residential sector and 10% for agriculture.

Michael Sivak, research professor at the U-M Transportation Research Institute, said that these overall trends occurred because of two underlying trends: 1) absolute emissions from the industrial sector decreased by 11%, despite the increases in population and GDP, and 2) the absolute emissions from transportation, commercial, residential and agricultural sectors increased 17%, 21%, 20% and 10%, respectively.

Sivak and colleague Brandon Schoettle examined data from the U.S. Environmental Protection Agency from 1990 to 2014.

"Because of the major progress in reducing emissions from industry, we can expect an increased emphasis on reducing emissions from the other economic sectors, including transportation," Sivak said. "In addition, because of the large increase in the contribution of medium- and heavy-duty trucks to total emissions, we can expect increased emphasis on reducing emissions from these classes of vehicles."

According to the study, absolute emissions of medium- and heavy-duty trucks jumped 76%, from about 15% of transportation emissions in 1990 to 22% in 2014. At the same time, the relative contributions of passenger cars, light-duty trucks and commercial and other aircraft decreased — although cars and light trucks still account for 61% of transportation emissions.

Overall, the relative contribution of medium- and heavy-duty trucks to total emissions has grown to 6%. This compares with a slight rise to 16% for passenger cars and light-duty trucks, despite a large increase in the number of these vehicles on the road today.