by Michael Sivak.

In this analysis, I examined recent changes in the contribution of transportation to total greenhouse gas emissions in the United States relative to the other sources of emissions (industry, commercial, residential, and agriculture). Also of interest were the changes in the relative contributions of various transportation modes, both to transportation emissions and to total emissions.

The examined data were for 1990 and 2017. The raw data came from EPA. The analysis is an update of a study published in 2016. (The data for 1990 were recently revised by EPA.)

The data in the table below show that the largest relative changes from 1990 to 2017 in greenhouse gas emissions by economic sector occurred in industry (a decrease from 36.3% to 29.9% of total emissions) and transportation (an increase from 24.1% to 29.2%). (Total emissions exclude emissions from U.S. territories.)

The next table shows emissions by transportation mode as a percentage of transportation emissions. The largest relative changes were an increase in emissions for medium and heavy trucks (from 15.1% to 23.3%), a decrease for cars and light trucks (from 63.1% to 58.7%), and a decrease for non-commercial aircraft from (5.1% to 2.4%).

The final table shows emissions by transportation mode as a percentage of total emission. The largest relative changes were an increase in emissions for medium and heavy trucks (from 3.6% to 6.8%), an increase for cars and light trucks (from 15.2% to 17.1%), and a decrease for non-commercial aircraft from (1.2% to 0.7%).

The following are the two main findings of this analysis. First, the relative contribution to total emissions from industry decreased from 1990 to 2017, while those from the transportation sector increased. Second, the relative contribution of emissions from medium and heavy trucks to total emissions from all sources increased substantially, while those from cars and light trucks increased only modestly.

There are two main implications of the findings. First, because of the large relative decrease in emissions from the largest emitter—industry, we can expect an increased emphasis on reducing emissions from the second largest emitter—transportation. Second, because of the large relative increase in the contribution of medium and heavy trucks to total emissions, we can expect an increased emphasis on reducing emissions from these vehicles.

Michael Sivak is the managing director of Sivak Applied Research and the former director of Sustainable Worldwide Transportation at the University of Michigan.