Taking action to reduce greenhouse gases, particularly carbon dioxide, is a must to mitigate climate change. And the importance of doing it on a local level is more important now than ever before.

Two professors from Penn’s School of Design, John Landis and Erick Guerra, and former Penn professor David Hsu, now at the Massachusetts Institute of Technology, recently published a report in the Journal of Planning Education and Research that examined ways to cut CO2 emissions through two sectors: housing and transportation.

“These are two places where if you wanted to take a regional approach to reducing greenhouse gases, you could,” says Landis, the Crossways Professor of City and Regional Planning.

He notes how important it was to merge these two fields for the study: “Usually there are people looking at housing, and others looking at energy efficient transportation. We wanted to put the two of them together.”

Knowing there wouldn’t be a one-size-fits-all answer to lessening greenhouse gas emissions for every metropolitan area, the researchers came up with various models or scenarios—and determined, using real, varied, and historical data, their potential success—for 11 distinct locations: Philadelphia, Atlanta, Boston, Chicago, Cleveland, Denver, Houston, Los Angeles, Miami, Phoenix, and Seattle.

Overall, the researchers estimated a significant effect—on average, a 31 to 46 percent reduction by 2030—by adopting energy-conservation and retrofitting standards for residences. (The latter, higher percentage would be obtainable if paired with former President Barack Obama’s Clean Power Plan, which the Trump administration is attempting to repeal.)

Due to projected population growth, emissions from transportation are slated to still increase, even if cities are built up compactly, enabling people to walk and take public transit more and drive less. But the researchers found that if these infill efforts are combined with an improved fuel-economy standard of 40 miles per gallon, emissions would fall by an average of 31 percent by 2030. In other words, cleaner vehicle technologies are vastly important.

With all this in mind, Landis says, it’s believed that fast-growing metropolitan areas, such as Phoenix, Seattle, and Houston, have the potential to significantly reduce greenhouse emissions on a regional level, much more than cities that aren’t growing quite as quickly, such as Philadelphia and Cleveland. In order to do this, Landis explains, cities and communities must get organized, as well as commit to sustainability.

The researchers first submitted their article “Intersecting Residential and Transportation CO2 Emissions: Metropolitan Climate Change Programs” in June 2016. They subsequently added “in the Age of Trump” to the end of the title after the new president was elected.

“When the Obama administration was in office, these local efforts to either improve energy efficiency of houses or getting people out of cars were really just icing on the cake,” Landis says. “They were helpful but not essential. Now with the Trump administration, where they are trying to undermine the Clean Power Plan and roll back on fuel economy standards, these local efforts are much more important.”