As a strengthening U.S. economy has boosted freight volumes, more commercial trucks are taking to the open road—all of them subject to new, stricter federal emissions standards.

So, are trucks powered by natural gas instead of diesel an effective way to reduce emissions? A new study urges caution until methane emissions in the “well-to-pump” production of natural gas can be reduced.

The study—published this week in “Environmental Science & Technology” and conducted by researchers with nonprofit advocacy group the Environmental Defense Fund—found that shifting to natural gas from diesel in the nation’s trucking sector “leads to damages to the climate” for anywhere from 50 to 90 years, depending on the type of natural-gas engine.

Over time, methane—which is released from wells, fuel tanks, and other points in the natural gas production process—has much stronger warming effects than CO2, researchers said.

“Natural gas trucks have the potential to reduce overall climate impacts compared to diesel,” study co-author Jonathan Camuzeaux said in a statement, “but only if we clean up the highly potent greenhouse gas emissions from the systems that produce and deliver the fuel.”

Jason Mathers of EDF added that policy makers shouldn’t promote switching to natural gas “until we are more certain about the magnitude of methane loss and have acted sufficiently to reduce emissions and improve natural gas engine efficiency.”

In a statement, Matthew Godlewski, president of Natural Gas Vehicles for America, a group representing companies that produce natural gas and natural gas-powered vehicles, engines and equipment, said the study “clearly demonstrates that there is a role for natural gas in addressing climate change.” NGVAmerica pointed to recent academic studies finding that methane emissions have been decreasing at hydraulically-fractured wells and in local distribution systems in recent years.

Mr. Godlewski called the EDF study “confusing” because it was conducted apart from research efforts on which the two groups are collaborating to investigate the same issue. One of those efforts, known as the “Pump to Wheels Methane Leakage” study, seeks “to end speculation about actual in-use methane leaks from natural gas stations and vehicles,” Mr. Godlewski said in the statement.

In a post on its website, EDF addressed the studies that are in progress, saying they were “distinct and separate” from the one published this week, which was meant to be “complementary” to other research projects, including “Pump to Wheels.”

About 2.1 million heavy-duty trucks handle the nation’s freight hauling, only a small percentage of which are powered by natural gas. And while demand for natural gas vehicles is growing, it’s not happening as quickly as some had anticipated.

Forecasters with ACT Research LLC said they expect a big increase in the number of new heavy-duty trucks this year: 338,000 compared with about 297,000 added in 2014. Only about 3% of new trucks will run on natural gas—roughly the same level as last year.