In his State of the Union Address last month President Obama called for government support of natural gas powered trucks as a way of reducing foreign oil consumption. He may want to reconsider that in light of a new report published in Science. It suggests that switching from diesel to natural gas in trucks could increase greenhouse gas emissions.

The report adds to the growing debate about whether natural gas is good for the environment. Burning it releases about half as much carbon dioxide as burning coal. But methane, the main component of natural gas, is itself a powerful greenhouse gas. If it leaks out of pipes and wells it offsets the climate benefits that come from burning it rather than other fossil fuels. No one really knows how much methane is leaking, but the amount will determine whether policies promoting a shift to natural gas–a major emphasis of the Obama administration–make sense, at least from a climate perspective.

Some researchers have gone as far as saying that methane leaks are so large that we’re better off burning coal. The new study doesn’t go so far. Analyzing 20 years of data, it concluded that the Environmental Protection Agency has underestimated the amount of methane leakage, but concludes that burning natural gas is still better than burning coal.

But it says that the higher rate of leaks means that burning natural gas instead of diesel in trucks isn’t a good idea. That’s because burning diesel is considerably cleaner than burning coal. The savings from burning natural gas in trucks don’t offset the increased methane levels from leaks.

The new study isn’t definitive. It requires extrapolating from relatively sparse measurements of leaks. Ideally, we should employ technology for more direct and widespread measurement of methane leaks to know their actual impact (see “Measuring the Climate Impact of Natural Gas”). That’s expensive though, so it could be a long time coming. Meanwhile, the study notes, it’s clear what needs to be done—stop the leaks. It found that most of the leaks come from relatively few pieces of faulty equipment (see “A Pragmatic Approach to the Debate on Whether Natural Gas is Good for the Climate” and “Three Technologies Could Solve the Methane Leak Issue”).