One of the main advantages of natural gas is that it is supposed to be far cleaner than oil or coal. Right now Congress is even considering a T. Boone Pickens-inspired bill aimed at converting the nation's truck fleet to run on natural gas. If it's passed, it will be in large part on the assumption that such a move will help the nation reduce climate-changing greenhouse gases.

But evidence continues to mount that natural gas is not as clean as we like to think.

In January, a ProPublica investigation found that large amounts of "fugitive" emissions were left out of common comparisons between coal and gas and that if these emissions were counted the advantages of natural gas dwindled. Our report found that the Environmental Protection Agency's emissions estimates from hydraulic fracturing in shale formations were 9,000 times higher than the agency had previously estimated. We also quoted Robert Howarth, a Cornell University professor, saying that he would soon release research that showed that the emissions from gas were even worse.

More details of Howarth's research, which is reportedly scheduled to be published in the journal Climatic Change, were released by The Hill and The New York Times this week. Howarth's conclusion -- that shale gas production is actually far dirtier than coal in terms of greenhouse gas emissions -- is attracting national attention.

Howarth's findings are based in part on the EPA's revelation that far more gas escapes into the atmosphere in production fields than was previously known, and on a mathematical tweaking of the intensity of methane gas' effect on the atmosphere. Howarth, whose figures for total emissions exceed even the EPA's revised estimates, calculates the impact of methane in the atmosphere over a 20-year period, saying the urgent need to address short-term climate change justifies that calculation. Over 20 years, methane is considered 72 times as powerful as carbon dioxide in its effects on climate change. Using that approach, Howarth concludes that gas may be between 20 and 100 percent dirtier than coal.

The EPA uses a different factor, calculating methane's effect on the atmosphere over 100 years, in part because the gas degrades over time. Using the 100-year time frame, methane's potency is diminished by about one third. ProPublica used this calculation in January and determined that in some cases -- where an old and inefficient power plant was used to burn the gas, for example -- natural gas may hold a 25 percent advantage over coal throughout its lifecycle, far less than the 50 percent advantage generally touted.

Howarth's calculations erase even that small advantage though. When he used the 100-year measure, he concluded that the greenhouse gas emissions footprint of coal and gas were equal.

There's plenty of parsing left to do here before clear answers emerge about exactly how natural gas stacks up. But one thing is increasingly certain: Without sustained efforts to shut off and capture leaked emissions in the gas production fields, whatever advantages natural gas does present will be diminished.