Originally Posted by sxotty
Methane is a far more potent GHG than CO2 and there will be plenty of methane leaking to the environment if we have tons of CNG vehicles running around. They are better for the immediate area they operate in though. So local effects are lessened global effects see little change.
More on sxotty's point above via a National Geographic article on LNG truck refilling stations
Measuring the environmental benefit of the switch to natural gas is complicated, even though it's clear that burning of natural gas produces about 30 percent less carbon dioxide than diesel fuel. One of the primary issues is leakage of methane, the main component of natural gas, which is 25 times more potent than CO2 as a greenhouse gas over 100 years. The U.S. government says when methane leakage in the production process is accounted for, a switch from gasoline to natural gas as a vehicle fuel still cuts greenhouse gas emissions, but only 6 to 11 percent.
The issue is even more complex, however, because methane's global warming potential is far worse when examined over a shorter time frame; it is 72 times more potent than CO2 as a heat-trapping gas over 20 years. Many scientists say it is vital to consider this short-term impact. A study published last year in the Proceedings of the National Academy of Science concluded that based on estimated leak rates, the atmosphere would be worse off for 80 years before any climate benefit would be realized from a switch to natural gas for gasoline-powered cars. It could take even longer to see a benefit from a switch for heavy-duty trucks, the study said.
So the nonprofit Environmental Defense Fund, working with partners in industry, has begun a deeper study of methane leak rates when natural gas is used as a fuel for heavy-duty vehicles. Chief scientist Steve Hamberg said the group will gather data on methane leakage from incomplete combustion in trucks, leaks from fittings in the system for delivering and storing natural gas, and from the filling stations themselves. "If you are switching fuels in the transportation sector, you need to be especially careful about methane losses through the supply chain if you want to be sure you are not increasing climate effects," he said. "This is not to say it can't be done." But because the energy content of diesel and natural gas are relatively close, "it doesn't take a lot of leakage of methane to result in higher net emissions from methane," he said.
A 6-11% benefit is definitely too small to convince me that switching to CNG is a good idea, not to mention the whole bit about being worse off for the first 80 years.
I will now cease my proselytizing for the fuel.