Liquefied natural gas leaves a greenhouse gas footprint that is 33% worse than coal, when processing and shipping are taken into account, according to a new Cornell study.

“Natural gas and shale gas are all bad for the climate. Liquefied natural gas (LNG) is worse,” said Robert Howarth, author of the study and the David R. Atkinson Professor of Ecology and Environmental Biology in the College of Agriculture and Life Sciences. “LNG is made from shale gas, and to make it you must supercool it to liquid form and then transport it to market in large tankers. That takes energy.”

The research, “The Greenhouse Gas Footprint of Liquefied Natural Gas (LNG) Exported from the United States,” published Oct. 3 in Energy Science & Engineering.

The emissions of methane and carbon dioxide released during LNG’s extraction, processing, transportation and storage account for approximately half of its total greenhouse gas footprint, Howarth said.

Over 20 years, the carbon footprint for LNG is one-third larger than coal, when analyzed using the measurement of global warming potential, which compares the atmospheric impact for different greenhouse gases. Even on a 100-year time scale – a more-forgiving scale than 20 years – the liquefied natural gas carbon footprint equals or still exceeds coal, Howarth said.

The findings have implications for LNG production in the U.S., which is the world’s largest exporter, after it lifted an export ban in 2016, according to the paper. Almost all of the increase in natural gas production since 2005 has been from shale gas. Howarth said the exported LNG is produced from shale in Texas and Louisiana.

The liquefication process – where the extracted natural gas is cooled to minus 260 degrees Fahrenheit – makes LNG easier to transport on tanker ships.

But that mode of transportation comes at an environmental cost. The ships with two- or four-stroke engines that transport LNG have lower carbon dioxide emissions than steam-powered ships. But as those stroke-engine vessels burn LNG during storage and transportation, methane slips through as emitted exhaust gas, putting more into the atmosphere.

Methane is more than 80 times more harmful to the atmosphere than carbon dioxide, so even small emissions can have a large climate impact, Howarth said.

That’s why, he said, the modern LNG tankers with two- and four-stroke engines have more greenhouse gas emissions than those tankers powered by steam. Regardless of better fuel efficiency and lower carbon dioxide emissions, methane still escapes in the tanker’s exhaust.

And significant methane emissions occur in the natural gas liquefication process, a figure close to 8.8% of the total when using the global warming potential. Methane emissions from tankers vary from 3.9% to 8.1%, depending on the ship.

“Almost all the methane emissions occur upstream when you’re extracting the shale gas and liquefying it,” Howarth said. “This is all magnified just to get the liquefied natural gas to market.

“So liquefied natural gas will always have a bigger climate footprint than the natural gas, no matter what the assumptions of being a bridge fuel are,” Howarth said. “It still ends up substantially worse than coal.”

The research was supported by a grant from the Park Foundation. Howarth is a fellow at the Cornell Atkinson Center for Sustainability.