It is renowned as the least polluting fossil fuel. For this reason, it is a candidate to become the energy source to focus on for a smooth and orderly transition. However, fossil gas is not (always) less impactful on the climate than oil and coal. It greatly depends on where it is produced and how. A crucial difference lies in the way its climate impact is calculated. When considering liquefied natural gas (LNG) produced and exported from the United States, the carbon footprint of LNG is 33% higher than that of coal, with 160 grams of CO2 equivalent per megajoule (gCO2eq/MJ) compared to 120.
Calculating the Carbon Footprint of LNG
This is the conclusion reached by a study from Cornell University. What parameters did the authors consider? In addition to the emissions generated once burned, the researchers also quantify those associated with the extraction of fossil gas, processing, transportation, and storage. Together, these stages generate about half of the carbon footprint of LNG.
The balance makes LNG worse than coal for two main reasons. The first depends on the raw material. Most of the gas that is liquefied for export in the United States is shale gas from Texas and Louisiana, which has a higher footprint. Then there is the liquefaction process for transport, which is energy-intensive because it requires cooling to -160°C. In this phase, methane is released: 8.8% of total emissions are generated. Transportation is another significant factor in the overall footprint of LNG, varying between 3.9% and 8.1% depending on the type of ship.
“Almost all methane emissions occur upstream when shale gas is extracted and liquefied,” explains Robert Howarth, the lead author of the study. “All of this is amplified just to bring liquefied natural gas to market. Therefore, LNG will always have a greater climate impact than natural gas,” he adds.
When calculating the carbon footprint of LNG over the first 20 years in the atmosphere, it is 33% larger than coal’s. But even on a 100-year time scale, which “dilutes” the weight of methane emissions, the carbon footprint is equal to or slightly above that of coal. Methane has a climate-warming potential 82.5 times that of CO2 in the first two decades in the atmosphere. The CH4 molecule is less persistent than that of CO2, so over a 100-year period, the climate-warming potential of methane drops to 28 times that of carbon dioxide.