Liquefied natural gas (LNG) exports from the United States carry a 33% larger climate impact than coal over a 20-year period, despite persistent industry efforts to brand it a cleaner fossil fuel, Cornell University environmental scientist Robert Howarth concludes in a peer-reviewed paper in the journal Energy Science & Engineering.

The difference is mostly due to methane leaks from U.S. shale gas production, the paper states, and to a lesser extent, methane slippage in ocean-going tankers that burn some of their cargo en route to their final destination. Methane is a shorter-lived greenhouse gas the carbon dioxide, but packs about 84 times the global warming potential over the crucial 20-year span when humanity will be scrambling to get climate change under control.

“Proponents of increased exports of LNG from the United States to both Europe and Asia have often claimed a climate benefit, arguing that the alternative would be greater use of coal produced domestically in those regions, with increased emissions of carbon dioxide,” Howarth writes. “In fact, even though carbon dioxide emissions are greater from burning coal than from burning natural gas, methane emissions can more than offset this difference.”

Howarth’s 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,” with most of the production coming from fracking operations in Texas and Louisiana, writes Cornell Chronicle, the university’s in-house newsletter.

Liquefying gas by cooling it to -260°F/-162°C makes it easier to ship, “but that mode of transportation comes at an environmental cost,” Chronicle explains. “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.”

With a two-stroke engine tanker powered by LNG, the paper says, 47% of the climate footprint comes from extracting and transporting the gas, while another 36% occurs when the gas reaches its final customer and is used as directed. Liquefaction accounts for 8.8%, tanker transport for another 5.2%.

“This whole process is much more energy intensive than coal,” Howarth told the Guardian. “The science is pretty clear here: it’s wishful thinking that the gas miraculously moves overseas without any emissions.”

All of which makes it a “global priority” to rapidly move away from all fossil fuels, gas as well as coal, rather than buying into the “massive infrastructure expenditures” that would be needed to expand LNG exports, Howarth concludes in his paper. “A far better approach is to use financial resources to build a fossil-fuel-free future as rapidly as possible.”

The Guardian says this latest paper by Howarth, whose cautions and critiques about shale gas production date back at least to 2012, “caused something of a firestorm before its publication, with a draft of the study highlighted by climate campaigners such as Bill McKibben to the extent it was reportedly a factor in a decision earlier this year by the Biden administration to pause all new export permits for LNG projects.” That has the gas lobby, congressional Republicans, and some energy experts maintaining the paper overstates LNG emissions.

“It’s hard to swallow,” Louisiana energy consultant and researcher David Dismukes told The Guardian. “Does gas have a climate impact? Absolutely. But is it worse than coal? Come on.”

Howarth said that pushback triggered “more peer review than I’ve ever had before,” with eight other scientists involved in five rounds of review. “I don’t consider the criticism valid at all,” he said “It feels like a political job.”

Duke University climate scientist Drew Shindell, who was not involved in the study, said Howarth’s findings are plausible.

“Bob’s study adds to a lot of literature now that shows the industry’s argument for gas is undermined by the option to go to renewables,” Shindell said. “The debate isn’t really about whether gas is slightly better or worse than coal, though. It should be about how both are terrible and that we need to get rid of both of them.”