A study in the scientific journal Nature points to greater anthropogenic contribution towards atmospheric methane levels than previously thought, spurring the beginnings of a backlash against use of liquid natural gas (LNG) as marine fuel.
Atmospheric methane (CH4) emissions degrade to carbon dioxide (CO2) after a period of around 20 years. However, according to the Intergovernmental Panel on Climate Change (IPCC), during this time methane increases the greenhouse effect by 86 times as much, molecule-for-molecule, as carbon dioxide, thanks to its far higher global warming potential (GWP).
Incomplete combustion of LNG fuel, as well as leaks from the upstream oil and gas industry, are two of the various anthropogenic outlets for methane into the atmosphere. However, some methane is emitted from natural processes. Incorporating scientific findings from field work in Greenland, a new study measured the methane (CH4) content in air extracted from bubbles within permafrost ice sheets. “Here we use preindustrial-era ice core 14CH4 measurements to show that natural geological CH4 emissions to the atmosphere were… an order of magnitude lower than the currently used estimates,” indicated the Nature article’s authors.
“This result indicates that anthropogenic fossil CH4 emissions are underestimated by about 38 to 58 teragrams CH4 per year, or about 25 to 40% of recent estimates. Our record highlights the human impact on the atmosphere and climate, provides a firm target for inventories of the global CH4 budget, and will help to inform strategies for targeted emission reductions,” the study’s abstract concludes.
It is not the only disheartening development over methane in recent weeks, emerging in the wake of another study by the International Council on Clean Transportation (ICCT), which determines that over a 20-year lifecycle – the period of time during which LNG is expected to perform as a ‘bridge’ fuel to lower-carbon alternatives — LNG confers no benefit for climate change whatsoever, with any reduction in carbon emissions offset by those of methane.
“Using a 20-year GWP, which better reflects the urgency of reducing GHGs to meet the climate goals of the International Maritime Organization (IMO), and factoring in higher upstream emissions for all systems and crankcase emissions for low-pressure systems, there is no climate benefit from using LNG, regardless of the engine technology,” the ICCT determined. “High pressure injection dual-fuel engines using LNG emitted 4% more lifecycle GHG emissions than if they used MGO.
“The most popular LNG engine technology is low-pressure dual fuel, four-stroke, medium-speed, which is used on at least 300 ships; it is especially popular with LNG-fueled cruise ships. Results show this technology emitted 70% more lifecycle GHGs when it used LNG instead of MGO and 82% more than using MGO in a comparable medium-speed diesel engine.”
The study also examines the application of LNG over a 100-year timeframe, but the results are scarcely more positive: “Over a 100-year time frame, the maximum lifecycle greenhouse gas benefit of LNG is a 15% reduction compared with MGO, and this is only if ships use a high-pressure injection dual fuel (HPDF) engine and upstream methane emissions are well-controlled. However, the latter might prove difficult as more LNG production shifts to shale gas, and given recent evidence that upstream methane leakage could be higher than previously expected. Additionally, only 90 of the more than 750 LNG-fueled ships in service or on order use HPDF engines.”
However, DNV GL Maritime CEO Knut Ørbeck-Nilssen hit back at the latter study at a conference on Tuesday, indicating that on a timescale of 100 years, a medium-speed diesel vessel burning LNG would only be 28 times worse than one burning MGO.
“The pathway to carbon neutral fuel starts with gas, capitalising on existing infrastructure for at least one-to-two generations,” Nilssen said, adding that he continues to support the carbon levy proposal by the International Chamber of Shipping, to fund research and development and support early adopters of new fuels.