There are many ways to categorize the emissions of greenhouse gases (GHGs) such as carbon dioxide (CO2). One informative approach to compare emissions from fuel combustion is to distinguish stationary from mobile sources.
As the name suggests, stationary sources of emissions are in a fixed location. Examples of stationary sources include power, plants, petroleum, refineries, chemical plants, smelters, cement plants, steel mills, boilers, furnaces, and pulp and paper mills.
Mobile sources of emissions move from place to place and are typically associated with transportation activities. Examples of mobile sources include cars and trucks, aircraft, ships, trains, military vehicles, and construction and agricultural equipment.
Two important points about the data are presented here. First, this article uses information on CO2 emissions. Other GHGs, notably methane, are excluded. Second, emissions are measured at the point of combustion and thus do not reflect emissions across the full life cycle of a fuel. In transportation parlance, the data here are “tank-to-wheel” or “tank-to-wake.” Full life cycle emissions of all GHGs would change the emissions factors and in some cases the relative ranking of fuels.
Transportation accounts for about 24% of CO2 emissions from the world’s energy system.1 About three-quarters of total transportation CO2 emissions are from passenger road vehicles and road freight vehicles.2 More than 90% of the energy used in the transportation sector is derived from fossil fuels: gasoline for personal vehicles, diesel fuel for personal vehicles and trucks, jet fuel in aviation, and residual (bunker) fuel in maritime shipping.
Liquefied natural gas is increasingly used in maritime transportation as a fuel for container ships, bulk carriers, cruise ships, ferries, and LNG carriers themselves.3 When gasified and burned, LNG yields more than a 50% reduction in CO2 emissions compared to residual fuel at the point of combustion. However, some of the methane escapes unburned to the atmosphere, a very undesirable outcome given that the global warming potential of methane is about times that of CO2 over 100 years.4 This is known as “methane slippage.” As much as six percent of the methane “slips” into the atmosphere, about twice the rate assumed in current and pending regulations.3 These leaks must be plugged for LNG to realize its potential as a cleaner fuel compared to petroleum-based fuels.
Coal has the highest CO2 emissions factor among the most widely used fuels in stationary combustion. This is a principal reason behind the phase-out of coal in electricity generation that is underway in many countries.5 At the point of combustion, the CO2 emissions intensity of natural gas is 57% lower compared to bituminous coal. However, methane leakages across the natural gas supply chain may negate much or all of the advantages that natural gas has at the point of combustion.6
1 Hannah Ritchie (2020) – “Cars, planes, trains: where do CO2 emissions from transport come from?” Published online at OurWorldInData.org. Retrieved from: ‘Link
2 International Energy Agency, “Transport sector CO2 emissions by mode in the Sustainable Development Scenario, 2000-2030,” May 27, 2019, Link
3 Comer, Bryan, et al., “Fugitive and Unburned Methane Emissions from Ships,” International Council on Clean Transportation, Explicit ApS, Netherlands Organization for Applied Scientific Research, January 2024, Link
4 Intergovernmental Panel on Climate Change (IPCC), “Technical Summary,” In: Climate Change 2021 – The Physical Science Basis: Working Group I Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press; 2023:35-144, https://tinyurl.com/yc823pcs
5 Jaeger, Joel, “These 10 Countries Are Phasing Out Coal the Fastest,” Word Resources Institute, November 30, 2023, https://www.wri.org/insights/countries-phasing-out-coal-power-fastest
6 Gordon, Deborah, Frances Reuland, Daniel J. Jacob, John R. Worden, Drew Shindell, and Mark Dyson. “Evaluating Net Life-Cycle Greenhouse Gas Emissions Intensities from Gas and Coal at Varying Methane Leakage Rates.” Environmental Research Letters 18, no. 8 (July 2023): 084008. https://doi.org/10.1088/1748-9326/ace3db
