...AND GREENER...:

Around the world, commercial air travel accounts for over 2 percent of energy-related CO2 emissions, according to the International Energy Agency. That number is set to soar in the coming years as more oil-burning planes and more passengers hit the skies.

In the near term, airlines and plane manufacturers are working to curb emissions by designing more fuel-efficient engines, electrifying ground operations and increasing their use of ​"sustainable aviation fuel" made from used cooking oil, forestry residues, carbon dioxide and other feedstocks. Last year, alternative fuels accounted for less than 0.1 percent of the total jet fuel used by major U.S. airlines.

Although plant- and waste-based fuels can be cleaner to produce than petroleum-based fuel, they still emit carbon dioxide when burned in engines. Hydrogen does not -- that's why airlines and manufacturers are joining efforts to develop H2-powered aircraft. Fuel cells in particular don't generate harmful nitrogen oxides or fine particulate matter, since they don't burn fuel.

A retrofitted fuel-cell aircraft would emit about one-third less CO2 over its lifetime than an aircraft burning ​"e-kerosene," a type of sustainable aviation fuel made from electricity, water and carbon dioxide, according to the ICCT analysis.

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