Hydrogen Contrails Will Have Lower Climate Impact, Study Says

Contrails from hydrogen-fueled aircraft are expected to form more readily and be more widespread than those from jet-fueled aircraft but will have less of a climate impact due to their lower density, predicts a Japanese study.

Findings from the theoretical study, conducted by researchers from the University of Tokyo and the Japan Aerospace Exploration Agency (JAXA), have emerged emerge as multiple projects to better understand the characteristics of contrail formation from hydrogen powered aircraft enter the advanced planning stage.

Although hydrogen is widely viewed as a future zero-carbon fuel, concerns remain over its potential climate effects—particularly from contrails. Although hydrogen combustion produces no soot, which in current jet fuel provides the condensation nuclei for the ice crystals at the heart of contrails, it generates 2.5 times more water vapor.

Ice crystals produced by today’s kerosene-fueled jet aircraft can grow and spread out to produce an aircraft-induced cirrus cloud. Although these reflect sunlight during the day, producing a cooling effect, at night, the cloud traps and warms the planet. This imbalance, or “radiative forcing,” caused by contrails is considered to have an overall warming effect on climate.

To predict the radiative forcing impact of hydrogen-induced contrails, the JAXA and University of Tokyo study combined global-climate and ice-nucleation models to simulate the exhaust plume of a hydrogen aircraft. The combination enabled researchers to evaluate the effects on microphysical processes such as sedimentation, condensation and sublimation.

Using a JAXA supercomputer, the models were then overlaid with global air traffic data from 2011 to assess the worldwide impacts and compare the results with contrail emissions from conventional jet-fueled aircraft.

The results indicated that for hydrogen-fueled aircraft, radiative forcing was around 23 mW/m2 compared to 58mW/m2 for kerosene aircraft—a 38% reduction. Other findings from the study, which were released in late September at the International Society for Air Breathing Engines (ISABE) conference in Toulouse, showed the annual average number and size of ice particles was also reduced, resulting in a reduction in the optical depth of the contrails.

The study also found that hydrogen contrail formation occurs at lower and warmer layers in the atmosphere due to the larger amount of water in the engine exhaust.

For hydrogen aircraft, the absence of soot emissions results in a lower ice number density—a measure of the number of ice nuclei in a given volume of air—compared to kerosene aircraft, says researcher Ryo Yoshida of the University of Tokyo Department of Aeronautics and Astronautics. “Consequently, the initial ice particle size is larger, condensation is less, and particles grow less efficiently. This leads to smaller ice number densities and particle sizes, reducing the optical thickness of the contrails.”

Yoshida adds that the results indicate “the lack of soot particles reduces the optical depth of the contrail and reduces the greenhouse effect.” He cautions that experimental verification, “including actual flights” and detailed examination of optical thickness and other factors is required to verify the study’s conclusions.