New research, published in the journal, Atmospheric Chemistry and Physics, on 27 June 2019, warns that the global heat-trapping effect of contrail cirrus clouds, the thin white clouds that jet airplanes leave behind as they cross the sky, will triple by 2050. That is unless airlines and airplane builders significantly reduce emissions, or air traffic patterns change.
The research paper can be accessed here.
Air travel impacts on climate change in two distinct ways:
- Through greenhouse gases (GHGs) released as airplane release fuel;
- Via the heat-trapping effect of the condensation that forms, as hot gases and soot from partially burned jet fuel, activate water particles, that freeze and form contrail clouds. These clouds can persist for more than half a day, and under certain atmospheric conditions, they can even merge, and spread across thousands of square miles, expanding the heat-trapping effect across wide areas.
In 2005, air traffic was responsible for about 5% of human-caused climate warming. Contrail cirrus clouds are already the largest contributor to aviation’s climate impact.
Whilst the commercial aviation industry is targeting a reduction in net aviation carbon dioxide (CO2) by 2050, relative to 2005 levels (see IATA; link), the plans do not address the climate change warming caused by contrail cirrus clouds.
The co-author of the study, and a climate change researcher at the German Aerospace Centre, DLR, Ulrike Burkhardt, states, “usually people say clouds are cooling the surface. For lower clouds that’s true. They reflect sunlight. But high clouds that are optically thin are most likely to warm the atmosphere.”
The researchers also highlight that the impact of contrail cirrus clouds on global warming will be stronger over North America, and Europe, the busiest air traffic areas on the globe, but it will also significantly increase in Asia as air travel increases.
The research methodology usesECHAM5-CCMod, an atmospheric climate model, with an online contrail cirrus parameterization, which includes a microphysical two-moment scheme, to investigate the climate impact of contrail cirrus for the year 2050. The research takes into account the predicted increase in air traffic volume, changes in propulsion efficiency and emissions (in particular soot emissions), and the modification of the contrail cirrus climate impact due to anthropogenic climate change.
…and what to do? The new study highlights that cleaner aircraft emissions would help solve some of the problem. Reducing soot pollution emitted by aircraft engines decreases the number of ice crystals in contrail cirrus clouds, which in turn reduces their climate impact. The researchers highlight, however, that even a 90% reduction of soot would probably not be enough to limit the climate impact of contrail clouds to 2006 levels.
The researchers also showed how improvements in fuel and propulsion efficiency, adjusting flight paths and the use of alternative fuels could shake up the equation. For example, rerouting flights or changing altitude to avoid regions most sensitive to the effects of contrail formation could limit their short-term heating impacts.
Air travel is not going to stop. It’s growing fast and will continue to grow for the foreseeable future. Bill Hemmings, aviation and shipping program director, for the NGO Transport and Environment, states “the non-CO2 impacts from aviation is the elephant in the room nobody wants to talk about”.