“Contrails formed by aircraft can evolve into cirrus clouds indistinguishable from those formed naturally. These ‘spreading contrails’ may be causing more climate warming today than all the carbon dioxide emitted by aircraft since the start of aviation.Aviation is at present responsible for about 3% of all fossil fuel carbon dioxide emissions, but an estimated 2–14% of anthropogenic climate forcing. Furthermore, its contribution to climate forcing could triple by 2050, according to some scenarios. As such, mitigating the impact of aviation on climate has become a subject of considerable public and political interest. The debate is complicated, however, by the fact that aviation’s climate impact results from a number of different factors, as well as by the large uncertainty in the effect that some of these factors have on climate. Writing in Nature Climate Change, Burkhardt and Kärcher present a global modelling study that quantifies the climate effect of ‘spreading contrails’ — the least well quantified of all the aviation-related climate-forcing agents.” Olivier Boucher, “Atmospheric science: Seeing through contrails,” Nature Climate Change 1: 24–25, 29 March 2011, reviewing the paper by Ulrike Burkhardt & Bernd Kärcher, “Global radiative forcing from contrail cirrus,” Nature Climate Change 1: 54–58, 29 March 2011.
“Condensation trails (contrails) in the wake of an aircraft are formed by the mixing of hot, moist air coming out of the engine with cold ambient air. When the atmosphere is supersaturated with respect to ice, the line-shaped contrails can spread to form cirrus cloud, which has a warming effect on climate. Its relevance to the climate system remains unknown. Burkhardt and Kärcher used a model that simulates this spreading process to assess the warming effects of contrails and the cirrus clouds that form from them. Their results indicate that so-called spreading contrails cause an order of magnitude more climate warming than the line-shaped contrails alone, and are the largest single climate-forcing agent associated with aviation. However, contrail spreading is not the only mechanism that could explain this increase. It has also been suggested that aircraft-emitted aerosols could serve as ice nuclei and facilitate the formation of cirrus cloud. To understand the impact of aviation on climate, it is necessary to quantify the importance of these two mechanisms. This, however, is not a straightforward task.
These findings are important if the calculations of Burkhardt and Kärcher are correct. They provide a basis to develop mitigation strategies to reduce the impact of aviation on climate. For instance, it has been suggested that flight routes or flight altitudes could be planned and altered in real time to avoid parts of the atmosphere that are supersaturated with respect to ice, but such a strategy is likely to lead to an increase in fuel consumption. Moreover, the results by Burkhardt and Kärcher might also justify the development of a novel engine concept that seeks to condense a fraction of the water vapour in aircraft emissions in a cooling unit before it leaves the engine. Reducing the content of water vapour in the engine exhaust would make contrail formation less likely.
The work of Burkhardt and Kärcher offers some exciting pointers as to how the impacts of aviation on the climate system might be reduced, but the uncertainties remain large. Given the urgency of the issue, it is important that research on the climate impacts of contrails and on how contrails could be mitigated through technological advances or operational changes in the aviation industry are pursued in parallel.”