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Thursday, March 31, 2011

Nature surprise: Aircraft contrails “may be causing more climate warming today than all the carbon dioxide emitted by aircraft since the start of aviation.”

by Joseph Romm, Climate Progress, March 30, 2011

A new study finds that all those aircraft condensation trails you see across the sky may, on any given day, be warming the planet more than all the CO2 emitted by all the planes since the Wright Brothers first flew over a century ago.

The study is “Global radiative forcing from contrail cirrus” (subs. req’d) in Nature Climate Change by Ulrike Burkhardt1 and Bernd Kärcher of the DLR German Aerospace Center. That the climate forcing from airplanes is considerably greater than just that of their CO2 emissions has been known a long time.

What this study adds is an analysis of an “important but poorly understood component of this forcing,” namely contrail cirrus — a type of cloud that consist of young line-shaped contrails and the older irregularly shaped contrails that arise from them.”  It turns out that “the radiative forcing associated with contrail cirrus as a whole is about nine times larger than that from line-shaped contrails alone.”  On the bright side, “contrail cirrus cause a significant decrease in natural cloudiness, which partly offsets their warming effect.”

Nature CC’s news story has this explanation and satellite images:

Aircraft-engine emissions are mostly composed of carbon dioxide, water vapour, nitrogen oxides, sulphur oxides and aerosol particles. As well as the direct effect that these emissions have on climate, aviation has an added impact induced by the formation of condensation trails (contrails) in the wake of the aircraft. These line-shaped trails 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.  [Fig. 1, click to enlarge]

Satellite infrared images of contrails spreading into cirrus clouds over the UK.
The young contrails, which appear as a spring shape and sharp lines in the first image, gradually spread into cirrus clouds, which appear as bright white areas in the lower images. The time of each image and the satellite used to take it are shown in the inset of each frame. 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.
Here are some more details:
Burkhardt and Kärcher developed a process-based model of how contrails form, grow (through the depletion of water vapour in the surrounding air), spread and finally disappear (through mixing and fall-out of the ice crystals). By tracking the fate of contrail and natural cirrus separately, the authors can quantify the radiative forcing from spreading contrails (including young line-shaped contrails), which they estimate to be 38 mW m−2. This can be compared with a radiative forcing of 4 mW m−2 from young contrails alone and 28 mW m−2 from aviation carbon dioxide. Interestingly, spreading-contrail cirrus clouds cause a reduction in natural cirrus, because they modify the water budget in the upper troposphere; however, this reduction in natural cirrus is relatively small (-7 mW m−2).
The lead author Ulrike Burkhardt told Reuters, “You can get rid of contrails very quickly. You can’t get rid of CO2 quickly.”  As Nature put it:
Overall, and despite their short lifetime, contrails may have more radiative impact at any one time than all of the aviation-emitted carbon dioxide that has accumulated in the atmosphere since the beginning of commercial aviation. It is important to note, however, that the emitted carbon dioxide would continue to exert a warming influence for much longer than contrails, should all aircraft be grounded indefinitely.
Yes, well, that ain’t gonna happen.

The question arises as to whether changing the flight pattern of aircraft or perhaps their engine technology could ameliorate this problem:
These findings are important, because 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 ice89. Even though this would help to reduce both young and spreading contrails, such a strategy is likely to lead to an increase in fuel consumption. It would be important to make sure that, given the large difference in atmospheric lifetime of carbon dioxide and contrails, the associated carbon dioxide penalty does not offset in the longer term the gain obtained by avoiding contrail formation10. 
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 engine11. The condensed water could be vented in the form of large ice crystals or droplets that would fall quickly through the atmosphere. Reducing the content of water vapour in the engine exhaust would make contrail formation less likely.
Certainly this is an important area for continued work.

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