A radiative forcing is something that disturbs the balance between incoming and outgoing radiation in the atmosphere. A positive forcing warms the surface, and a negative one cools it. Contrails, and the clouds which result from them (see the BBC link, mentioned in the previous post) are the largest radiative forcing associated with aviation. Despite their similar appearances, especially when longer lasting contrails begin spread out and resemble clouds, the radiative forcing for contrails is 9 times larger then for similar shaped, naturally formed cirrus clouds (Burkhardt and Kärcher 2011).
Contrails can be expected to have the same effects as cirrus clouds i.e to reflect incoming short wave radiation and to retain long wave outgoing radiation from the surface - this is due to the small ice crystals that contrails are made up of. Small differences in the properties between contrails and cirrus clouds for example the size of ice crystals however, creates some ambiguity and differences in the forcing effects of contrails. Contrails have a smaller vertical depth and are formed at cooler temperatures meaning that contrail induced cirrus clouds can form and persist high in the troposphere even when there are no natural cirrus clouds.
Ascertaining the global coverage of contrails is difficult as only young contrails, which still exhibit the classic line formation (see background picture to this blog) can be distinguished from natural cirrus cloud. It is impossible to tell from satellite imagery whether cirrus clouds have been induced by aircraft or are natural. It is also thought that global atmospheric circulation maintains a higher level of cirrus cloud over Europe in comparison to the USA, where there is more air traffic (Burkhardt and Kärcher 2011).
The aftermath of 9/11 presented a unique research opportunity in that there were no commercial flights in the 3 days that followed. Travis et al. (2002) compare the diurnal temperature range for the 11th - 14th September 2001 with that from 1971- 2000. They found there was an anomalous increase in diurnal temperature (1.1°C), which they partially attribute to the lack of contrails in this 3 day period. It has also been suggested that the presence of contrails, which ties up water vapour in the atmosphere, means that this vapour can not be used to expand natural cirrus clouds - and so can prevent their expansion and counter their own climatic impact (Burkhardt and Kärcher 2011).
The aftermath of 9/11 presented a unique research opportunity in that there were no commercial flights in the 3 days that followed. Travis et al. (2002) compare the diurnal temperature range for the 11th - 14th September 2001 with that from 1971- 2000. They found there was an anomalous increase in diurnal temperature (1.1°C), which they partially attribute to the lack of contrails in this 3 day period. It has also been suggested that the presence of contrails, which ties up water vapour in the atmosphere, means that this vapour can not be used to expand natural cirrus clouds - and so can prevent their expansion and counter their own climatic impact (Burkhardt and Kärcher 2011).
These examples show the difficulty presented in pinpointing the effects contrails have on radiative forcing and the difficulty of assessing their impacts due to their close correlation with naturally occurring cirrus clouds.
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| Contrails as seen from the International Space Station. Credits ESA/NASA |
#contrailversial
ReplyDeleteDo you think that comparing a 3 day temperature record to such long records is at all useful given the temperature variations that occur frequently on short timescales. These guys think 8 years is too short: http://onlinelibrary.wiley.com/doi/10.1002/joc.1136/abstract
ReplyDeleteYes, I agree 3 days is a bit short but never has such an opportunity arisen before, and they did not have the luxury to extend the observation time!
ReplyDeleteThey address the short term, daily changes in atmospheric conditions and know exactly where natural clouds that could affected radiative forcing were over the 3 day period. Even accounting for the large variations in cloud observed, they attribute the absence of contrails for the abnormal average diurnal temperatures recorded.
It does seem strange how contrails that last up to a few hours at their most extreme are responsible (or at least partly) for such observations. I have found an earlier study (see below) which has examined the cloudiness over the American Midwest from 1901-1977, where they attribute observed increased cloudiness from about 1960 onwards to contrails and natural cirrus clouds expanded by jet aircraft as I have discussed.
http://journals.ametsoc.org/doi/pdf/10.1175/1520-0450%281981%29020%3C0496%3AMCSATT%3E2.0.CO%3B2
3 days is short for one particular metro region... But the same 3 days over the the entire country could be telling... if all the areas were contrails are concentrated had the same differences... When normally they don't coincide.
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