First of all, this question is not about the conspiracy theory based chemtrails as in the linked question, from the answers there and from the question Equation for predicting contrail formation I understand how contrails form and how they could be predicted. Also, the evidence (as in the links and discussions within the answers to the linked questions, reveal that much of the chemistry of contrails is water and some soot.

Observations of contrails (from the ground and from satellite imagery) in the webpage Contrails by Professor Steve Ackerman of the University of Wisconsin, he notes that contrails

rapidly dissipate or spread horizontally into an extensive thin cirrus layer.

and also states that

NASA and the DOE are sponsoring a research program to study the impact contrails have on atmospheric chemistry, weather and climate.

Given it is long known that clouds affect the atmospheric radiation budget, is there evidence of atmospheric radiative effects of contrails?

  • $\begingroup$ This may be a good start to look for answers. Search results for "contrail" in AMS journals: journals.ametsoc.org/action/… $\endgroup$ Dec 11 '14 at 21:00
  • $\begingroup$ @IRO-bot thank you for that - I would imagine the magnitude of radiative effects would be very minor. $\endgroup$
    – user889
    Dec 11 '14 at 22:51
  • 1
    $\begingroup$ That would be my guess as well, although my knowledge is very limited in this area of research. $\endgroup$ Dec 11 '14 at 23:32
  • $\begingroup$ @IRO-bot it's parallel to my expertise, I might self answer this one (and certainly would accept any well researched answer). $\endgroup$
    – user889
    Dec 11 '14 at 23:45

We had a surprising opportunity to study this very question during the period of September 11-14, 2001, when all air traffic was grounded across the United States.

The research was inconclusive, but they found that there was a 1.8 degree celsius increase across the US during this time frame compared to the three days before and after that time frame. However, this shift may have been coincidental. It was found by others that this temperature shift was within the normal range for the time of year.

Ultimately, the question of whether these high-level clouds impact the weather and temperatures is still an open debate.

The source for this answer is from a Nature.com article. However, they take their sources from the original research:

  • $\begingroup$ Also, another article saying the same thing. There's simply no answer to this question yet. It's still being debated. $\endgroup$
    – Richard
    Dec 12 '14 at 11:53

Cirrus cloud is already widespread in the tropics, and is slowly spreading polewards in response to global warming. If the air temperature is around -40 deg C, as it commonly is at the cruising altitude of modern jets, then ice crystals form almost immediately as the water vapour crystallizes around nuclei (soot particles from the burnt aviation fuel). High level winds then disperse the ice crystals to form additional cirrus. The effect is pretty small, with high-level contrail cirrus covering about 0.1% of the global surface area, compared to about 20 to 25% coverage that was there in the first place. The effect seems larger because most high level jet traffic is at mid latitudes. See http://www.flightradar24.com/27.36,-64.11/3 (but beware - this page is addictive). The next question is 'what effect does increased cirrus have on the global heat balance'? Ice crystals reflect incoming solar radiation, but also reflect outgoing thermal radiation - acting as a kind of Greenhouse blanket. According to theory, the latter should slightly exceed the former, thus constituting a positive feedback to global warming. In the tropics the cirrus is known to cause slight warming, but at other latitudes the jury is still out. The ice crystals in cirrus vary according to size, shape, orientation with respect to incoming solar radiation, cloud density and cloud thickness. Also, it is known that tropical, mid-latitude and polar cirrus all behave differently, so there is still some research required to get a good handle on the global effect of cirrus in general, and contrails in particular. Liou gives a good summary of current (?) knowledge in http://people.atmos.ucla.edu/liou/Cirrus_&_Climate.pdf This is an area of active research. Does anyone have a more up-to-date assessment?


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