Contrails, digested

I’ve written a post on the NCAS Climate blog. Check it out here: http://ncas-climate.nerc.ac.uk/ncas-science-blog/237-science-blog-contrails-digested

contrails NASA/courtesy of nasaimages.org

Contrails and cirrus, as seen from the International Space Station, NASA/courtesy of nasaimages.org

Alternatively, read the text here. But do click through to the NCAS blog too, as there are some other interesting posts about climate science (including oceans, crops, blue blobs of death…) by other climate scientists.

Contrails, digested

I noticed that a new journal has just been launched – Nature Climate Change – and thought it would be a good source of inspiration for a blog post. Luckily for me, there was an interesting paper about contrails by Burkhardt and Kärcher in the first issue, AND there was a piece in the news and views section by Boucher about the paper. So I thought I’d do a kind of a ‘digested read, digested’ for this paper about contrails.

For the uninitiated, contrails are the line-shaped clouds that you sometimes see in the wake of aircraft in the sky. Contrails form when hot, moist exhaust from aircraft at cruising altitudes is emitted into the cold, dry air, and the water condenses (hence the name, which is a contraction of condensation trail). Contrails can spread out from their original line shape to cover much larger areas with cirrus (thin, wispy) cloud. This cirrus reflects some sunlight back to space (which cools the atmosphere), but it also absorbs infrared radiation coming from the Earth’s surface (warming the atmosphere), so the net effect is thought to be a warming. The interesting thing is that aircraft have the potential to punch above their weight (compared to other modes of transport) because of these contrails. Burkhardt and Kärcher developed a model of the formation, spreading and dissipation of contrails, so that they could see what effect the contrails had on the radiative forcing (a measure of the radiative imbalance of the atmosphere caused by a particular forcing agent; a positive value means a warming to the atmosphere, and a negative value means a cooling). What they found from their model might come as quite a surprise: that one of the biggest effects on climate from aircraft comes from the spreading out of contrails into cirrus clouds.

They found that the radiative forcing from the contrail cirrus as a whole was 9 times larger (37.5 mW m-2) than for line-shaped contrails alone (4 mW m-2). This is in comparison to a radiative forcing from aircraft emissions of carbon dioxide of 28 mW m-2. They also found that the contrail-induced cirrus reduced the amount of natural cirrus (-7 mW m-2), so the net effect was a 31 mW m-2 radiative forcing from contrail-induced cirrus. Put another way by Boucher: “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.” This work gives an interesting starting point for further investigation into both the climate effects of contrails, and potential climate change mitigation strategies. The Boucher article gives ideas on possible ways of reducing the radiative forcing caused by aircraft. I rather like the idea of reducing the water vapour in aircraft exhaust emissions (maybe releasing the water as ice instead), so the condensation trails don’t condense in the first place.

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