Can aircraft trails affect climate?: Grounding planes after the 11 September attacks may not have …

Published online 31 December 2008 | Nature | doi:10.1038/news.2008.1335

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Can aircraft trails affect climate?

Grounding planes after the 11 September attacks may not have caused unusual temperature effects.

Anna Barnett

When all commercial air traffic in the United States was grounded after the terrorist attacks of 11 September 2001, scientists got an unexpected opportunity to test ideas about the climate effects of the condensation trails left behind by jets.

A study in 2002 suggested that these contrails could have a significant effect on daily temperature patterns (see 'http://www.nature.com/news/2002/020808/full/news020805-7.html">Air-traffic moratorium opened window on contrails and climate'). But a new analysis now claims that altered US temperature patterns during the three flight-free days can be explained by natural variations in cloud cover, rather than the absence of planes.

Aircraft contrails can spread into cirrus-like clouds high in the atmosphere. Similar to natural clouds, they are thought to have an overall warming effect on the planet. But they can also moderate daily temperature extremes by trapping heat that escapes from the ground and reflecting sunlight. This raises the lowest overnight temperatures and, to a lesser degree, reduces the highs during daylight hours, scientists have suggested.

With air traffic projected to grow by 2–5% per year in the near future — amounting to at least a tripling in traffic by 2050 — the effects of contrails are expected to become an increasingly important factor in climate change. But atmospheric scientists are still unsure about the scale of the contrails' impact.

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… Examining patterns of cloud cover and temperature in early September at US weather stations from 1971 to 2001, Hong and his colleagues found that thicker, low clouds are the dominant influence on temperature extremes, whereas high clouds such as contrails have a minor effect at most. …