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

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

How big is the effect of contrails on climate change? GETTY

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 20021 suggested that these contrails could have a significant effect on daily temperature patterns (see '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.

Theory or fact?

Two studies1,2 noted that when planes stopped flying on 11–14 September 2001, the average daily temperature range in the United States rose markedly, exceeding the three-day periods before and after by an average of 1.8 °C. The unusual size of the shift, says David Travis of the University of Wisconsin–Whitewater, who led both of the earlier studies, implied that an absence of contrails gave the temperature range a significant boost. But that idea, he says, was "more like a hypothesis" than a firm conclusion.

Research led by Gang Hong, an atmospheric scientist at Texas A&M University in College Station, now suggests that this hypothesis is wrong3. 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. They add that the 2001 temperature swings seem to be within the range of natural variability over those decades.

Hong's work doesn't prove that the contrails have no effect on temperature, just that they are unlikely to have a major role, says Ulrich Schumann, director of the Institute of Atmospheric Physics at the German Aerospace Center in Oberpfaffenhofen, near Munich.

But that contrail effect, Schumann cautions, has been exaggerated in public discussions because the events of 2001 were so shocking. "Some not very good scientific arguments were misused," he says.

Up in the air

The research by Hong and his colleagues, published in Geophysical Research Letters, follows other studies arguing that the September 2001 temperature variations can be explained by the clear, dry weather on the crucial days4, and that climate-modelling results do not support the contrail effects claimed by Travis's group5.

Travis, however, stands by his findings. Absent contrails were never assumed to be the sole cause of the large temperature ranges, he emphasizes. "We've always said it's a combination of the lack of airplanes and the natural weather conditions," he says.

Because Hong's analysis studied high-level clouds in general — and not contrails in particular — Travis says that specific conclusions cannot be drawn about the role of contrails from the survey. "Their calculation of cloud influences was without any consideration of contrails," he says. "They were just looking at natural clouds, and not actually at the presence or absence of contrails."

He adds that Hong's study shows that an increase in the average daily temperature range of 1.8 °C is extremely rare. And although previous events on that scale can be explained by local meteorological effects, says Travis, the jump seen in 2001 cannot be accounted for in this way.

Travis maintains that contrails are an important factor in climate change. "Eventually these impacts will be felt," he says.