Contrails and Aviation-induced Cirrus Clouds

TOM CLARKE

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Contrails form when water vapour in the exhaust from jet-engines freezes high in the troposphere where airliners cruise. Being efficient at scattering light, contrail ice crystals have a greater 'greenhouse' effect than the carbon dioxide gas also produced by jet engines, says Myhre.

Measuring contrails' impact is not an exact science. Depending on a plane's altitude, and the temperature and humidity of the atmosphere, contrails can vary enormously in their thickness and duration, and therefore in their reflecting or insulating power. Most last minutes or hours, some last days. During the day, persistent contrails trap slightly more heat than they reflect back into space - at night they continue to trap heat. In areas with dense air traffic, such as Europe and North America, contrails could be warming the atmosphere by up to 0.1�C, Minnis and colleagues found in 1999².

Although warming by contrails is minor - about 75 times less than man-made CO₂ - "the future climatic impact of aircraft is very important", warns Myhre - with a predicted fivefold increase in air traffic in the next 50 years. Myhre and Stordal combined satellite images with data on the journey length and fuel consumption of air traffic. Comparing this with models of how contrails scatter light, they estimated how much heat contrails trap or reflect.

Like others before them, the duo found a net warming effect. But taking previous measurements of the reflecting properties of ice crystals in icy cirrus clouds into account, they found that when light hits contrails at low angles - like at dawn and dusk - they in fact reflect light, causing a cooling effect. The researchers plan to keep watching contrails as jet technology and flight patterns are set to change. Although cleaner burning, next-generation jet engines are expected cruise at higher altitudes and to make more contrails.

Minnis, P., Schumann, U., Doelling, D. R, Gierrens, K. M. & Fahey, D. W., Global distribution of contrail radiative forcing. Geophysical Research Letters, 26, 1853 - 1856, (1999).

� Nature News Service / Macmillan Magazines Ltd 2001

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Such temperature changes could have a profound impact, affecting growing patterns of some major crops and increasing the likelihood of larger insect populations, according to the researchers.

It may also be the clearest indication that human activity can alter the climate.

"We know for a fact that because of the contrails there are less clear days in the U.S. than there have ever been," said David Travis, a University of Wisconsin climatologist who headed the study. "We believe that human activity has much greater potential to change regional climate than it does to change global climate."

The range between the daytime high and nighttime low temperature is called the diurnal temperature, and it has decreased by 3 degrees on average around the nation, and by 5 degrees in the Midwest, Northeast and Northwest where air traffic is the heaviest, Travis said.

But whether the contrail effect changes the average 24-hour temperature is still under study. Based on three decades of records from 4,000 weather stations, scientists are trying to determine if the cooling during the day or the heating at night has a greater influence on the average daily temperature. So far the highs and lows seem to average out.

During the three-day grounding, the diurnal temperature increased by 3 to 5 degrees, the only time it has made such a significant change in the last 30 years, Travis said.

Satellite images of the U.S. showed that the sky was clearer than usual during the three days.

"During the three-day period when there were no commercial flights and only a few contrails from military planes, we got this sudden increase in the temperature range due to the fact that we suddenly had clearer skies across the country," Travis said.

The diurnal temperature range rose sharply on Sept. 11 and fell again on Sept. 14 when commercial flights resumed, he said.

The diurnal temperature range for Sept. 11-13 for the last 30 years across the U.S., prior to 2001, was 35 degrees. But for the same period last year after the terrorist attacks, it rose to 38 degrees.

Cirrus clouds affect temperature by reflecting some sunlight back into space during the day and by reducing the amount of infrared heat escaping from the earth at night, Travis explained. Scientists have long suspected that contrail clouds affect ground temperatures but there was little data comparing contrail-free skies with today's jet traffic. Satellite images of the Earth for Sept. 11 to 13 provided the first view of the skies nearly free of contrails in 50 years.

"This result, if it is corroborated by additional studies, represents the first large-scale evidence of the effect of contrails on climate," said Patrick Minnis, a senior research scientist at NASA's Langley Research Center in Hampton, Va.

Minnis used satellite images to study the individual growth of contrail clouds during the airport shutdown, a task that was previously impossible because the numerous contrails that crossed paths blurred the findings.

Studying satellite images of lone contrails from military craft on Sept. 12, Minnis was able to determine that the cirrus clouds formed from these contrails lasted an average of six to eight hours.

Six to eight contrails can grow to form a cloud cover the size of Massachusetts, he said. These clouds now make up 4 to 6 percent of the cloud cover on any given day, a huge increase from the 1 to 2 percent previously estimated.

On a typical day, an estimated 13,000 planes crisscross the nation at altitudes greater than 25,000 feet, the elevation at which cirrus clouds begin to form, Minnis said. As a result, contrails are more common between airports than they are around airports.

Cirrus clouds form in the upper atmosphere when there is sufficient moisture and temperatures of 30 to 40 degrees below zero.

Contrails are formed from jet engine exhaust when particles from burnt fuel and water vapor, which is flash-frozen into tiny droplets, serve as nuclei to which moisture already in the atmosphere can cling and grow into clouds.

Jets frequently fly at altitudes where there is not enough moisture to form clouds naturally, but there is enough moisture to sustain cloud growth once the initial cloud is formed from a contrail, Minnis said.

Unlike the warming from greenhouse gases, which are thought to have a global impact, contrail clouds act regionally over areas with the most flights, Travis said. How contrail clouds fit into the global warming scenario is unclear, but their ability to moderate extreme temperatures will complicate the issue.

Also complicating the global warming issue is a recent report by the General Accounting Office estimating that jet engine emissions may account for about 3 percent of greenhouse gases.

"Our winters have not been as dramatically cold in recent years as they have been historically," Travis said. "Some of what has been attributed to global warming may be due to contrails that throw down a blanket of clouds to warm the nights."

Reducing the extremes of day and night temperatures could have a subtle impact on ecosystems, Travis said. It may, for instance, increase insect populations that thrive better in moderate temperatures.

Some crops could also be affected. Cranberries and citrus orchards need a certain number of cold nights and warm days to ripen properly, Travis said.

Travis said he has mixed feelings about contrails. On the one hand they have been a prime focus of his research interest for the last 10 years. On the other hand they almost spoiled a recent vacation in the Pacific Northwest.

"There were so many contrails in the sky we weren't having any nice days," he said. "The contrails were making the days kind of gloomy and overcast. There aren't as many sunny days as we had when we were kids."

Copyright � 2003, Chicago Tribune

Contrails are Bad News