CLOUDS CAUSED BY AIRCRAFT EXHAUST MAY WARM THE U.S.
NASA scientists have found that cirrus clouds, formed by
contrails from aircraft engine exhaust, are capable of increasing
average surface temperatures enough to account for a warming trend
in the United States that occurred between 1975 and 1994.
"This result shows the increased cirrus coverage, attributable
to air traffic, could account for nearly all of the warming
observed over the United States for nearly 20 years starting in
1975, but it is important to acknowledge contrails would add to and
not replace any greenhouse gas effect," said Patrick Minnis, senior
research scientist at NASA's Langley Research Center in Hampton,
Va. The study was published April 15 in the Journal of Climate.
"During the same period, warming occurred in many other areas where
cirrus coverage decreased or remained steady," he added.
"This study demonstrates that human activity has a visible and
significant impact on cloud cover and, therefore, on climate. It
indicates that contrails should be included in climate change
scenarios," Minnis said.
Above Image: These images from the NOAA-17 satellite taken
during the morning of March 20, 2004, show how a contrail outbreak
can appear to be regular cloud cover. The image on the left is a
standard infrared satellite image, while that on the right uses two
infrared wavelengths to reveal contrails. Many of the contrails
appear to be from planes either coming from or going to central and
southern California. Other contrails are due to aircraft on
southeast-northwest tracks into Seattle and Portland. The contrails
and resulting cirrus clouds are blown eastward by winds during the
Minnis determined the observed one percent per decade increase
in cirrus cloud cover over the United States is likely due to air
traffic-induced contrails. Using published results from NASA's
Goddard Institute for Space Studies (New York) general circulation
model, Minnis and his colleagues estimated contrails and their
resulting cirrus clouds would increase surface and lower
atmospheric temperatures by 0.36 to 0.54 degrees Fahrenheit per
decade. Weather service data reveal surface and lower atmospheric
temperatures across North America rose by almost 0.5 degree
Fahrenheit per decade between 1975 and 1994.
Minnis worked with colleagues Kirk Ayers, Rabi Palinkonda, and
Dung Phan from Analytical Services and Materials, Inc., of Hampton,
Va. They used 25 years of global surface observations of cirrus
clouds, temperature and humidity records from the National Centers
for Environmental Prediction (NCEP) reanalysis dataset. They
confirmed the cirrus trends with 13 years of satellite data from
NASA's International Satellite Cloud Climatology Project.
Both air traffic and cirrus coverage increased during the period
of warming despite no changes in the NCEP humidity at jet cruise
altitudes over the United States. By contrast, humidity at flight
altitudes decreased over other land areas, such as Asia, and was
accompanied by less cirrus coverage, except over Western Europe,
where air traffic is very heavy.
Cirrus coverage also rose in the North Pacific and North
Atlantic flight corridors. The trends in cirrus cover and warming
over the United States were greatest during winter and spring, the
same seasons when contrails are most frequent. These results, along
with findings from earlier studies, led to the conclusion that
contrails caused the increase in cirrus clouds.
"This study indicates that contrails already have substantial
regional effects where air traffic is heavy, such as over the
United States. As air travel continues growing in other areas, the
impact could become globally significant," Minnis said.
Humidity is the amount of water vapor in the air and determines
how long contrails remain in the atmosphere. Contrails that persist
for an extended period of time are most likely to impact the
Above Image: This enhanced infrared image from NASA's Terra
satellite shows a widespread outbreak of contrails over the
southeastern United States during the morning of January 29, 2004.
Satellite data are critical for studying the effects of contrails.
The crisscrossing white lines are contrails that form from planes
flying in different directions at different altitudes. Each
contrail spreads and moves with the wind. Contrails often form over
large areas during winter and spring.
Contrails form high in the atmosphere when the mixture of water
vapor in the aircraft exhaust and the air condenses and freezes.
Persisting contrails can spread into extensive cirrus clouds that
tend to warm the Earth, because they reflect less sunlight than the
amount of heat they trap. The balance between Earth's incoming
sunlight and outgoing heat drives climate change.
NASA's Earth Science Enterprise funded this research. NASA's
Earth Science Enterprise is dedicated to understanding the Earth as
an integrated system and applying Earth System Science to improve
prediction of climate, weather, and natural hazards using the
unique vantage point of space.
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