A new NASA study claims that man-made cirrus clouds formed by commercial jet engine exhaust might be responsible for increased surface temperatures detected in the U.S. between 1975 and 1994.

Climate data shows that cirrus cloud cover over the U.S. has increased by 1 percent per decade, and the report says the rise is likely due to commercial air traffic.

“High-altitude air traffic has the potential for increasing cirrus coverage and
thickening existing cirrus clouds by generating additional ice crystals,” said study lead author Patrick Minnis, senior research scientist at NASA Langley Research Center in Hampton, Va.

Cirrus clouds, whether natural or artificial, play an important climatological role because they trap heat in the atmosphere by reflecting infrared radiation emitted from the earth’s surface.

The study, published in the April 15 issue of the Journal of Climate, estimates that cirrus clouds from jet-engine condensation trails (contrails) increased the temperature of the lower atmosphere by anywhere from 0.36 degrees F to 0.54 degrees F per decade, which tends to agree with National Weather Service data that shows the temperature of the surface and lower atmosphere rising by almost 0.5 degrees F per decade between 1975 and 1994.

Hot Air

Contrails result from turbulence generated by jet aircraft engine exhaust, which is hot and moist. The exhaust’s water vapor comes from the combustion of hydrogen in aircraft fuel. Contrails form when the hot, humid exhaust air mixes with cooler ambient air, similar to the vapor cloud you create when you exhale in cold weather and see your breath.

Since the exhaust takes a moment to cool and mix with the surrounding air, there is normally a 150-foot to 300-foot gap behind an aircraft before the contrail appears. Upper atmospheric winds distort and spread the contrails once they have been formed.

“Contrails occupy a fairly special niche in atmospheric thermodynamics because they produce ice clouds at ambient relative humidities that are less than those required for most natural cirrus cloud formations,” Minnis said.

Humidity in the air also determines how long contrails remain in the atmosphere. Persistent trails sometimes form large patches of fibrous clouds indistinguishable from natural cirrus, cirrocumulus or cirrostratus clouds. Contrails that persist for an extended period are most likely to affect the climate. Earlier studies determined that at flight altitudes and conditions that can support contrail-generated cirrus clouds, the contrails exist 10 to 20 percent of the time. It has been estimated that in certain heavy air-traffic corridors, cirrus cloud cover has increased by as much as 20 percent.

Fly Now, Gray Later

The Minnis study suggests that contrails, which are already responsible for significant regional climate effects where air traffic is heaviest, could also become an international issue.

“As air travel continues growing in other areas, the effect may become globally significant,” he said. Using 25 years of global surface observations of cirrus cloud, temperature and humidity records from the National Centers for Environmental Prediction (NCEP), Minnis confirmed the cirrus trends with 13 years of satellite data from NASA’s International Satellite Cloud Climatology Project.

According to Minnis, 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 U.S.

“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,” he said.

The trends in cirrus cover and warming over the U.S. were greatest during winter and spring, seasons when contrails are most frequent. These results led to the conclusion that contrails are the cause of cirrus cloud increase.

Aircraft and Atmosphere

Minnis, who has researched clouds for about 25 years, began looking closely at contrails when NASA became interested in the early 1990s with the question of whether aircraft affect the atmosphere.

“We did field experiments in 1996 where we found that jet contrails can dissipate and cover areas as large as a state, suggesting that these could be climatologically significant,” Minnis said.

He estimated that groups of contrails that begin as thin gossamer lines drawn across the sky like chalk marks can spread to cover more than 7,500 square miles in just a few hours.

“We had one contrail that covered 2,317 square miles just in one day, and we’ve seen outbreaks where contrails cover areas as large as Kansas and Nebraska combined, lasting anywhere from 10 to 14 hours,” he said.

Clouding the Issue

However, the NASA report raised as many questions as it answered, igniting a debate within climatology circles.

“The idea that the earth is warming and high cloudiness is increasing and therefore part of the warming is due to increasing high cloudiness is not logically valid, if one is considering observations only,” said Andy Detwiler, a professor of atmospheric sciences at South Dakota School of Mines and Technology.

“Correlation does not equate to causation,” he said. “So many processes affect the temperature of the earth that contrails could easily be acting to cool the earth and yet the overall temperature trend could be increasing.”

Minnis agreed that correlation does not equate to causation. “That’s why we used the climate model estimates of cirrus effects to compute the temperature changes,” he said. “We did not simply relate cirrus to the observed changes, although the seasonal correlation in the theoretical and observed warming strongly supports our conclusion.”

A paper presented at the 2002 Conference on Aviation, “Range and Aerospace Meteorology” by David Travis, chair of the Department of Geography and Geology at the University of Wisconsin-Whitewater, clouds the issue further. Travis reported that contrails tend to warm the earth, but only at night; in daytime they cool the earth.

Travis took the unique opportunity following the grounding of all commercial aircraft in U.S. airspace for the three-day period of September 11 to 14, 2001, to evaluate the potential role of jet aircraft contrails on climate.

His study found a distinct increase in temperature for that three-day period, when contrails were absent in the sky, suggesting that the lack of aircraft flying in the upper troposphere helped reduce the cirrus cover over the U.S. during the time of aircraft groundings and thus allowed a temporary temperature increase until flights resumed the afternoon of September 14.

“There is no doubt that contrails produce a warming effect at night and also during portions of the daytime hours under certain conditions,” Travis said. Whether one dominates or they simply offset to cause a net change of zero is not clear.

Travis said the most important finding in the NASA report is the high cloud increase over the U.S. that has occurred during the past 25 years without any corresponding increase in ambient moisture.

“This allows us to point the finger further than ever before at an anthropogenic source, such as jet aircraft, for the added moisture and instability needed to create these additional high clouds,” he said.

However, the conclusion that the bulk of the warming in the U.S. for the past 25 years can be attributed to jet contrails is probably a bit overstated, Travis said.
Although there is strong evidence that jet contrails can produce a net warming similar to natural cirrus clouds, there are still a number of uncertainties about how much warming truly occurs during the daytime hours, he said.

In response, Minnis said the Travis study and others considered only immediate surface temp- erature effects in their studies, ignoring immediate warming of the upper troposphere, which eventually leads to surface warming.

“It’s true that the cirrus or contrail albedo effect [the reflection of sunlight by the contrails] produces an immediate cooling at the surface, which leads to the depressed maximum surface temperature that Travis attributes to contrails,” Minnis said. “However, at the same instant, the greenhouse effect from contrails and cirrus warms the layer of the atmosphere between 16,000 feet and 33,000 feet much more than it warms the surface.”

 Further, the warming in the atmosphere eventually is transferred to the surface and overtakes the cooling inferred by surface temperature measurements used by Travis.
“That’s why we used the results of a climate model that included the circulation changes and other feedbacks that lead to overall warming, and why we looked at atmospheric and surface temperatures. The daily temperature range provides only a piece of the equation,” Minnis said.

Joyce Penner, professor of atmospheric, oceanic and space sciences at the University of Michigan, and one of the authors of the 1999 IPCC paper, said Minnis calculated an effect on temperature that is large enough to explain recent growth rates, but that this “may or may not be reasonable.”

“If aerosol effects from fossil-fuel burning and biomass burning on lower altitude clouds are as large and negative as some models estimate, then this might be large enough to balance the large positive effects that Minnis calculates,” she said.

In response, Minnis said the aerosol effects from fossil fuel and biomass burning on clouds over the U.S. have probably not changed much since the 1970s.

“They might be more important in less developed areas or over the ocean where the air is more pristine, but the air is not a great deal dirtier now than it was in the early 1970s over the U.S.,” he said.

If that’s the case, then the aerosol effect would not be a cooling trend over the U.S.–it would just be a baseline, constant cooling that would go away if all of the low-level particulate emissions disappeared.

Other climatologists agree that more work is necessary to establish a clearer relationship regarding the effect of contrails on other climate variables.

“The Minnis paper appears to have shown the contrail effect could be important but has not related it to other better known and probably larger effects,” said Robert Dickinson, professor of dynamics and climate, in the Georgia Tech School of Earth and Atmospheric Sciences.

Indeed, Minnis did not take into account changes in aerosol concentrations, greenhouse gases and the geographical distribution of clouds, instead estimating temperature changes based on an application of limited general circulation model calculations and assuming cirrus coverage to be the only parameter changing during the period of study.

Global Warming

Still, Minnis is not the first to connect jet contrails to the issue of global warming. In 1999 the Intergovernmental Panel on Climate Change (IPCC), a body established by the World Meteorological Organization and the United Nations Environment Program to understand climate change, estimated that contrails from the world fleet of 12,000 civilian jetliners contribute as much to global warming as the carbon dioxide their engines emit as they burn aviation fuel.

The IPCC report stated that aircraft emit gases and particles directly into the upper troposphere and lower stratosphere, where they have an effect on atmospheric composition. These gases and particles alter the concentration of atmospheric greenhouse gases, including carbon dioxide, ozone and methane, and they trigger formation of contrails that may increase cirrus cloudiness–all of which contribute to climate change.

The expected 2- to 5-percent annual growth of worldwide jet air traffic over the next 50 years necessitates continued assessment of contrail climate effects.

“Better measurements of humidity, cloud distribution and contrail properties, in addition to more precise specification of flight paths and [better knowledge] of cirrus and contrail formation in general climate models, are needed to more rigorously determine the contrail climate impact,” Minnis said.

Minnis said contrails could be diminished or eliminated if jetliners reduced their altitude from about 33,000 feet to somewhere between 24,000 feet and 31,000 feet, depending on the weather.

But this solution would be expensive because lower altitude means denser air and higher air resistance, requiring airplanes to burn more fuel and cutting into their range capability. Burning more fuel would not just cost more, it would probably mean more carbon dioxide emissions, which conceivably could very well negate any benefit gained from contrail elimination.