Could contrails influence our climate?

By Jeff Mitton

I have always had my eyes on the sky.

My maternal grandfather noticed my habit when I was 8 years old, and he gave me a telescope to foster my interest. I used it to watch birds and the commercial air traffic going to and from Newark and LaGuardia airports. Those were low-flying, lumbering planes driven by propellers.

As the years went by, the commercial fleets turned to faster, higher-flying planes powered by jet engines. They appeared as glinting dots, given that they were so much higher, but their contrails always pointed directly to them.

Earlier this summer, I was driving along Interstate 70, returning to Colorado from a camping and photography trip in Utah. The sky was cloudless, but the number of contrails and their persistence was striking. I noticed them because I photograph natural subjects and landscapes and, in my mind, contrails mar landscape photographs. Traffic was nil between Green River and Grand Junction, so I counted them: 26 contrails.

Contrails often dissipate, sometimes within minutes, but they did not dissipate that day. They slowly diffused, becoming more diaphanous but growing wider. Then slowly but perceptibly they coalesced, and untracked portions of blue sky disappeared. The contrails had persisted, accumulated and coalesced into a cirrus cloud that covered the entire sky. I was driving beneath a man-made cloud.

Incidentally, the persistence of contrails is dependent on the relative humidity at 25,000 to 37,000 feet, for if the air is dry, the ice crystals in the contrail sublimate (turn from solid to gas, skipping the aqueous phase) quickly. Apparently, the troposphere was humid that day, sustaining the contrails and perhaps propagating the suspended ice.

Clouds influence the heating and the cooling of the earth. During the day, high-elevation cirrus clouds can reflect some portion of the light from the sun, reducing heating of lower levels of air and the ground. But at night, clouds act as a blanket, reducing radiation of heat into space. So what is the net impact of contrail cirrus clouds?

The tragedy of 9/11 triggered a series of events that provided insight into the role that contrails play in climate change. Because the terrorists used hijacked commercial jets to bring down the World Trade Center towers, all commercial flights were canceled in North America for three days — the contrails disappeared. Atmospheric scientists have used these three days to measure the impact of contrail cirrus clouds, and they found that the days were two degrees Fahrenheit warmer and the nights were two degrees cooler.

So we have learned that contrail cirrus clouds make the days cooler but the nights warmer, and those results make intuitive sense, given what we know about the effects of clouds. But we do not have clear insight into the effect of contrails on climate change, other than they reduce the daily amplitude of temperature fluctuations.

Contrails are primarily ice particles that form from water emitted by jet exhaust, but the exhaust also contains oxides of nitrogen and sulphur and soot particles. The oxides and soot particles help water vapor form water droplets, enhancing the likelihood of cloud formation.

Contrails seem to be fairly innocuous, and they do not yet have clear impacts on climate change. But I am suspicious of the long-term effects of emitting so much soot and nitrogen and sulphur oxides 25,000 to 37,000 feet above sea level.

We never imagined that small amounts of gases from our refrigerators could eat enormous holes in the ozone over Antarctica, but it happened. We never imagined that heating and air conditioning our homes and traveling by car, bus, train, boat and plane would cause the climate to change, but it did. It behooves us all to have our eyes on the sky.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado Boulder. This column originally appeared in the Boulder Daily Camera.

July 21, 2014