What role do condensation trails play in our climate?

Condensation trails (short: contrails) are artificial ice clouds formed by airplanes at a height of around 10-13 km in the atmosphere. They are frequently seen on satellite pictures, as are natural ice clouds, as a result of their brightness and low temperatures. Contrails can be distinguished from natural cirrus clouds in these pictures primarily due to their linear orientation parallel to flight paths. The example below comes from an European overflight of satellite NOAA-12 on the 4th of May 1995 at 7:48 UTC (source: DLR-Oberpfaffenhofen). As the resolution of the picture is just over 1 km, only old, slightly dispersed vapour trails are identifiable.

Why do contrails form?
At the altitude where planes cruise (around 10-13 km) the temperature is very low, between -70 ºC and -40 ºC. Generally speaking, the temperature decreases with height as you climb from the surface of the earth to this altitude, however above this height it starts warming again. This temperature minimum, known as the tropopause, denotes the transition between the troposphere and the stratosphere above.
As the temperature decreases, the maximum amount of water vapour the air can hold becomes much smaller – any excess water vapour condenses and forms clouds. The water vapour content of a parcel of air as a percentage of the maximum possible for that parcel is known as the relative humidity – completely saturated air thus has a relative humidity of 100%. 100% relative humidity at the tropopause is only 1/1000 of water vapour density at ground level, as the air aloft is much colder and thus has a much smaller capacity. Levels for other gases however decrease much more slowly with altitude, so, for example, the density of oxygen or CO2 at the tropopause is just 1/3 of its value at the Earth’s surface. The burning of 1 kg of kerosene in an airplane engine produces 1.25 kg of water vapour and 3 kg of CO2, as well as amounts of nitric oxide and soot. Whilst this additional water vapour would be negligible compared to the background amount at ground level, near the tropopause it often leads immediately to condensation behind the airplane. The formation of these contrails becomes more likely the colder the surrounding air is.

If the relative humidity of the surrounding air at the altitude of the airplane is low, once the contrails have formed they cannot survive long as they evaporate through mixing with the surrounding air. In contrast, if the surrounding air is already above a relative humidity value sufficient for the formation of ice crystals, the ice particles in the contrails can survive for a much longer period of time. Such ice clouds can then spread, covering large areas and remaining visible for hours or even days.

Why can we see contrails?
Both the visibility of the contrails and their climatic influence basically depend on their optical properties. These properties are determined by the number, size and shape of the ice crystals formed in the cloud. Two opposing effects have roles to play here: in the region of the spectrum occupied by sunlight, the ice crystals have a cooling effect, reflecting solar radiation back to space; in the thermal region of the spectrum the ice clouds have a warming effect on the climate system as a whole, as the low temperatures at the tropopause reduce the amount of thermal radiation emitted out to space. The net effect of these two processes depends on various factors such as the altitude and the thickness and other properties of the cloud particles, so cannot be simply stated for the general case. Their small optical thickness does mean however that they generally act to strengthen the anthropogenic greenhouse effect.

Currently it is not clear whether contrails are significantly different from natural cirrus clouds in this respect. The exhaust fumes from airplane engines also increase the number of condensation nuclei in the air, onto which the water vapour in the exhaust gases can condense, later freezing to ice as they cool. This causes an increase in the amount of thin cirrus clouds, and likewise increases their effect on the Earth’s radiation budget.

The climatic influence of contrails
The increase in the degree of ice cloud cover due to contrails has a potentially important effect on the climate. Data from satellites and other observations shows significant sporadic, local increases in coverage from long-lived ice clouds formed as part of contrails. Over central Europe and the transatlantic flight path an average coverage of about 0.5% has been measured, off the main flight paths the amounts are smaller still. This value should be compared with an average coverage of 20% for natural ice clouds. Climate models predict that this increase in cloud coverage, currently tiny, leads to an equally small contribution to the greenhouse effect of 0.05ºC. The same simulations suggest that a tenfold increase in this additional coverage would however result in a much larger contribution to climate change. With the rapid increase in flight traffic that has been predicted, this effect cannot be ruled out as being a future player in climate change.

Along with the direct effect of the contrails themselves, it has been suggested that the extra condensation nuclei emitted in the exhausts might have a climatic influence once the contrails themselves have evaporated away. Their addition could cause the number of ice particles at the tropopause to increase so much that the later formation of natural cirrus clouds is made much more likely. These additional clouds can no longer be directly ascribed to the airplane emissions and hence are not included in studies of the contrail effects. The number of cirrus clouds observed in the last decades has increased however, which may be indicative of such an effect.