Weather: Do aircraft contrails contribute to surface warming?

Tuesday, August 4, 2009

The evidence is not all in, but so far it suggests that, while contrails may have some significant effects in heavily travelled regions, the global impact is likely to be small.

by HENRY HENGEVELD

If you look up at the sky on a clear day, odds are you will see a number of white streaks across the sky, like pencil strokes against the blue backdrop of endless space. Here and there, the lines are broken or have broadened into thin, almost transparent wisps. Often, you can see the line still being formed as you watch, a silver dart glistening slightly ahead of its leading point. Another jet leaving its contrail!

Jet contrails form when the compressed, hot and humid exhaust from the aircraft engines mixes with the extremely cold air present at high altitudes, then rapidly expands in its new low-pressure environment. This cooling and expansion of exhaust causes the water vapour within it to condense directly into ice crystals that collect as a long streamer in the wake of the airplane, much like the exhaust of a car on a cold winter day.

Contrails only form in the upper troposphere, where temperatures are below -35 C. If the surrounding air is dry, the crystals quickly sublimate back into water vapour. However, when humidity in the upper troposphere is relatively high, the crystals not only persist but become seeds for condensation of water vapour within the air itself. If a cross wind is present, they can rapidly expand, gradually becoming wide bands of cirrus cloud that can persist for hours and even days. On the other hand, if the aircraft briefly passes through a drier stretch of sky, this process is interrupted, creating a break in the line. 

With the global volume of air travel currently increasing at about two to five per cent per year, such contrails are becoming a standard presence in the atmosphere, and can now be observed over most of the planet. In some of the most heavily travelled regions, on days ideal for contrail formation, much of what otherwise would be clear skies now becomes veiled with criss-crossing patterns of cirrus clouds that can be readily observed from space.

Climatologists have long speculated on how such contrails might affect the flow of sunlight and heat energy in and out of the atmosphere, and thus affect our weather at the surface. Clouds are known to reflect and scatter incoming sunlight, which causes a surface cooling effect. On the other hand, they also insulate the earth's surface from heat loss to space by absorbing outgoing heat radiation and sending much of it back to the surface. That's why, every thing else being equal, cloudy nights are generally much warmer than clear ones.

Studies also show that, for denser low-level clouds, the cooling effect of reflected sunlight is greater than the warming effect of reduced heat loss. Hence, their presence caused a net average cooling. Cirrus clouds, however, are much wispier, and allow much of the incoming sunlight to pass through, but remain effective absorbers of outgoing heat. Their presence, therefore, should cause a net warming of surface temperatures. Can we thus conclude that jet contrails are a cause for surface warming?

Following the demolition of the World Trade Center towers on Sept. 11, 2001, all aircraft travel across the United States was halted for three days, freeing the skies of contrails. Thus, out of the disaster arose an opportunity to study their effects on surface temperatures. Several American researchers compared weather station records for those three days with what would normally be expected. They indeed discovered that maximum daytime temperatures were warmer and minimum nighttime temperatures cooler during the contrail-free period, and that the temperature range for those days had increased by 1.8 C – proof, they argued in a research paper, that contrails significantly affected surface temperatures.

However, subsequent studies have challenged these results. Canadian air traffic was also significantly curtailed during the Sept. 11-14 hiatus. Yet there was no evidence of a corresponding response in Canadian temperature data.

Another team of American investigators examined other factors that might have contributed to the increase in the diurnal temperature range across the United States, and pointed out that the increase was well within the normal variability caused by unrelated changes in low cloud cover.  Hence, while they note that contrails may indeed cause a slight warming effect, the U.S. data during the September 2001 event did not prove this.

However, since global air traffic is projected to at least triple within the next 40 to 50 years, climatologists were not yet ready to give up on better understanding the weather implications of contrails. In the United Kingdom, for example, they developed a volunteer observing network to record the daily variations in contrail presence and patterns over the main transport corridors, so that researchers could compare these in greater detail with local weather data.

Meanwhile, a team of researchers at England's University of Reading have developed a sophisticated radiation model to help analyze the impact of the large number of contrails that form over southeast England, as aircraft enter the North Atlantic flight corridor. They discovered that about 60 to 80 per cent of the net regional increase in surface radiation from contrails was attributable to night-time flights, even though these flights accounted for only 25 per cent of daily air traffic.

Furthermore, while winter flights account for only 22 per cent of annual air traffic, they contributed half of the annual mean forcing, a term used to describe mechanisms that alter the global energy balance and "force" the climate to change. The warming effect of contrails was most evident at altitudes of about 10 kilometres. At this altitude, each one per cent increase in air traffic caused an increase in related surface radiation of about 0.25 per cent.

However, researchers estimated that the net global warming effect from contrails caused by all current air travel worldwide to be only an average of about six milliwatts per square meter – or less than one per cent of that due to past increases in greenhouse concentrations. If these results withstand the test of further studies, they imply that the local effects of contrails on surface temperature might be significant in most heavily travelled regions, but that the global effects are likely to be small. 

The real world is far more complex than it may first appear – even for jet contrails!

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada.