Shifting Flights From Night-time To Daytime Could Minimize the Climate Effect of Contrails
27 Jun, 2006 10:37 am
Aviation is the second largest transport sector after road traffic. Air traffic has grown dramatically and continues to do so. Predictions by EUROCONTROL see a doubling of today's air traffic by the year 2020. This implies a considerable increase in aircraft emissions. Air traffic affects climate in numerous ways.
Contrails affect the radiation balance of the Earth in two ways. They reflect part of the incoming solar (shortwave) radiation back into space, thus implying a cooling effect on the system (negative radiative forcing). Contrails also trap outgoing longwave (infrared) radiation emitted by the Earth and the atmosphere, thus enhancing the natural greenhouse effect of the atmosphere and implying a warming (positive radiative forcing). On average, the longwave effect dominates, and contrails are a contributor to climate warming, albeit currently a small one.
Our study  was part of ongoing efforts in Reading to look into the different mechanisms by which air traffic affects climate and how these climate effects can be minimized. One way to reduce contrail forcing is by avoiding the formation of contrails. Another way is to minimize the radiative forcing of contrails when they are present. Contrail radiative forcing depends on a variety of parameters, e.g., the amount of air traffic and the atmospheric conditions, as well as the optical properties of the contrail. Our study aimed to understand which parts of the diurnal and annual variation of these parameters have the largest impact on the radiative forcing of contrails.
We chose "Herstmonceux", a village in East Sussex, as the location for our study. Analyzing routine radiosonde measurements for this points, we could identify days with contrail favorable atmospheric conditions. We combined this data with flight data from the AERO2k database, which holds information about the amount of air traffic depending on height, time of day and day of year. The result was a diurnally as well as annually varying vertical profile of contrail cover.
We used a sophisticated radiative transfer model to simulate the interaction of longwave and shortwave radiation with the atmosphere and the contrail and first had a look at the seasonal variation of contrail radiative forcing.
As the humidity in mid-latitude cruising altitudes reaches its lowest values in summer contrail formation is less likely in summer than in winter. It is almost twice as likely for an aircraft flying over Herstmonceux to leave a contrail in winter than in summer. Even though air traffic has a maximum in summer, contrail radiative forcing is larger in winter than in summer. This shows the dominant role of variations in atmospheric conditions, compared to variations in air traffic. We found that half of the annual mean contrail radiative forcing can be attributed to flights during the three winter months of December, January, and February.
Next we had a look at the diurnal variation of contrail radiative forcing. Flight restrictions at night time mean that approximately 75% of the total air traffic over Herstmonceux occurs between 6:00 and 18:00 GMT. During these daytime hours the shortwave cooling effect and the longwave warming effect of the contrails nearly cancel each other out, leaving a comparably small net effect. However, for the remaining 25% of flights between 18:00 and 6:00 GMT, the greenhouse warming effect of the contrails remains unbalanced, as the sun is mostly down during this period. As a consequence flights during nighttime hours have a disproportionate effect on the diurnal mean contrail radiative forcing. They account for 60 to 80% of the mean contrail climate effect, even though only one in four flights occurs at night.
Our results suggest that shifting flights from night-time to daytime could be one measure to minimize the climate effect of contrails.
 Nature, vol 441, p 864-867 (15 Jun 2006)