Aircraft Contrails & Non-CO2 Emissions: The Facts

Besides CO2, what are the other aviation emissions that have effects on the climate, and how can Business Aviation operators, once aware of these, reduce them with minor adjustments to their operations?

Until now, the aviation industry’s main focus in its efforts to achieve environmental sustainability and minimize its effects on climate change has been to reduce the volume of carbon dioxide emissions it produces.

Those efforts – ongoing and gradually increasing in scope – involve a welter of different strategies, including new techniques and technologies to improve operational efficiency, as well as ramping up the production and use of sustainable aviation fuel and (where possible) beginning using propulsion methods which aren’t based on burning fossil fuels.

But while aviation’s continuing drive to reduce its CO2 emissions is entirely worthwhile and important in minimizing its effects on the Earth’s atmosphere and climate, the industry now needs to be mindful of other non-CO2 emissions it also routinely produces.

As the state of scientific knowledge regarding aviation’s effects on the atmosphere and climate advances, it has become clear that long-persisting contrails produced by aircraft traveling at their most efficient cruising altitudes – usually from about 35,000ft to about 45,000ft – have an overall climate-forcing effect at least equal to that of CO2.

Factors Influencing the Effects of Persistent Contrails

In fact, in certain atmospheric conditions and at certain times of day, the overall climate-forcing effect of persistent contrails can be even greater than the total effect caused by CO2, according to Kennedy Ricci, President of aviation environmental sustainability company 4Air.

Contrails formed in the evening and at night, and which then persist overnight, can have the greatest climate-forcing effects of all. Why should this be?

Professor Steven Barrett, Director of MIT’s Laboratory for Aviation and the Environment, and Interim Head of MIT’s Aeronautics and Astronautics Department, explains that persistent contrails can have greater or lesser impacts on the climate depending on the time of day at which they exist and on a range of other factors.

Those factors include the length of time for which contrails persist – the longer the contrails remain in the air, the longer they can affect the climate and are spread by high-altitude wind shear.

Another factor influencing contrail effects is where above the Earth they are created. Contrails “occur predominantly in northern mid-latitudes and the upper troposphere”, according to the Intergovernmental Panel on Climate Change, and thus they have greater effects at those latitudes.

(One of the reasons for this predominant formation is that most long-distance flights over land and over the Atlantic, Pacific and Indian oceans route through the middle latitudes.)

Barrett notes that in the morning (particularly the early-morning daylight hours), contrails reflect sunlight falling on the Earth from above and so they can produce a cooling effect on the atmosphere overall.

But as the ground temperatures on the Earth’s surface gradually warm during the day, persistent contrails block the ability of the atmosphere to radiate the increasing heat into its upper reaches, trapping the heat beneath them. This heat remains in the lower atmosphere, influencing physical climatic processes.

In this context, the atmosphere’s upper reaches means the ozone layer and the stratosphere. In the regions where it exists, the ozone layer stretches from an altitude of about 20km above the Earth’s surface to about 30km above.

In the middle latitudes, the stratosphere begins at about 33,000ft – where a gradual layer of troposphere-stratosphere mixing called the tropopause is located above the troposphere below – and the stratosphere extends up to about 50km above the Earth’s surface.

The heat trapped by persistent contrails can exert climate-forcing effects because the atmosphere and the earth’s surface – particularly the surface of the ocean – absorbs this heat, increasing the overall temperature of the surface and the troposphere. The additional heat energy fuels and intensifies atmospheric physical processes such as water evaporation, cloud formation and wind production.

Sunlight does not fall on any part of the Earth when it is night there. So, if there is no radiative barrier in the atmosphere above, at night that part of the Earth’s surface can radiate surface heat into the stratosphere more easily.

However, if persistent contrails form at or near night over that part of the land or ocean surface, the surface cannot radiate the heat it has absorbed during the day into the upper atmosphere. Indeed, contrails do tend to form at night and persist longer through the night than they do during the day.

Having identified the problems that persistent contrails create, how do they form, and how can business aircraft operators help reduce them through small adjustments to their operations? Continue reading this article in the AvBuyer August digital edition by clicking the button below…